ExxonMobil
Safety in LPG Design
LHHA(CO) if filled by
Pipeline or by Ship
Water Draw-off to Location
LI
LHA
PRV
min. 15 m from Fence
PI
LHHA
Concrete Wall
TI
Distance Valve to Fence15 m
to ESS
min. 0.9 m
LPG
Tank Slope 1:100
EBV
Removable Part
Removable Fill
First Edition by
Exxon Company, International
Florham Park, New Jersey
 ExxonMobil
Fairfax, Virginia
ExxonMobil Proprietary Information
This manual was produced using Doc-To-Help® Version 4, WexTech Systems, Inc.
Revision History
First Edition - January, 1996
Second Edition –October, 1999
Third Edition ExxonMobil – March, 2001 now including Mobil EPT 15-T-02 and Mobil SCS Volumes 1 and 2
For future additions, suggestions and changes please contact either:
Detlef Robertz/CentralEurope/ExxonMobil@xom or
Herbert Pui-Shui Ho/HKPRCTWN/ExxonMobil@xom
Contents
1
PREFACE
1.1
1.2
2
2-1
2.1
2-1
2-1
2-2
2-2
2-2
2-3
2-4
2-9
2-9
2-15
2.3
2.4
Exposures from and to the Site
2.1.1 Planning Considerations and Criteria
2.1.2 Topography and Prevailing Wind
2.1.3 Road Access and Traffic Situation
LPG Plant Layout
2.2.1 Space Requirements
2.2.2 Equipment Spacing to Maximize Separation
Electrical Equipment Specifications
2.3.1 Electrical Area Classification
Emergency Shutdown System
BULK STORAGE
3-1
3.1
3.2
3-1
3-2
3-2
3-3
3-6
3-11
3-12
3-14
3-15
3-15
3-16
3-16
3-17
3-17
3-23
3-23
3-25
3-26
3-27
3-28
3-29
3-30
3.3
3.4
3.5
3.6
4
1-1
1-2
PLANT SITE
2.2
3
1-1
Introduction
Objectives for Safety in LPG
Storage in Plants and Industry
Mounded and Above Ground Storage
3.2.1 Dimensional Sizing of Drums
3.2.2 Materials Selection for Tanks
3.2.3 Design Basis for Mounded Drums
3.2.4 Testing Requirements
3.2.5 Horizontal “Bullets” and Spherical Tanks
3.2.6 Spill Containment
3.2.7 Vacuum Conditions
Refrigerated LPG Storage
Overpressure Protection for Tanks
3.4.1 Contingencies to be Expected
3.4.2 Refinery and Upstream Pressure Relief
3.4.3 Pressure Relief in Marketing Terminals
Emergency Block Valves on Bulk LPG Tanks
3.5.1 Tank EBV's in Liquid Service
3.5.2 Tank Shutoff Valves in Vapor Service
Tank Instrumentation
3.6.1 Tank Level Measurement
3.6.2 Pressure and Temperature Indicators
3.6.3 Grounding Connections for Tanks
3.6.4 Product Odorization
PUMPS & COMPRESSORS
4-1
4.1
4-1
4-1
4-11
4-12
4-14
4.2
Pumps
4.1.1 Pump Types Commonly Used
Compressors
4.2.1 Compressor Types Used
4.2.2 Compressor Sizing
Safety in LPG Design
Contents
i
5
PIPING AND VALVES
5-1
5.1
5-1
5-1
5-4
5-4
5-5
5-5
5-6
5-7
5-8
5-9
5-9
5-9
5-10
5-10
5-11
5-11
5.2
6
PRODUCT TRANSFER
6-1
6.1
6-1
6-3
6-4
6-5
6-6
6-6
6-8
6-10
6-10
6-13
6-15
6-15
6-21
6.2
7
7-1
7.1
7-1
7-2
7-8
7-8
7-10
7-11
7-13
7-14
7-15
7-15
7-16
7-17
7-17
7-22
7-30
7-33
7-33
7-33
7-35
7.3
7.4
7.5
7.6
7.7
Cylinder Purchasing Specifications
7.1.2 Cylinder Specifications
Cylinder Valve Purchasing Specifications
7.2.1 Manufacturing Design Standards
Regulator Purchasing Specifications
7.3.1 Manufacturing Design Standards
Hose Purchasing Specifications
7.4.1 Hose clips
Cylinder Filling Plant
Cylinder Filling
7.6.1 Cylinder Processing and Filling
7.6.2 Manual Filling System
7.6.3 Automated Filling System
7.6.4 Integrated Automated Filling Plant
7.6.5 Purchasing Guidelines for Filling Plants
7.6.6 Third Party Cylinder Filling Plant
Cylinder Distribution
7.7.1 Distribution Center
7.7.2 Dealer and Reseller Cylinder Storage
FIRE PROTECTION
8.1
ii
Principles of Product Transfer
6.1.1 Loading or Unloading with Pumps
6.1.2 Loading or Unloading with Compressors
6.1.3 Using Pumps Versus Compressors
6.1.4 Static Electricity in Unloading and Loading
6.1.5 Hard Arms
6.1.6 Hoses in Product Transfer
Loading and Unloading
6.2.1 Truck Loading and Unloading
6.2.2 Rail Car Loading and Unloading
6.2.3 Marine Loading and Discharge
6.2.4 Marine Pier Installations
6.2.5 Pipeline Dispatch and Receipt
LPG CYLINDERS
7.2
8
Piping in Plants
5.1.1 Piping Arrangements
5.1.2 Piping Location
5.1.3 Piping Integrity
5.1.4 Pressure Ratings
5.1.5 Pipe Sizing
5.1.6 Pipe Connections
5.1.7 Small Piping Connections
5.1.8 Installation
Valves in Piping
5.2.1 Valve Integrity
5.2.2 Shutoff Valves
5.2.3 Backflow Check Valves
5.2.4 Thermal Relief Valves
5.2.5 Emergency Block Valves for Piping
5.2.6 Valve Packings
8-1
Passive and Active Fire Protection
8.1.1 Fireproofing, a Passive Fire Protection
8.1.2 Fire Resistant Coatings
Contents
8-1
8-1
8-2
Safety in LPG Design
8.2
9
Fire Protection System Design Philosophy
8.2.1 Firewater System, an Active Fire Protection
8.2.2 Protection Requirements
8.2.3 Flammable Gas Detectors
8.2.4 Fire Detectors
8-7
8-8
8-16
8-19
8-20
TRANSPORTATION
9-1
9.1
9.2
9-1
9-1
9-1
9-2
9-7
9-11
9-12
9-13
9.3
9.4
9.5
Means of Product Movement
Road Bulk Transportation Equipment
9.2.1 Truck Design and Procurement
9.2.2 Basic Design Considerations
9.2.3 LPG Truck Discharge System
Road Cylinder Transportation
Rail Tank Cars
Marine
10 CUSTOMER INSTALLATIONS
10.1
10.2
10-1
Cylinder Bank Installations
10.1.1 Design of Cylinder Banks
10.1.2 Installation of Cylinder Banks
10.1.3 Vaporization Rate in Cylinders
10.1.4 Icing or Sweating on Cylinders.
Containers at Customer Sites
10.2.1 Spacing and Location of Containers
10.2.2 Designing Customer Storage Systems
10.2.3 Sizing Of Containers and Vaporization Rates
10.2.4 Sizing Containers for Vaporizing Liquid
10.2.5 Enforced Vaporization by Means of Vaporizers
10.2.6 Installation of containers
10.2.7 Container Fittings and Piping
10.2.8 Container Valves and Accessories
11 AUTOMOTIVE LPG
11.1
11-1
Automotive LPG Stations
11.1.1 Design of Automotive LPG Equipment
11-1
11-1
12 LPG PROPERTIES
12.1
12.2
12-1
Product Properties
LPG Hazards
12-1
12-5
13 GLOSSARY OF TERMS
Safety in LPG Design
10-1
10-1
10-2
10-2
10-5
10-5
10-5
10-7
10-8
10-9
10-12
10-14
10-17
10-18
13-1
Contents
iii
1
PREFACE
1.1
Introduction
The manual is primarily intended for assisting managers and engineers responsible for
design, construction, or modification of LPG facilities. It should also be useful as a
ready reference for members of management who have a need to understand LPG
equipment and design practices.
The LPG guidelines included in this manual are intended for application to new
construction or alterations to existing facilities. The manual covers guidelines for the
safe receipt, storage, loading, transport, and unloading of LPG for refining, upstream,
and marketing bulk storage, as well as marketing cylinder filling plants and depots. The
manual is not intended to cover refinery onsites, upstream gas plant facilities, or
offshore platforms. Design practices among the functions are quite similar, although in
a few cases some differences appear due to the scale of operations in refineries or
upstream plants compared with marketing plants. These are noted. When deviations
from the guidelines become necessary they should receive safety review and approval
by the country/cluster.
If local regulatory requirements or current country/cluster practices are more
restrictive, however, they will of course supersede the guidelines described herein.
Local regulations have to be followed as a minimum.
The manual references EMRE Global Practices (GPs), EMRE Design Practices (DPs)
and ExxonMobil Engineering (EMRE) reports. GPs are typically referenced to provide
detail on the implementation of design concepts. Within GPs, individual paragraphs are
identified as to their purpose, i.e. safety, operability, reliability, maintainability, etc.
Deviation from “safety” paragraphs should receive formal safety review and approval.
The lead design engineer would control deviation from “other” paragraphs. DPs are
referenced to provide considerations for design. Their application requires engineering
judgment and the lead designer should determine when deviations require formal safety
review. EMRE reports are referenced to provide additional design considerations on
special topics. The lead design engineer should determine their applicability by
reviewing or contacting EMRE.
Industry standards used in this manual include NFPA (National Fire Protection
Association), LPGA (Liquefied Petroleum Gas Association), API (American Petroleum
Institute), NPGA (National Propane Gas Association), and IP (Institute of Petroleum,
UK).
The material contained in this manual is considered “Proprietary” and its distribution
limited according to company procedures for safeguarding proprietary company
information. If there is a strong business reason to give portions of the material to
customers or contractors, only the minimum portion containing pertinent information
shall be released.
Safety in LPG Design
PREFACE
1-1
Quality Assurance
Whenever possible, equipment for LPG Marketing applications should be purchased
from manufacturers who have submitted their product to a recognized governmental or
independent laboratory for analysis, evaluation, and performance testing in LPG service,
and have been granted approval as indicated by listing and/or labeling. Organizations
performing or sponsoring the type of testing recommended include US Underwriters
Laboratories, the UK Board of Trade, Lloyds Underwriters, the German BAM
(Bundesanstalt für Materialprüfung) or similar institutions, which are widely recognized.
It is desirable to have selected equipment covered by a testing program that provides
periodic quality assurance of the manufacturer that design, materials, and procedures are
unchanged from the initial approval. The Underwriters Laboratories listing program
incorporates this feature.
Approved LPG equipment will often be available for standard Marketing equipment,
e.g. cylinders, cylinder valves, filling equipment, small pumps and seals, and regulators.
Equipment for larger scale LPG operations in refining and upstream is often not
submitted for formal LPG approval, because it is manufactured in accordance with
industry standards for hydrocarbon service, which includes LPG. When equipment has
not been approved for LPG, its mechanical design and materials of construction
reviewed by the project engineer, with input from specialists when necessary, to
determine suitability for LPG use, and to ensure that the device will function
satisfactorily for the intended service life.
The conversion of valves and accessories originally designed and fabricated for another
service is not recommended. All valves and accessories shall be supplied by the
manufacturer as assembled and tested at the factory, without subsequent modifications
1.2
Objectives for Safety in LPG
Following are objectives to maintain high standards of equipment design and operations.
1. Minimize or eliminate LPG incidents
2.
Develop and coordinate inter-functional LPG safety training programs.
3.
Monitor regulatory trends.
4.
Update manuals at regular intervals.
5.
Publish information on lessons learned from incidents that occurred in the
company or industry.
Procedures for LPG
Inter-functional training programs for LPG safety have been developed and coordinated
through the LPG Safety Network.
The LPG design guidelines contained in this manual are based on current ExxonMobil
and industry design philosophy and practices, and on the results of company LPG risk
assessment programs. Where differences exist between country/cluster design practices
and those presented in this manual, it is suggested that country/cluster management
consider adopting the practices described herein. ExxonMobil issues this manual to its
country/clusters for suggested use in design of Liquefied Petroleum Gas (LPG) facilities
used in Upstream, Refining and Marketing operations. Relevant aspects of safety for
LPG design are documented in this third edition (2001) of the “Safety in LPG Design”
manual. LPG safety design and operations manuals will be updated on a 3-yearly basis.
More urgent updates will be communicated through the LPG Safety Network and
through the BestNet. Recommended changes to guidelines and practices will be
developed as appropriate.
1-2
PREFACE
Safety in LPG Design
New directives and standards are coming from legislative bodies and industrial
associations, which every nation is directed to follow. Regulatory developments will be
controlled by following the OIMS systems.
Distribution of industry and company incidents and lessons learned will be done on a
timely basis through the LPG Safety Network.
Responsibilities
ExxonMobil LPG Technical Advisors provide guidance and assistance to help plants to
implement procedures and practices described in the LPG manuals and solve their
problems. The ExxonMobil LPG Advisors are primary contacts for Marketing
country/cluster LPG issues such as incidents and lessons learned, training needs,
suggested revisions to LPG safety manuals. They attend industry LPG meetings,
monitor technical and regulatory trends, evaluate changes in industry procedures and
practices for applicability. They communicate to the country/clusters as appropriate.
Representatives from Upstream, Refining, Marketing, and Marine will have an influence
on company LPG standards reflected in the “Safety in LPG Design” and “LPG Safe
Operations Guidelines.” Ultimate decisions on technical issues will be made by the
ExxonMobil LPG Technical Advisors and ExxonMobil Research and Engineering.
Verification and Feedback
Yearly verification of SHE performance of LPG operations by the SHE organization.
Summary of incidents, lessons learned. Identify areas for system improvement and
implement.
Safety in LPG Design
PREFACE
1-3
2
PLANT SITE
2.1
Exposures from and to the Site
Before choosing a plant site it is important to study all relevant facts that may be of
influence on the choice. Such items of influence are exposure to and from the
neighborhood, topography, prevailing wind direction and the road traffic situation.
It is considered appropriate to locate an LPG plant in an industrial zone. The plants in
the neighborhood may preferably be refineries or storage plants or similar industries
where ignition sources are rare or under control. Therefore, neighborhood to any type of
facility which incorporates obvious ignition, sources, or which employ hazardous
processing methods (e.g. storing powerful oxidizing agents), shall be avoided. If known
exposures are more severe, increased clearances may be necessary.
Warning signs “NO SMOKING,” “FLAMMABLE GAS” shall be posted at all
LPG handling areas and outside the gate. The locations of the signs shall be
determined by local conditions, but the lettering shall be large enough to be visible and
legible from each point of transfer.
2.1.1
Planning Considerations and Criteria
The input data required to start the design of an LPG system shall be available from the
ExxonMobil marketing unit responsible for the proposed facility. This data shall
include the following:
1.
Products to be handled and peak volumes in bulk and in cylinders.
2.
Total storage capacity required for each product.
3.
Requirements of local government agencies with jurisdiction.
4.
Allowances for future expansion.
5.
Product sampling requirements.
6.
Toxic materials in plant.
7.
Site data including limitations or restrictions.
8.
Possibility of storage and handling of other petroleum products within the
facility.
9.
Evaluation of any surrounding external hazards at the site.
10. Capacity and frequency of each type of LPG delivery to the site from tank
trucks, railroad cars, marine vessels and pipelines.
11. Number of tank truck or railroad tank car unloading positions required.
12. Maximum and minimum flow rates for each type of equipment.
Safety in LPG Design
PLANT SITE
2-1
13. Method used to verify quantity of shipment: weight, volumetric measurement
or meters.
14. Types of unloading pumps or compressors carried by transporting equipment.
15. Number, size and type of connections on transporting equipment.
16. For cylinder filling, the type, number and layout of scales, and the size,
number and type of cylinders to be filled.
17. Cylinder painting, stenciling, washing and testing facility requirements. If
possible, this task should be contracted to third party companies.
18. Method of receipt, storage and handling of cylinders: Manual handling or
palletization, forklift, hand truck or other.
2.1.2
Topography and Prevailing Wind
Considering the basic characteristics of LPG vapor, it is desirable to locate a plant in an
area which is free of depressions and contours radiating from the plant which might
convey vapors to a point of exposure.
In the event of an accidental discharge of product within the plant, LPG tends to
vaporize rapidly. Dissipation of vapors below the lower flammable limit develops
more rapidly if the plant site is elevated slightly above the general terrain, or is slightly
inclined. Dissipation of vapors in a flat location will mainly depend on wind or time. In
low-lying areas vapors may stay trapped despite wind. This potential shall be
considered in connection with gas leaks.
Any site considered for a marketing plant shall be analyzed with respect to the
prevailing wind direction. Prevailing winds may have an influence on potential
exposures. Based on prevailing wind, sites shall, whenever possible be located down
wind (meteorological data) of population centers or known ignition sources. Tanks shall
always be located downgrade and downwind from possible ignition sources.
2.1.3
Road Access and Traffic Situation
Any streets adjacent to the plant site shall be analyzed to determine that a safe traffic
flow in and out of the plant is possible. The access road shall be of such width that
vehicles could enter and leave the plant without creating a hazardous condition.
Consideration shall be given to the traffic conditions on the access road during peak
traffic flow in and out of the plant. Visibility, which is influenced by road curves etc.,
may play an important role in these considerations.
2.2
LPG Plant Layout
Following are some concepts that govern the layout/plot plan of the bulk LPG storage
facilities.
Bulk LPG Tanks shall be located together to minimize piping and general site size.
Before the final location for bulk storage is selected, a site survey shall be conducted
and if deemed necessary, soils investigation shall be done. It should be defined whether
the storage is above or below ground and whether cylindrical or spherical tanks shall be
used. These investigations shall provide sufficient data on the bearing capacity,
drainage characteristics, estimates of settlement, and remedial measures, if necessary.
Future expansion plans shall also be considered. Bulk transports that discharge or
receive product within the plant shall be provided with a clear access through the plant,
which can be negotiated without, at any time, reversing the unit. Sufficient space shall
be provided at a loading or unloading location to allow for positioning the transport with
minimal chance of collision with other vehicles or fixed objects. Loading or unloading
2-2
PLANT SITE
Safety in LPG Design
facilities shall be protected with guardrails or stanchions to prevent damage from
vehicles. Vehicles shall be facing an exit while loading or unloading.
Pipeline or marine loading and discharge operations will utilize the least amount of
land since pipelines need only metering and control equipment and ships need only
shore-based compressors for the unloading operation. However, a considerable amount
of waterfront space shall be allocated for marine berth(s). With product delivered by
rail tank car, sufficient space shall be allotted to the rail line and the racks and
compressors for the unloading operation.
Pumps and compressors shall be grouped together and arranged such that the piping
is as simple and direct as possible. Electrical control panels and other support
equipment shall be located in accordance with the electrical area classifications.
Figure 2.1 a: - Example for plant layout in sloped terrain
The Cylinder Filling Plant, with storage area, loading dock and access area, shall be
laid out to ensure:
2.2.1
1.
Optimum traffic pattern/parking for cylinder transport trucks.
2.
Minimum interference between truck and fork lift traffic.
3.
Clear separation of filled/empty cylinders.
4.
A minimum of necessary cylinder handling. Cylinders should “flow” from
the empty to filled area.
5.
Optimize natural ventilation by positioning the cylinder filling plant such that
natural air currents will be utilized in the most effective manner.
6.
A separate area is designated for cylinders requiring repair or refurbishment.
Space Requirements
Space requirements will normally incorporate the following functions:
Safety in LPG Design
1.
Gate house and security fence.
2.
Administration and Control Building.
3.
Bulk product receiving and dispatching areas, including weighbridge.
4.
Tank truck loading and unloading area with access roadway.
5.
Railroad siding, including loading and unloading area.
6.
Tanker or barge loading or unloading facilities.
7.
Stenching facilities for odorization of the LPG.
PLANT SITE
2-3
8.
Bulk product storage.
9.
Cylinder handling and storage (new, empty, filled, scrapped, truck parking).
10. Cylinder filling and inspection processes.
11. Other fuels storage or manufacturing facilities.
12. Pumps and compressors for loading and unloading trucks, rail cars, tankers
and barges.
13. Pumps normally used for filling cylinders.
14. Cylinder repair and re-qualification processes (if located in filling plant).
15. Maintenance shop and warehouse.
16. Storage area for reserve supply of cylinders and customer containers.
17. Staff, customer and plant vehicle parking areas.
18. Fire water systems.
19. Firefighting access.
All of the above should be interrelated to the spacing requirements which are discussed
below. A qualified company outside the filling plant may preferably carry out the
cylinder repair and re-qualification process.
Figure 2.1-.b: Filling plant with unloading rack, tanks, pump/compressor house and filling area
2.2.2
Equipment Spacing to Maximize Separation
The most important objective of spacing is to separate risks by zoning. The higher a
risk, the larger the spacing. Spacing requirements between tanks will help to limit the
spread of fire, should it occur. In case of accidental leakage, spacing between
equipment and the fence will help to disperse flammable mixtures below lower
flammability limits (LFL) such that ignition by uncontrollable external ignition sources
(e.g. passing cars) may be prevented. Another aspect that influences spacing is the
requirement for safe access of the operator to perform an emergency
shutdown/activities but also for normal operations and maintenance. Also traffic
patterns for truck and rail loading shall be considered. The safe location of the control
room and firefighting pumps may be of importance during an emergency.
The equipment spacing requirements detailed below are minimum figures, which in
general will satisfy the above objectives. Whenever a special consideration or particular
factors (e.g. plot space available) require deviation from the spacing rules a safety
specialist shall perform a risk assessment. Additional safety and firefighting
equipment requirements may compensate for a lack of spacing. So, if for instance, LPG
is stored in vicinity to housing, passive fire protection may be the answer to reducing the
risk. This may be achieved by mounded storage of the tanks.
2-4
PLANT SITE
Safety in LPG Design
Where sufficient space is available, the ground can be contoured and sloped such that
the liquid from a potential leak will flow away from the storage tank to a remote
location. In case of a subsequent fire, this would be desirable since the liquid from the
leak would burn at a place away from the tank. Admittedly for Propane and hot climates
this is not a major mitigation factor since the bulk of the leaking gas will flash right at
the leak. But for Butane and colder climates it may mitigate the situation considerably.
2.2.2.1
Spacing at Refining and Upstream Gas Plants
Spacing of storage and loading and unloading facilities at Refining and Upstream Gas
Plants shall be in line with spacing requirements documented in the Design Practices DP
XV-G, “Equipment Spacing” and GP 9-1-1, “Spacing and Dikes for Storage Vessels and
Tanks.”
2.2.2.2
Spacing Requirements for Marine Berths
Planning for Marine berths requires considering a number of issues affecting spacing.
Generic marine issues such as berth layout, approaches to the berth, dredging etc. may
affect the location of the berth and spacing requirements regarding other shore-based
facilities. Information on such issues is available in the Marketing Engineering Standard
EE.3M.86 “Marine Facilities, Design, Specification and Evaluation.” If multiple LPG
berths are planned, additional spacing considerations between berths shall be taken into
account. The minimum spacing criteria for fire and safety considerations is 30 m.
However, in most cases, other plant design considerations shall require berth spacing
greater than 30 m. More detailed descriptions of such considerations and additional
commentary is provided in the ERE report EE.131E.79 “Suggested Design
Considerations for Refrigerated Liquefied Gas Facilities.” Additional clarifications are
provided in “Clarifications of Recommendations Arising from the ‘Betelgeuse’
Incident” (83 EEEL 514 or 83 CMS3 R9). EMRE's Marine Section should be consulted
from the early stages of the project to ensure that the appropriate issues have been
considered regarding berth layout and spacing.
2.2.2.3
Spacing at Marketing Plants
Spacing to the property line for Marketing Plants is consistent with API 2510, as
indicated in the table below. Marketing plants may store considerably smaller volumes
of LPG than Refineries or Upstream sites.
Individual Tank
Capacity, m3
Spacing to
Property Lines, m
8 - 110
< 260
< 340
< 450
< 760
15
20
30
40
60
< 3800 > 3800
90
120
Table 2.2.2.3-a: Spacing to property lines
At some locations, where risk exposure is considerably lower than usual the design
engineer may deviate from the spacing requirements. A typical example would be a
marketing plant on a riverbank surrounded by industrial plants (no housing). When
additional storage is required at existing sites, and the spacing guidelines cannot be met,
risk assessment techniques may be used to evaluate reduced spacing. As explained
earlier, compensation for reduced spacing may be achieved by adding active/passive fire
protection. The ExxonMobil LPG Technical Advisor or an EMRE Safety Engineer
should be consulted when planning to deviate from spacing requirements. The rationale
for deviating shall be documented in the design memorandum.
In addition to property line spacing, Marketing sites have other spacing guidelines not
covered by DP XV-G. Additional guidelines, as well as spacing from DP XV-G
commonly applied at Marketing plants, are covered by the following table.
Safety in LPG Design
PLANT SITE
2-5
Property
Lines
Office
Building
Cylinder
Filling
Loading,
Unloading
Sphere
Horizontal
Bullet
Mounded
Tank
Pump or
Compressor
Sphere
Note 1
30
30
30
Note 2
Note 3
Note 6
Note 9
Horizontal Bullet
Note 1
30
15
15
Note 3
Note 4
Note 7
Note 10
Mounded Tank Valving
15
15
15
15
Note 3
Note 5
Note 8
5
Mounded Tk. Covered Part
3
3
3
3
Note 6
Note 7
Note 8
3
Truck Loading/Unloading
30
30
30
15
30
15
3
3
Rail Unloading/Loading
15
30
30
15
30
15
3
3
Cylinder Filling
30
30
Note 11
30
30
15
Note 12
5
Note 11
Note 11
30
30
30
30
Note 12
30
Spacing Distances,
in Meters
Firewater Tank or Pump
Table 2.2.2.3-b: Spacing within Marketing LPG bulk storage plant
1.
Note 1: Above ground tank spacing from property lines according to
the above Table 2.2.2.3-a: Spacing to property lines.
2.
Note 2: ¾ Diameter of larger sphere.
3.
Note 3: ¾ Diameter of sphere.
4.
Note 4: 1 Diameter of larger bullet (or 1.5 m min.).
5.
Note 5: 1 Diameter of bullet.
6.
Note 6: At sphere bund wall.
7.
Note 7: Next to bullet toe wall.
8.
Note 8: Spacing between mounded drums does normally not involve
any fire hazard considerations, therefore, the following is
recommended:
a. Tanks up to 135 m3 water capacity shall have a minimum
spacing of 1.5 m between the tanks.
b. Tanks over 135 m3 water capacity: The site conditions and
the needs for safe installation, testing, maintenance, and
removal shall determine the spacing between adjacent
tanks.
Note 9: Pumps, compressors, and other equipment (including piping
not related to LPG tanks) shall be outside bund walls.
9.
10.
11.
12.
Note 10: Pump drawing from individual bullet may be outside toe wall
and minimum 3 m from bullet. Other pumps or compressors
5 m.
Note 11: No minimum. Provide spacing appropriate for access.
Note 12: From drains, vents, and valving or flanges 15 m; from
covered part of mounded drum 3 m.
For spacing to atmospheric storage of other fuels or refrigerated storage see GP 9-1-1.
Spacing of groups or “stacks” of cylinders to the property line is discussed in Chapter
7. Dikes spacing to spheres shall be according to GP 9-1-1 “Spacing and Dikes for
Storage Vessels and Tanks.” Toe wall spacing to shell of bullet shall be 3 m. Also refer
to section 3.2.6 “Spill Containment.”
2-6
PLANT SITE
Safety in LPG Design
Minimum Distances
Flammable Liquid
Storage Tank
Bullet up to 135 m3
Bullet over 135 m3
Flash Point lower
than 37 oC
6 m to bund wall
15 m to bund wall
Flash Point from 37
to 65 oC. Tank size
up to 3,000 liters
Safety distances for LPG
tank or 3 m to the tank /
bund wall, whichever is
the less
6 m to tank, bund wall
or diversion wall
Tank size over 3,000
liters
3 m to bund wall or
diversion wall and 6 m to
tank
15 m to tank, bund wall
or diversion wall
Table 2.2.2.3-c: Minimum separation distances for LPG horizontal tanks (bullets) from other
flammable liquid storage
Non pressurized hazardous and Flammable
Storage Tank Type and Product
Minimum separation
distances
Refrigerated LPG Tank
¾ diameter of the larger
tank or sphere
Storage Tank with product flashpoint 37 oC
or less
1 diameter of the larger
tank or sphere
Storage Tank with product flashpoint more
than 37 oC
½ diameter of the larger
tank or sphere
Table 2.2.2.3-d: Minimum separation distances for LPG sphere tanks from other flammable liquid
storage.
LPG tanks shall not be installed within the bunded area for flammable or combustible
liquid storage tanks. The minimum distances of separation between a LPG horizontal
tank and a storage tank containing a flammable liquid shall be according to Table
2.2.2.3-c. For LPG spherical tank and a tank containing flammable liquid, Table
2.2.2.3-d shall be used. Open drains, gullies or ducts located within the tank safety
distances in Tables 2.2.2.3-a and b, carrying water runoff from the ground underneath
aboveground LPG tanks shall be provided with an LPG trap or be sealed to prevent LPG
liquid and vapor from passing through.
No permanent source of heat shall be located within 1.5 m of a LPG tank. LPG tanks
shall not be located directly beneath electrical power cables. LPG tanks shall be located
such that a break in overhead electrical lines shall not cause exposed ends to fall onto
any tank or equipment. No horizontal separation shall be required between an
aboveground LPG tank and underground tanks containing flammable or combustible
liquids installed in accordance with NFPA 30.
If, in industrial installations, LPG and oxidizing gases or hydrogen are stored on the
same premises, the following minimum distances shall be observed:
Chlorine
LPG
Oxygen* (> 0.75 t)
LPG (> 1.9 m3)
Gaseous Hydrogen* (> 7 kg)
LPG (> 1.9 m3)
*see NFPA 58 for smaller capacities
Safety in LPG Design
PLANT SITE
300 m
15 m
15 m
2-7
Depending
on Tank Size
Electrical
Insulation
Remote
Impoundment
min. 15 m
min. 15 m
min.
3 m
RAIL CARS
SPHERE
Distance depends
on drum size
MOUNDED
TANK
Dike
BULLET
MOUNDED
TANK
BULLET
3/4 Sphere
Diameter
one bullet diameter
15 m
Depending
on Tank Size
15 m
RAILWAY
Toe Wall
Earth Mound
Depending
on Tank Size
min. 30 m from
Sphere or Bullet
min. 15 m from
LPG bulk storage
30 m
Propane Loading
Butane Loading
FIRE
WATER
TANK
FILLED
CYLINDERS
FIRE
PUMPS
30 m
CYLINDER
FILLING
EMPTY
CYLINDERS
OFFICE
BUILDING
30 m
min. 15 m from manhole of Mounded Drum
min. 30 m from Sphere/Bullet or Cylinder filling
Pumps or compressors must be outside spill containment
and may be 3 m away from tanks they take suction from,
but 5 m from other tanks (like next bullet or next sphere dike)
One Way
GATE
Figure 2.2.2: Spacing in marketing LPG bulk plant with cylinder filling
2.2.2.4
Siting of Aboveground Tanks and Equipment
Pressurized LPG tanks shall not be located within buildings, within the spill containment
area of flammable or combustible liquid storage tanks as defined in NFPA 30, or within
the spill containment area for refrigerated LPG tanks.
Rotating equipment and pumps taking suction from the LPG tanks shall not be located
within the spill containment area of any storage facility.
Horizontal tanks used to store LPG may be oriented so that their longitudinal axes do
not point toward other tanks, process equipment, control rooms, loading or unloading
facilities, or flammable or combustible liquid storage facilities located in the vicinity of
the horizontal tank.
Horizontal LPG tanks shall not be stacked one above the other. Horizontal tanks used
to store LPG shall be grouped with no more than six tanks in one group. Where
2-8
PLANT SITE
Safety in LPG Design
multiple groups of horizontal LPG tanks are to be provided, a minimum horizontal shellto-shell distance of 15 m shall separate each group from adjacent groups.
2.3
Electrical Equipment Specifications
Electrical equipment and wiring shall comply with the specifications of, and be installed
in accordance with the requirements of the local electrical codes. For reference on
reliability in electrical design, see the related electrical GPs.
2.3.1
Electrical Area Classification
Normal electrical equipment can be considered an ignition source. Therefore, where
flammable liquids, gases or vapors are handled, or stored, special electrical equipment
shall be installed, which normally will not serve as an ignition source. The industry has
produced standards to differentiate the ignition potential of electrical equipment.
Following are the levels of protection:
1.
Equipment that will never produce a spark, even if it fails.
2.
Equipment that, when operating normally, will not produce a spark, but may
do so if it fails.
3.
Equipment that will produce a spark during normal operation.
The likelihood of encountering flammable vapors in plants governs the level of
equipment needed. Plants shall be divided into separate areas according to the
likelihood of flammable LPG vapors being present.
FILLING HOSE
FLAMMABLE
VAPORS
FLAMMABLE
Figure 2.3.1-a: Example for Zone 1 area
Based on experience, minimum distance requirements between points of potential gas
release and electrical installations have been developed. These minimum distance
requirements are defined in both NFPA 497A and API 500. Classifications used in LPG
and other Hydrocarbon service are called Zone 0, Zone 1 and Zone 2 distances. Zone 0
is an area where an explosive gas atmosphere is continuously present, or present for a
long period. Definition of Zone 1 and 2 follow below. Different Zones require different
quality electrical installation. Areas that require no special electrical equipment are
called “Unclassified.” Following are definitions for the electrical classification areas.
Zone 1 areas are defined as locations where ignitable concentrations of flammable gases
or vapors are likely to occur in normal operation. Below grade spaces such as trenches,
Safety in LPG Design
PLANT SITE
2-9
pits and sumps are typical Zone 1 areas. This may be by frequent releases or by
infrequent releases or small releases combined with inadequate ventilation. The
Example in Figure 2.3.1-a shows a solvent drum filling area, for LPG it would be
around the filling nozzles.
FLAMMABLE VAPORS
PUMP SEAL LEAK
VAPORIZING LIQUID
Figure 2.3.1-b: Example for Zone 2 area
Zone 2 areas are defined as locations where an ignitable concentration of flammable
gases or vapors is not likely to occur in normal operations. If it does occur it will be
infrequent and will exist for a short period. Examples for Zone 2 are areas adjacent to
Zone 1 (and not separated by a vapor barrier), areas normally prevented from explosive
mixtures by positive ventilation, and areas where abnormal operation or equipment
breakdown might create an explosive mixture.
Figure 2.3.1-c: Example for Unclassified area
Unclassified is defined as locations where there are little or no hazards from flammable
gases or vapors under normal or abnormal operating conditions. Plant roads, adequately
ventilated LPG cylinder storage areas, and maintained, adequately ventilated piping
systems, which may include valves, fittings, meters and flanged or threaded connections
(per GP 16-1-1) are examples of unclassified areas.
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PLANT SITE
Safety in LPG Design
Following selected example drawings from NPFA 497A showing how the electrical
classifications apply. It is important to notice that they also include the space above the
potential leak source. The hatched areas indicate that in these spaces only certified
(Zone 1 or 2) electrical equipment can be installed.
SOURCE OF
POTENTIAL LEAK
7.5 m
UNCLASSIFIED
7.5 m
UNCLASSIFIED
7.5 m
0.6 m
15 m
BELOW GRADE LOCATION
SUCH AS A SUMP OR TRENCH
30 m
ADDITIONAL
ZONE 2 AREA
ZONE 2
ZONE 1
UNCLASSIFIED
Figure 2.3.1-d: Electrical classification area around pump seal.
The contour of the envelope roughly approximates the flow that gases may follow in
case of leak. It is important to notice that the complete area below a potential leak is
considered classified. However, if a vent exits through a roof the hemisphere of a 7.5 m
radius may be considered Zone 2 area. Pits and trenches, unless ventilated by force,
shall be considered as Zone 1 areas. The outer 0.6 m Zone 2 region in the figure above
is additional area to reflect crawling of heavier-than-air vapors along the ground. This
area would normally be included for LPG applications, as explained in NFPA 497A.
7.5 m
SOURCE OF
POTENTIAL
LEAK
7.5 m
UNCLASSIFIED
UNCLASSIFIED
7.5 m
0.6 m
BELOW GRADE LOCATION
SUCH AS A SUMP OR TRENCH
15 m
30 m
ZONE 1
ZONE 2
ADDITIONAL
ZONE 2 AREA
UNCLASSIFIED
Figure 2.3.1-e: Electrical classification area around an elevated source of potential release
Notice that a sump on a pier is a Zone 1 area due to potential collection of vapors. At a
Pier the Zone two area extends to the water level. Electrical installations are rarely
found below the pier level but this may be important for small vessel traffic.
Safety in LPG Design
PLANT SITE
2-11
The area around the valves of rail cars and trucks is Zone 1 because of frequent making
and breaking of loading and unloading connections. Depending on conditions, a 7.5 m
diameter zone may be required around the pressure relief valve as indicated in the truck
drawing.
15 m
15 m
7.5 m
7.5 m
7.5 m
UNCLASSIFIED
15 m
7.5 m
0.6 m
PIER
SUMP
WATER LEVEL
ZONE 1
ZONE 2
UNCLASSIFIED
Figure 2.3.1-f: Electrical classification area around marine unloading facility
2.3.1.1
Electrical Codes
The US National Fire Protection Association Codes #70 (National Electrical Code) and
#58 (LP-Gas Code), or UK Institute of Petroleum “Model Code of Safe Practices in the
Petroleum Industry,” Parts 1, (Electrical) and 9, (Liquefied Petroleum Gas),
supplemented by Health and Safety Executive publications HSG 34, “The Storage of
LPG at Fixed Installations” and HSG 22, and British Standard 5345 are commonly used
as the design basis for electrical systems.
1.5 m
ZONE 1
ZONE 2
7.5 m
UNCLASSIFIED
Figure 2.3.1-g: Electrical classification area around LPG rail car
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PLANT SITE
Safety in LPG Design
Truck
PRV
7.5
Truck
ESS
m
Truck
ESS
7.5
m
1.5 m
Electrostatic
Bonding Cable
EBV
EBV
ZONE 2
ZONE 1
UNCLASSIFIED
.
Figure 2.3.1-h: Electrical classification area around LPG truck
Standard
Continuous
Hazard
Intermittent
Hazard
Hazard under
abnormal
conditions
IEC/CENELEC/
EUROPEAN
Zone 0
Zone 1
Zone 2
NORTH
AMERICA
Division 1
Division 2
Table 2.3.1.1-a: Comparison of Area Classification
IEC / CENELEC /
EUROPEAN
NORTH AMERICA
(CLASS 1)
Acetylene
II C
A
Hydrogen
II C
B
Ethylene
II B
C
Propane/Butane
II A
D
GAS
Table 2.3.1.1-b: Gas Grouping for Area Classification Protection Techniques
2.3.1.2
Electrical Installations
Both European and American practices are acceptable. The requirements for electrical
installations shall be in accordance with NFPA 70 or equivalent. Current-carrying
Safety in LPG Design
PLANT SITE
2-13
conductors shall be made of copper. Electrical wiring shall be installed such that the
system is free from short circuits and from grounds. All protection devices shall be
properly sized, selected and installed. An overall electrical study for the entire electrical
system shall be undertaken by qualified electrician or electrical engineer. Internal parts
of electrical equipment shall not be damaged or contaminated by foreign materials.
There shall be no damaged parts that may adversely affect safe operation or mechanical
strength of the equipment. Conductors of dissimilar metals shall not be inter-mixed in a
terminal or splicing connector where physical contact can occur between the dissimilar
conductors.
Live parts of electric equipment shall be designed to guard against accidental contact
using any of the following means:
1.
Approved enclosures.
2.
Locations in a room or similar enclosure accessible only to qualified persons.
3.
Suitable partitions arranged so that only qualified persons will have access to
space within the reach of live parts.
4.
Location on platform so elevated and arranged as to exclude unqualified
persons.
5.
Elevation of 2.5 m or more above the working surface.
Parts of electric equipment which in ordinary operation produces arcs or sparks shall be
enclosed or separated and isolated from all combustible material. Circuit breakers for
electrical equipment shall be legibly marked to indicate its purpose.
2.3.1.3
Emergency Shutdown Systems
The emergency shutdown system in a liquid transfer operation shall close all emergency
shutoff valves and stop all pumps when activated. The location of Emergency
Shutdown Pushbuttons is described below under “Emergency Shutdown Systems.” The
shutdown buttons shall be RED in color of the Push-to-Activate, Pull-To-Reset type.
They shall be clearly marked for the purpose for which it is intended and protected
against accidental activation. All emergency shutoff valves shall be provided with both
open and closed position indicators. All wiring and logic diagrams shall include a
written description of the proposed operation. Each sequence trip and alarm shall be
described in detail.
2.3.1.4
Instrumentation
Instrumentation shall meet the requirements of the applicable national codes. All
instruments, pneumatic or electronic, shall fail to the safest position or lock in place
upon air or power failure. Enclosures and cabling for all instrumentation shall conform
to the requirements of the electrical area classification of the area of installation.
Instrument installations shall meet all area classifications and code requirements. All
electronic instrumentation shall be grounded at a single, common point separate from
the plant ground grid.
Cable and conduit shall be routed in underground trenches where practical. Armored
cable may be installed by direct burial methods. Where underground routing is not
practicable, overhead cabling shall be routed in cable racks. All terminal strips used
shall be of modular construction. Electric terminals shall be of the pressure-plate type,
with all “live” parts recessed into the insulated block.
2.3.1.5
Lightning Protection
Aboveground LPG tanks do not require lightning protection for tank integrity.
However, it is common practice in refineries and production plants to ground all towers
and drums. This is done to protect electronic instrumentation and control systems.
Therefore, grounding is recommended when electronic instruments are on the tank, but
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PLANT SITE
Safety in LPG Design
is optional if the tank has no electrical instruments or control systems. Grounding rods
shall be provided for tanks supported on non-conductive foundations.
2.3.1.6
Plant Lighting
Adequate lighting is needed for security as well as operations. It shall be provided to
illuminate operating facilities such as walkways and essential control valves and
devices. Any loading or unloading facility to be used after daylight hours shall be
provided with adequate lighting, as well as gates within the plant fence area. The
quality of the lighting installations as well as all other electrical installations shall
comply with the Electrical Area Classifications.
Adequate lighting shall be provided for the following:
1.
All storage and operating areas for normal operation.
2.
To illuminate storage tanks, tanks being loaded, control valves and other
equipment.
Facility gates.
3.
In addition, sufficient emergency lighting shall be provided to allow safe operations
during an emergency. Lighting shall be designed to provide the average maintained
illumination in Table 2.3.1.6.
Location
Lux
Footcandles
Cylinder inspection and filling
540
50
Cylinder processing plant (general)
320
30
Tank car, tank truck, loading point
320
30
Piers, loading point
110
10
Entrance gate
55
5
Table 2.3.1.6: Average maintained illumination
2.4
Emergency Shutdown System
There shall be an Emergency Shutdown System (ESS) by which the facility can be
shut down in case of emergency. At the following strategic locations throughout the
plant, emergency push-buttons shall be installed which relay a signal to the central
emergency shutdown system.
1.
One in a central area which is at least 15 m from LPG tanks.
2.
One at each loading or unloading position.
3.
One, located 15 m from each loading or unloading position.
The actuating system shall be designed to close valves upon failure of any system
component. More information on Electrical requirements are described under
“Electrical Classification” above. When one of these push-buttons is activated the
following shall happen:
1.
Safety in LPG Design
Shutdown power to all product pumps, compressors, and cylinder filling.
Rundown streams from processes are not included in this requirement. They
shall be handled individually based on “fail safe” considerations.
PLANT SITE
2-15
2.
Closing of all Emergency Block Valves (EBV) at unloading, loading, tankage
and cylinder filling. EBV’s in rundown streams from refinery or gas plant
process units are not included in this requirement. They shall be closed or
kept open individually based on “fail safe” considerations.
3.
An audible alarmshall be activated.
4.
The power to the firewater system shall be maintained throughout the
emergency/alarm.
5.
Emergency push buttons at the pier may shut down the pier lines only or they
may be tied into total plant shutdown. This may depend on distance to the
pier and on other factors. The necessity shall be determined in a Hazard and
Operability Analysis (HAZOP). In any case, the closure of the pier valves
shall result in an alarm at the plant.
6.
When the loading/unloading area is part of a refining or upstream site,
activation of the ESS shall not necessarily require a shutdown in the process
area. This shall be confirmed during the design.
MARINE PIER
RAIL CAR
EBV
EMERGENCY
PUSH BUTTON
EBV
EMERGENCY
PUSH BUTTON
SHORE
EBV
EMERGENCY
PUSH BUTTON
BULLETS
EBV
SPHERE
MOUNDED DRUMS
EBV
EBV
EBV
EBV
EMERGENCY
PUSH BUTTON
FIRE WATER
EBV
EMERGENCY
PUSH BUTTON
TRUCKS
EQUIPMENT
SHUT DOWN
POWER TO
FIRE PUMP
NOT
ESS
INTERRUPTED
EMERGENCY
SHUTDOWN
SYSTEM
EBV
CYLINDER
FILLING
ALARM
OFFICE
EMERGENCY
PUSH BUTTON
EMERGENCY
PUSH BUTTON
Figure: 2.4: Plant Emergency Shutdown System (ESS). Only control system shown.
2-16
PLANT SITE
Safety in LPG Design
3
BULK STORAGE
3.1
Storage in Plants and Industry
This chapter is intended to provide general technical guidance to engineers who are
responsible for the design and installation of bulk tanks for Liquefied Petroleum
Gas (LPG). The guidelines are intended to assist engineers in the development of
technical specifications, which meet company and industry standards for design,
fabrication, installation, and testing of such facilities. These specifications aim to
maximize the integrity and safety of these facilities. Achieving this objective is
dependent upon using design concepts, which are proven to be safe and conform to good
operating and maintenance practices throughout the operating life of such facilities. In
developing these specifications, designers shall also follow Global Practices (GP's),
Design Practices (DP's) specified in this section, and local regulations.
This guide tries to use consistent wording for LPG storage. “Tanks,” “mounded drums,”
“spheres” and “bullets,” are used for bulk storage at company plants or industry.
“Vessels” means ships and barges only. Note that the GP's, DP's and other codes do use
“vessel” for refining drums and towers. “Containers” are used in small bulk and
domestic use. “Cylinders” (also often called “bottles”) are used for small portable LPG
containment.
If LPG tanks for commercial, utility, or industrial customers are as large as plant bulk
LPG tanks they shall be designed to the same principles. Typically commercial
consumers or domestic users require containers. Such containers may be designed
according to requirements in the chapter “Customer Storage” of this Guide.
For all new designs or design modifications in LPG storage service, a review of all
applicable local regulations, codes, standards, practices and operating permits is needed.
Major pressure tank manufacturers have developed standard designs for different tank
capacities, permitting the purchaser to specify only the code required at the proposed
plant site, and the tank openings and fittings required. Standardization can provide
substantial savings over development of an individual design. However, the standard
shall meet all the requirements of the GPs specified in this section.
For new designs the use of “mounded drums” for pressure storage is required for some
countries (Europe). These are horizontal tanks placed on above ground foundations or
on sand beds but thereafter completely covered by an earth mound. This type of storage
is inherently safer since its passive fire protection makes it not vulnerable to external
fires.
The most frequent tank types used in the past and still in use in many countries today are
the horizontal cylindrical (“bullet”) tank and the sphere. These are above ground
tanks on concrete foundations. The horizontal “bullet” tank was the most common
design in the past for tanks sized between 28 and 282 m3. For larger capacities, often
Safety in LPG Design
BULK STORAGE
3-1
the most economic solution was the spherical tank. Depending on location and exposure
in some cases it may still be appropriate to use this approach for new designs, however,
more and more regulations ask now for retroactive passive fire protection by
fireproofing.
Vertical “bullet” type tanks have also been installed by industry. They may have their
merits when spacing is tight, however, from a safety and firefighting standpoint this is
an undesirable configuration and shall be avoided.
The following International Codes may be applicable to the design of pressurized LPG
tanks:
3.2
1.
The United States American Society of Mechanical Engineers, “ASME Boiler
and Pressure Vessel Code, Section VIII” Section VIII is subdivided into
Division 1 “Unfired Pressure Vessels” and Division 2 “Rules for Construction
of Pressure Vessels.” To be more economical, LPG horizontal tanks shall be
designed and fabricated according to ASME VIII Division 1 while LPG
sphere tanks shall be designed and fabricated according to ASME VIII
Division 2.
2.
API STD 2510 and NFPA 58.
3.
BS 5500 Specification for Unfired Vessels.
4.
BS 1501 Steels for Fired and Unfired Pressure vessels - Plates, or Equivalent.
5.
BS 1502 Specifications for Steels for Fired and Unfired Pressure vessels Sections and Bars.
6.
BS 1503 Specifications for Steel Forgings (including semi-finished forged
products) for Pressure Purposes.
7.
Finnish Standards (SFS) 3205, 3339, 3340, 3341, and 3342. Finnish
Government Statues 98/73, 636/77, 257/84, 258/84, 312/79, and 1106/81.
8.
Japan High Pressure Gas Law (HPGL). Japan Industrial Standards (JIS).
9.
Australian Pressure Vessel Code AS 1210.
Mounded and Above Ground Storage
The guidelines and technical considerations discussed below refer to facilities, which
utilize large horizontal mounded drums for storing LPG. Storage in aboveground tanks
presents the risk of a BLEVE. Storage of LPG in mounded drums avoids the risk of
external fire exposure. Mounded drums are long horizontal cylindrical tanks, with
dished heads, which are installed above grade level and covered completely with sand
bed fill and general fill material. The mounding of drums permits reduced spacing when
compared to the space needed for spheres/bullets, which is mandated by regulatory
requirements.
As of 1993 there are several mounded drum installations in company Refining and
Marketing facilities in Europe and the Asia Pacific region. These tanks have been used
to store LPG since 1982-83. Operating experience with the installations during the
years has been good.
3.2.1
Dimensional Sizing of Drums
The specific dimensions for the drums are based on LPG storage requirements, available
space on site, safety considerations, spacing from buildings, facilities, and other
equipment, orientation, and the costs associated with fabrication, transport and
installation. The capacities of the drums should be based on planned sales. However,
shipment parcel size, transport delays, seasonal effects, future business outlook or other
factors may have an influence on the capacity.
3-2
BULK STORAGE
Safety in LPG Design
LPG Vapor Return
Mounded drum sizes vary substantially in diameter and tangent length. The drum
sizes that are currently in use have diameters in the 4 to 6.5 meter range and tangent
lengths of 34 to 88 meters. The diameter and length chosen are dependent upon
transportation and site spacing. In addition, considerations such as field assembly and
fabrication of drum sections should be used to optimize LPG storage and inventory
needs at specific installations.
LPG Filling
Distance PRV to Fence15 m
Water Draw-off to Location
min. 15 m from Fence
PRV
LHHA(CO) if filled by
Pipeline or by Ship
LI
LHA
LHHA
Concrete Wall
TI
PI
to ESS
min. 0.9 m
LPG
Tank Slope 1:100
EBV
Removable Part with Sleeve
Removable Fill
Figure 3.2.1-a: Typical LPG mounded drum
LHHA(CO) if filled by
Pipeline or by Ship
LHHA
Distance PRV to Fence 15 m
Water Draw-off to Location
min. 15 m from Fence
LI
LHA
PRV
Distance to Fence 3 m
min. 0.9 m
to ESS
PI
Submerged Pump
TI
Tank Slope 1:100
Figure 3.2.1-b: Mounded drum, spacing alternative with submerged pump
In general, it is preferable to have the drums fabricated in the manufacturer shop and
transported to site. However, due to the large sizes involved, transportation, site access
and off-loading at the plant usually dictate the feasibility of shop manufacture or the
need for site assembly of major drum sections. Recent LPG mounded drum installations
have used both shop fabrication and field construction practices.
3.2.2
Materials Selection for Tanks
All materials of construction shall meet the requirements of Section II of ASME “Boiler
and Pressure Vessel Code”, or equivalent national code. Low melting point materials of
construction such as aluminum and brass shall not be used for LPG storage drums. It is
recommended that the drum materials consist of fully killed, grain refined and
normalized carbon steel plates and forgings, with adequate mechanical strength and
toughness properties for the storage of LPG. The presence of H2S in has led to wet H2S
cracking problems associated with hard welds (> 225 Brinell Hardness). The tendency
for in service cracking increases as the H2S concentration and strength of the material
increases. H2S is more of a problem in refining and less in marketing where H2S
Safety in LPG Design
BULK STORAGE
3-3
contents by specification is in the order of magnitude of 1 ppm. Minimum specified
tensile strength of the tank steel historically has been below 483 MPa. However, with
more recent technology development higher strength steel is used to keep the thickness
of spheres below 38 mm, so PWHT can be waived per ASME Code. Experience shows
that if a proper procedure is taken (e.g. pre-heating 90-150 ºC), high strength steel can
provide satisfactory service. The amount of H2S in the product to be stored is a very
important factor to determine metallurgic characteristics of the material of construction.
Steel specifications shall include chemistry control per GP 9-2-1, and requirements for
impact properties at the Critical Exposure Temperature (CET), and heat treatment
per GP 5-1-1. Impact Requirement for Materials shall follow GP 18-10-1 "Additional
Requirements for Materials." Additional information which may be useful to the
designer is available from GP 18-7-1 “Welding Procedures,” GP 5-3-1 “Hydrostatic
Testing of Vessel,” and GP 5-2-1 “Internals for Towers & Drums.”
Material of construction for the pressure parts shall comply with ASME Sec II D
Appendix 5. Alternative materials equivalent to the ASME Code material specification
may be used. However, alternative materials shall be provided with the following to
EMRE for approval:
1.
Nomenclature and complete chemical and physical properties of the proposed
material stated along with ASME equivalent. Any additional requirement
necessary for equivalence shall be stated.
2.
Where necessary to demonstrate the equivalence of alternative material, test
specimens shall be provided for Charpy V-Notch testing according to
applicable ASME material specification.
3.
Quenched and tempered steel is limited to a maximum tensile strength of 690
Mpa and an actual yield-to-tensile ratio of < 0.85.
The following shall NOT be used as material of construction for pressure parts:
3.2.2.1
1.
SA36, SA283 and other structural grade steel.
2.
Steel casting.
3.
Low melting point materials such as aluminum and brass.
Minimum and Maximum Design Temperature
The principal purpose for specifying impact requirements is to ensure that a catastrophic
brittle fracture of the drum will not occur during hydrotesting, start-up, shutdown, and
normal operations throughout its service life. Impact requirements are based on the
Critical Exposure Temperature (CET, also “Minimum Design Temperature”),
metal thickness of the drum component, and the material specification selected.
The CET for a LPG pressurized storage drum or sphere shall be based on the lower of
the following:
1.
2.
Lowest one-day mean temperature. This would account for filling the drum
or sphere up to the safety valve pressure limit on the coldest day.
The temperature equivalent to 25% of the design pressure on the vapor
pressure curve for the material to be stored.
The minimum design temperature shall be the minimum metal temperature expected in
service, taking into consideration ambient temperature and auto-refrigeration of the
stored product when it flashes to atmospheric pressure. For storing Propane, this
temperature will be –42 °C. In no case shall the minimum design temperature be higher
than –18 oC. In many situations, the owner prefers to set the minimum design
temperature at the lowest possible temperature due to depressurizing the LPG to
atmospheric pressure (–42 °C). This is almost always more conservative than the
criteria provided above. It adds an extra safety margin for protection against brittle
fracture and is recommended. Using modern carbon steels, this should not significantly
add to the cost for the drum, bullet or sphere.
3-4
BULK STORAGE
Safety in LPG Design
3.2.2.2
Post-Weld Heat Treatment
For aboveground bullets with a plate thickness below 38 mm Post Weld Heat Treatment
(PWHT) is not required. However, Post-Weld Heat Treatment is recommended for
mounded drums due to service considerations. These requirements are independent of
PWHT that may be required from Code considerations of plate thickness, and material
specification. The preferred method of PWHT for shop fabricated drums is to heat treat
the entire drum or major sections of the drum in a heat treating furnace. This minimizes
the thermal stresses, which can be introduced by local PWHT, which typically involves
banding the weld seams with electric resistance heating jackets. If this capability is not
available in the shop or if PWHT in the field becomes necessary, then local Post-Weld
Heat Treatment of the individual seams may be done, subject to careful control of
temperature and temperature gradients.
3.2.2.3
Materials Specifications
The recommended materials specifications for bullets and mounded drums are identical
to ASTM Specifications as follows:
SA 516 Grade 70 normalized
SA 333 Grade 1 or 6
SA 350 Grade LF2
SA 352 Grade LCB
SA 334 Grade 1 or 6
for the shell and heads
for nozzles
for flanges & fittings
for fittings
for tubing
Substitute Materials specifications may be made provided they meet with the
requirements in GP 18-1-1. Materials not covered in GP 18-1-1 should be evaluated on
a case by case basis.
3.2.2.4
External Corrosion Protection
The earth mounds used on mounded LPG Drums increase the potential for soils induced
corrosion and holing through. In addition, the design does not permit on stream
thickness measurements and/or visual inspection of the drum surface. Shop fabrication
is preferred when drum size permits. Any damage during transport to the coatings on
shop fabricated drum shall be repaired. It is extremely important, therefore, to consider
the following:
1.
Develop an earthwork specification for the mound. Specify sand bed fills,
general fills, and acceptance tests, in accordance with ASTM Standards and
Global Practice GP 4-9-1.
2.
Provide an adequate corrosion resistant coating on the external surface of
the drum per GP 19-1-1. Shop-applied coatings are preferred, but field
applications are also acceptable. Irrespective of type and application method
of coatings, a holiday test on the coating shall be carried out immediately
prior to back-filling on site. The following specification is the normal
requirement:
3.
4.
Safety in LPG Design
Surface Preparation:
Abrasive Blast Clean to
SSPC SP-10 Near White
1st Field Coat:
Coal Tar Epoxy @
6-8 mils DFT
2nd Field Coat:
Coal Tar Epoxy @
6-8 mils DFT
Install a cathodic protection system utilizing sacrificial anodes. Permanent
reference electrodes shall be buried along with the drum at each end of the
drum and above and beneath the drum at its midpoint.
Requirements for a cathodic protection system are a twenty-year life and a
maximum exposed steel surface of 10%. Insulating flanges shall separate
permanent lines connected to plant piping from this cathodic protection
BULK STORAGE
3-5
system. The cathodic system may be used to protect short runs of buried pipe
provided the pipe coating is equal or better than the tank coating.
5.
Any special plant refinery/production applications where the LPG is expected
to contain wet H2S, shall have the drums lined internally.
The exterior surface of aboveground tanks, including the steel supports, shall be grit
blasted to SSPC SP-10 standard or chemically treated and adequately painted.
After grit blasting horizontal LPG storage bullets and LPG sphere tanks, shall be
painted with a primer coat of alkyd zinc phosphate (75 microns dry film thickness),
build-coat of alkyd micaceous iron oxide (50 microns dry film thickness) and topcoat of
white alkyd enamel (30 microns dry film thickness).
1.
Sphere legs shall be fireproofed and left unpainted
2.
Bases and saddles of bullets that are concreted shall not be painted.
3.
If bases and saddles on bullets have exposed metal, they shall be painted with
same primer coat, build-coat and finished with topcoat.
The exterior of aboveground LPG tanks shall be inspected every 2 years. Repainting
shall be carried where necessary.
3.2.3
Design Basis for Mounded Drums
The mechanical design of large mounded drums is complex, due to its mounded
configuration, internal pressure, nozzle geometry and piping loads, type of support,
foundation pad design and soils settlement characteristics. In addition, if these facilities
are located in earthquake zones, the seismic loads substantially increase the complexity
of mechanical design considerations.
Most national codes entrust the responsibility to users for defining and specifying the
loading mentioned above. Therefore, it is recommended that users develop Mechanical
Specifications (MSpec) for these drums and ensure that all pertinent design criteria and
loads are specified.
The following guidelines are intended to assist users when developing MSpecs for
mounded drums.
3.2.3.1
Design Conditions for LPG Drums
Design conditions for LPG drums should preferably be based on Propane storage. This
will allow the flexibility to switch to Propane and will provide protection against
inadvertent loading of Propane to a lower design pressure drum. Based on local
regulations in several countries, the minimum design pressure is specified as 17.2 bar
gauge with a corresponding design temperature of 55 °C. Under design conditions, 1.6
mm corrosion allowance shall be added to the design thickness of a drum. In addition,
external pressure of 1 bar gauge is used to allow for the soil pressure from the earth
mound. In the absence of local regulations, the maximum design temperature shall be
taken as the highest ambient temperature that has been recorded over the last 10 years at
the nearest meteorological station. In no case shall this temperature be lower than 38
°C.
3.2.3.2
Design Codes Applied
Most nationally recognized Codes can be used for the design of mounded drums.
However, design, fabrication, inspection, and testing requirements shall be based, as a
minimum, on the requirements of Global Practice 5-1-1. Design and fabrication
inspection of LPG tanks shall be carried out by an internationally recognized and EMRE
Engineering approved third party inspection agency.
3-6
BULK STORAGE
Safety in LPG Design
3.2.3.3
Permanent Identification is Needed
For continuous reference, a non corrosive identification plate shall be fixed to the drum
in a suitable and clearly visible location. It shall be stamped with the following
information as a minimum:
Drum Serial Number:
Owner’s Name:
Designer’s Name:
Design Code:
Manufacturer’s Name:
3.2.3.4
Design Pressure, min./max.:
Design Temperature, min/max:
Product Stored:
Water Capacity in volume units:
Date of test and test pressure:
Earth Mound Design
The following supplementary requirements shall apply to mounded drums:
3.2.3.5
1.
Partial mounding is not recommended. Partial mounding continues to have
the design considerations of storage in aboveground bullets so that little or
nothing is gained.
2.
The depth of the mound over all surfaces of the drum shell shall be a
minimum of 0.9 m. The mound cover shall include the heads of the drums,
which shall not be exposed.
3.
The backfill used for mounding shall consist of washed sand totally free of
rocks or abrasive materials likely to damage the drum coating. The mounds
shall have good stabilization to prevent erosion by firewater or heavy rain.
Furthermore, it shall be capable of withstanding prolonged heat radiation or
jet flame impingement. This is important since the pressure relief valves on
such drums are not designed to provide protection against heat input by
external fire. Therefore, if the drum is being uncovered later, e.g. for
external inspection, LPG shall be taken out of the drum before it is
uncovered. If a drum has to be removed and the adjacent drum is still filled
with LPG the side of the filled drum shall still be covered by a mound of
0.9 m thickness.
Nozzles on LPG Drums
As a matter of principle the number of nozzles on mounded drums shall be kept to the
necessary minimum. This pertains in particular to the lower part of the drum.
Following nozzles are considered necessary:
1.
Pressure relief valve (PRV) connection to the vapor space.
2.
Water draw-off connection via the top (similar to GP 9-2-1, para. 9.5).
3.
Connection for the level indicator with high level alarm.
4.
Connection for the independent high level alarm.
5.
Connection for the fixed level gauge.
6.
Filling connection at the top of the drum.
7.
Withdrawal connection at the top or bottom of the drum.
8.
Vapor return connection at the top of the drum.
9.
Vent connection to atmosphere.
10. Connection for the temperature indicator.
11. Pressure indicator connection to the vapor space.
Careful engineering may permit the combination of some of the above fittings to
reduce the number of nozzles on the drum. The preferred location for nozzles is at the
manhole (see below). All nozzles on new drums are preferred to be flanged and not
smaller than 50 mm. This type of connection presents adequate integrity against
Safety in LPG Design
BULK STORAGE
3-7
mechanical damage or leak during fire. Fittings on underground or mounded tanks shall
preferably be accessible from above ground level.
As a matter of principle nozzles on above ground drums are preferably located in the
vapor phase section, however, those necessary for liquid withdrawal shall be at the
bottom. All tank nozzles’ connections and nozzles’ flange joints shall be welded.
Flanged joints on tanks (and pipelines) shall be designed based on the relationship of
pressure limits against temperature of carbon steel. The material class shall not be
inferior to that based on the pressure limit at the design temperature. Gaskets for
flanged joints shall be resistant to liquid LPG and shall be made of metal or other
suitable material confined in metal having melting point over 816 °C. Gaskets of natural
rubber or bonded with natural rubber shall not be used.
Figure 3.2.3.5-a: Submerged pump
The following shall be considered when designing nozzles on LPG drums. The
requirements below are intended to address the mechanical and structural design
considerations of nozzles on LPG mounded drums. They are not intended to cover the
instrumentation, controls and alarms that are recommended from a safety and operability
standpoint.
3-8
1.
All nozzles, with the exception of the pump suction, shall be installed at the
top of the drum. If the drum is located in an earthquake zone, the bottom
suction design shall be a last resort and requires additional assessments for
possible structural failure, corrosion and leakage. An access chamber shall
permit inspection of the bottom suction nozzle for the pump. The access
chamber may be filled with sand and closed by a cover, which can be
removed to permit inspection, with the drum in service.
2.
Installations where the suction line for an external pump enters through the
top shall be avoided. Even though a compressor may permit continuous
vapor return, often NPSH problems are encountered. Solar radiation may
cause vapor lock in the line and it may be impossible to empty the drum
below a certain level.
BULK STORAGE
Safety in LPG Design
3.
Since pumps have become highly reliable, the submerged pump is an
elegant solution for locating the pump suction nozzle on the top of the drum.
A bottom shutoff valve permits removal of the pump while the drum is filled.
4.
Shell penetrations between bullet supports shall be avoided. They are more
difficult to access in mounded drums. When the tank is above ground, they
are difficult to reach with cooling water in the event of a fire.
5.
It is recommended that drum larger than 56 m3 or exceeding 2 m diameter be
provided with a manhole of not less than 610 mm. Manholes on mounded
drums must be located at the top of the drum in form of an extended nozzle
and shall be protected from direct involvement in a fire by covering the
opening with a removable insulated lid. A manhole may be positioned below
the mound rim however, in such cases it shall be adequately protected from
corrosion by providing a steel skirt with cover. The skirt shall be attached at
a level below the flange ensuring that the nuts and bolts are not buried inside
the mound. In drum 56 m3 or smaller volumetric capacity, provision of a
manhole is optional unless a need for cleaning is anticipated, or required by
local regulations. The manhole shall be located to facilitate cross ventilation
with the tank nozzles. It is suggested to locate nozzles and connections for
instrumentation, PRV and other on the manhole in order to reduce the
penetrations to the tank shell and to facilitate later changes should they be
necessary. However, it should be checked whether the code is not restrictive
on nozzles on manholes.
Figure 3.2.3.5-b: Two-plane gusset
6.
The nozzle shall be of fully integrally reinforced and shall not permit the
use of reinforcement pads. All nozzles shall be flanged and valved. The
flanged joint shall not be buried inside the mound.
7.
Nozzles shall be adequately spaced to ensure that localized stresses would
satisfy the criteria of ASME Section VIII and GP 5-1-1 and GP 9-2-1.
8.
If the drums are frequently filled and emptied, cyclic loading effects shall be
considered in the nozzle attachment design (See GP 5-1-1).
9.
The nozzle to shell design shall be adequate to accommodate external piping
loads, as appropriate.
10. The following minimum nozzle wall thickness shall be used:
Nominal Nozzle Size
< 50 mm
75-150 mm
> 200 mm
Pipe Schedule or Thickness
Schedule 80 or 160
Schedule 80
12,5 mm
11. All nozzles 50 mm or smaller shall be gusseted in two planes in accordance
with GP 3-18-1.
12. Sampling connections may be provided on piping to tanks. Adequate
gussetting of small connections and piping in sampling lines shall be provided
Safety in LPG Design
BULK STORAGE
3-9
to minimize vulnerability to mechanical damage. The inlet piping to
sampling connections shall be double valved. For above ground drums,
sample locations shall not be under the drum. Connections shall be oriented
so that the purge vapors do not engulf the operator or approach an ignition
source. Ideally the purge shall be discharged into a closed system (blowdown
in refining). Consider the addition of a restriction orifice (< 2 mm) external
to the sampling valve.
13. Appropriate startup and shutdown connections shall be provided for
commissioning storage drums and taking them out of service. Pressure drums
may be purged by water filling, with a 50 mm vent connection to remove air
displaced as the water rises. A 50 mm vapor connection to an adjacent drum
in the same service, where available, allows vapor to be drawn in when the
water is drained. The same connection can be used for taking the drum out of
service. When water is injected, the residual liquid and vapor is displaced
over the top into another drum.
3.2.3.6
3.2.3.7
Drum Shell Design
1.
The non-uniform external loads transmitted to the drum shell, by the earth
mounds, should be analyzed using Finite Element Analysis (FEA) methods.
The FEA stresses should comply with ASME Section VIII Div 2. In addition,
a theoretical safety factor of 3.0, for buckling design, should be used as
acceptance criteria for minimum shell plate thickness. Review with the fixed
equipment specialists in EMRE or EMPRCo may determine when FEA may
not be required.
2.
When calculating shell thickness due to internal pressure no credit shall be
taken for the restraint provided by the earth mound on the drum shell.
Corrosion allowance as per section “Design Conditions for LPG Drums”
above.
3.
Stiffeners, used on the drum shell shall be attached to the internal surface.
Design calculations and details as well as fabrication and NDE (Non
Destructive Examination) requirements shall ensure that shear and lamellar
tearing does not occur (see also ASME Section V “Non-destructive
Examination”).
Drum Support and Foundation Design
The recommended type of supports for mounded drums is as follows:
1.
For locations, which are subjected to earthquake forces, mounded drums
shall be located on sand beds. Saddle supports are not recommended due to
the increased risk of failure from liquid sloshing and dynamic forces during a
seismic event.
2.
For other locations, saddle supports on rigid slabs supported on piled
foundations are permissible.
However, the mounded drum shall be
investigated for buckling, local circumferential bending and shear stresses per
GP 5-1-1.
3.
In addition, reinforced concrete retaining walls have been used to confine the
earth mound. Reinforced concrete walls shall have adequate drainage and the
concrete shall not touch the drum or it can cause accelerated corrosion. See
also GP 4-9-1 “Site Preparation/Earthwork.”
It is recommended that soils investigation and assessments be completed prior to
determining the most suitable type of foundation. The primary objective of these
investigations should be to reduce to an absolute minimum the potential for differential
settlement. The foundation design shall be based on the weight of the drum full of
water (needed for pressure test).
If these settlements are not kept to a practical minimum, they may introduce
unacceptably high stresses at the drum support locations and increase the risk of in-
3-10
BULK STORAGE
Safety in LPG Design
service failures. The sensitivity to such settlements is due to the large length to diameter
ratios and the relatively thin shells in these drums. Also the piping connecting to the
drum must be kept free floating and not embedded into concrete or other fixations. It is
important that connecting piping is not subjected to any stress from earth movement etc.
In view of the above considerations, a maximum differential settlement of 25 mm on a
16.5 m long drum has been used at one facility.
Where groundwater or possible flooding makes it advisable, anchorage shall be
provided to prevent flotation. Underground pipes and services, such as steam, water,
electricity and sewer, shall be at least 1.5 m horizontally from the mounded tank. Above
ground horizontal LPG tanks of more than 7.6 m3 water capacity shall be provided with
structural steel saddles designed to be mounted on flat-topped concrete foundations by
means of anchor bolts or other adequate devices. Lifting lugs for LPG horizontal tank,
where provided, shall be designed for taking 1.5 times of the total weight of an empty
tank. LPG horizontal tanks’ supports shall be of reinforced concrete, masonry or
fireproofed structural steelworks. Design of the supports and foundations shall take into
consideration:
3.2.4
1.
Ground conditions, including allowable bearing pressure and differential
settlement.
2.
Possibility of flotation.
3.
Expansion and contraction of the tank shell.
4.
The greatest combination of static loading due to weight of the tank, its
contents, weight of water used for testing, wind loading, vibration, thermal
effects and seismic conditions.
Testing Requirements
NDE and testing of LPG horizontal tanks shall be per ASME Code Section VIII,
Division 1.
For a tank which has a wall thickness of:
More than 38 mm, the minimum Non-Destructive Examination (NDE) shall be per the
following sequence:
1.
100% X-ray on all seams.
2.
Post Weld Heat Treatment (PWHT). Hardness test after PWHT shall not be
more than 225 HB unless waived by EMRE Engineering.
3.
100% ultrasonic test on all seams.
4.
100% Magnetic Particle Test (MT) on all seams.
5.
Hydrostatic test at 1.3 times of the design pressure. (The 1999 ASME
Section VIII, Div.1 specify 1.3 times of MAWP as Hydrostatic testing
pressure, since ASME has lowered the safety factor for materials from 4 to
3.5. For any alteration of a tank built prior to 1999, EMRE Engineering shall
be consulted.) Also refer to GP 5-3-1 "Pressure Testing of Unfired Pressure
Vessels", which has a clearer description for hydrostatic testing. It states: In
the hydrostatic test condition, the maximum membrane stress in the tank in
the un-corroded or corroded condition shall not exceed 90% of the specified
minimum yield strength for ferritic steels, nor 100% of the specified
minimum yield strength for austenitic steels or non-ferrous materials.
6.
100% MT on all seams.
Between 32 mm to 38 mm, the minimum NDE shall be per the following sequence:
Safety in LPG Design
BULK STORAGE
3-11
1.
If a service condition permits (e.g. H2S concentration under 50 ppm), PWHT
can be waived provided that the Metal is preheated prior to welding.
Otherwise, follow the procedures as above.
2.
100% X-ray on all seams.
3.
Hydrostatic test at 1.3 times of the design pressure.
4.
100% MT on all seams.
Less than 32 mm, the minimum NDE shall be per the following sequence:
1.
If a service condition permits (e.g. H2S concentration under 50 ppm), PWHT
can be waived. Otherwise, follow the procedures as above.
2.
100% X-ray on all seams.
3.
Hydrostatic test at 1.3 times of the design pressure.
4.
100% MT on all seams.
NDE and testing of LPG sphere tanks shall be per ASME Code Section VIII, Division
2. For a tank which has a wall thickness of:
More than 38 mm, the minimum Non-Destructive Examination (NDE) shall be per the
following sequence:
1.
100% X-ray on all seams
2.
3.
Post Weld Heat Treatment (PWHT). Hardness test after PWHT shall not be
more than 225 HB unless waived by EMRE Engineering.
100% ultrasonic test on all seams.
4.
100% Magnetic Particle Test (MT) on all seams.
5.
Hydrostatic test at 1.25 times of the design pressure.
6.
100% MT on all seams.
Between 32 mm to 38 mm, the minimum NDE shall be per the following sequence:
1.
If a service condition permits (e.g. H2S concentration under 50 ppm), PWHT
can be waived provided that the Material is preheated prior to welding.
Otherwise, follow the procedures as above.
2.
100% X-ray on all seams.
3.
Hydrostatic test at 1.25 times of the design pressure.
4.
100% MT on all seams.
Less than 32 mm, the minimum NDE shall be per the following sequence:
1.
If a service condition permits (e.g. H2S concentration under 50 ppm), PWHT
can be waived. Otherwise, follow the procedures as above.
2.
100% X-ray on all seams.
3.
Hydrostatic test at 1.25 times of the design pressure.
4.
100% MT on all seams.
Documentation from fabricator shall include MDR (manufacturer's data report) and all
test recordings according to ASME VIII requirement.
3.2.5
Horizontal “Bullets” and Spherical Tanks
Design procedures for horizontal “bullet” tanks and spheres are similar to those
described above for mounded drums except those items that are specific to mounded
drums. All connections shall be flanged. Existing screw type connections in older
Marketing tanks need not be retrofitted to flange connections. Older Marketing bullets
also may have 3 meters from the edge of the bullet to the water drain discharge point
3-12
BULK STORAGE
Safety in LPG Design
compared with the latest 4.5 meter recommendation in DP XXII-C. Existing drain
points do not need to be retrofitted.
For aboveground bullets the bottom nozzles should be positioned at the two ends and
not between foundations. This is to facilitate access of valves and better firewater
coverage if needed. If possible the bottom nozzles may be reduced in favor of top
nozzles. This may avoid the potential for liquid leaks. Top spray filling is desired for
bullets and spheres since it lowers the pressure during filling. Typical sphere and bullet
designs are shown in Figure “Typical horizontal bullet tank” and Figure “Typical
spherical tank.” Tanks used in loading and unloading service would normally have a
vapor balancing line, which is not shown in these figures.
REFERENCES
SAFETY
VALVE
7
DRAIN WITH ELL
RELIABLE GAUGE
MAINTAINABLE WITH
BULLET IN OPERATION
CSO
BLEED (GUSSETED)
LI
PI
PI
1
PLATFORM ACCESS
VIA LADDERS
7
3
1
GP9-2-1
2
GP9-1-1
3
GP15-1-3
4
GP3-2-3
5
GP3-5-1
6
GP14-3-1
7
GP3-2-4
GENERAL
LHA
LHA
1
(INDEPENDENT)
3
LHA AND LHHA FOR
ALL BULLETS
LHCO IF FILLED BY
RUNDOWN, PIPELINE
OR SHIP.
GP9-2-1
DPM XV-J
INSTRUMENTS
NO GAGE
GLASSES
3
PI
PdI
NON-FREEZE DRAIN
(IF COLD CLIMATE)
30 m
MIN.
WATER FLOOD
(IF LARGE BULLET)
FILL & DISCHARGE
1
THROTTLING
EBV(D)
(FIREPROOFED OR
FIRE SAFE ACTUATOR)
4
FILL
HYDRANTS
(PREFERABLY
WITH MONITORS)
1
FIRE-SAFE
5
QUICK CLOSING
SUPPORTS FIREPROOFED
4m
MIN.
SAMPLE CONNECTION
5
OUTSIDE
TOE WALL
SLO
H
WATER
DRAW-OFF
PE
5
TOE WALL
2
SLOPE
SLOPE
4
6
5
CATCH
BASIN
5
2
GROUNDED
1
TO SEWER OR
DRAINAGE
SYSTEM
NC
NOTES:
2
IF POSSIBLE ALL CONNECTIONS SHOULD BE
AT ONE END OF VESSEL AND THE CATCH
BASIN SHOULD BE LOCATED NEARBY.
DR, 3/2001
Figure 3.2.5-a: Typical horizontal bullet tank (from DP XXII-C)
REFERENCES
SAFETY
VALVE
8
RELIABLE GAGE
MAINTAINABLE WITH
SPHERE IN OPERATION
TYPICAL AREA LAYOUT
GP9-1-1
GP15-1-3
DRAIN WITH ELL
CSO
1
DELUGE
SYSTEM
3
4
GP3-2-3
5
GP3-5-1
6
GP14-3-1
7
GP4-2-1
8
GP3-2-4
4
INSTRUMENTS
LHA
STAIRWAY &
PLATFORM
1
(INDEPANDENT)
3
LHA IF FILLED BY RUNDOWN,
PIPELINE, OR SHIP
GENERAL
DPM XV-J
NO GAGE
GLASSES
3
SUCTION
CONNECTION
1
1
SUPPORTS
FIREPROOFED
TI
THROTTLING
5
FIREWATER
SPRAY SYSTEM
BELOW SPHERE
4
WATER
FLOOD
1
6
NON-FREEZE DRAIN
(IF IN COLD CLIMATE)
FIRE-SAFE
QUICK CLOSING
3
EBV(D) (FIREPROOFED
OR FIRE-SAFE ACTUATOR)
MIN 30 m
WATER DRAW-OFF
1
1
5
5
WATER DELUGE
INSTRUCTION SIGN
4
SAMPLE
CONNECTION
2 OR MORE FIRE HYDRANTS
(PREFERARLY WITH MONITORS)
4
5
SLO
PE
FILL
5
SLOPE
2
FILL &
DISCHARGE
COATED OR SLEEVED
GP9-2-1
7
PdI
LI
OUTSIDE
DIKED AREA
GP9-2-1
2
BLEED (GUSSETED)
PI
1
1
3
5
DIKE
4 m MIN
H
4
2
2
CATCH
BASIN
1
2
NC
TO SEWER OR
DRAINAGE
SYSTEM
GROUNDED
5
2
1
DR 1999
GREATER OF 30 m OR ONE DIAMETER
Figure 3.2.5-b: Typical spherical tanks (from DP XXII-C)
Note: GP 15-1-3 referenced in the both figures further refers to GP 9-7-1 for details on
installing level devices. GP 9-7-1 shows welding to an atmospheric tank. Welding shall
not be done to pressure tanks and an alternate means of level device support is
necessary.
Safety in LPG Design
BULK STORAGE
3-13
Aboveground pressurized LPG tanks need certain considerations as to siting and
location. These are explained under section 2.2.2.4 “Siting of Aboveground Tanks and
Equipment.” Horizontal bullets and spheres are more vulnerable to fire exposure than
mounded drums. Therefore, codes recommend installation of some kind of fixed fire
protection. Depending on risk, fire resistant coatings may be needed for bullets and
spheres that are normally equipped with fixed water spray or water deluge systems.
These requirements are discussed in depth in Chapter 8 under “Firewater Sprays,”
“Firewater Deluge for Spheres,” and “Fire Resistant Coatings.” Storage at refineries
and Upstream Gas Plants shall be provided with water flooding facilities to inject water
and displace LPG in the lower parts of tanks, in the event of a tank leak.
Spheres shall be built in accordance with GP 9-2-1. They are normally equipped with
top and bottom manholes. The filling and withdrawal connections for spheres are
normally at the bottom. LPG Sphere tanks shall be provided with steel support columns
and wind (earthquake) girders. Columns shall be mounted on concrete foundation with
anchor bolts. The columns shall be fireproofed from ground level to intersection of the
support with the tank shell (see also drawing in Chapter 8).
3.2.6
Spill Containment
Spill containment shall be considered for all locations and shall be provided in locations
in which either of the following would result in a significant hazard to important nearby
facilities, nearby properties, or public areas, per API 2510:
1.
The physical properties of the stored LPG make it likely that liquid material
will collect on the ground. The more hydrocarbons like Butane (or Pentane)
is contained in the LPG the more it will spill as a liquid.
2.
Climatic conditions during portions of the year make it likely that liquid LPG
will collect on the ground.
The ground beneath aboveground LPG tanks shall be of impervious construction and
graded to be sloping away from the tanks at minimum 1 percent gradient, to drain any
liquid spills to a safe area away from the tanks and piping. Diversion kerbs with a
height not exceeding 380 mm to avoid formation of gas traps may be permitted if
necessary for directing possible spillage away from bullet tanks.
If spill containment is to be provided, it shall be either remote impoundment or diking of
the area surrounding the spherical tank. If diking around the tank is to be used for
spill containment, the diked area shall be designed according to GP 9-1-1.
Remote impoundment collects any spill at a location away from the tank. This may be
of significant importance shall the vaporizing LPG ignite during the spill. However,
installation of remote impoundment may need more land. In any case, all materials for
components of spill containment shall be capable of withstanding the effects of a
thermal shock associated with spilling LPG. Furthermore, spill containment shall allow
for adequate venting of the vapor generated during an LPG spill.
If remote impoundment is to be used for spill containment, the remote impoundment
facility shall be designed as follows:
3-14
1.
Grading of the area under and surrounding the tanks shall direct any leaks or
spills to the remote impoundment area. Grading shall be at a minimum of
1 percent slope.
2.
Walls, dikes, trenches, or channels may be used to assist in draining the area.
3.
The remote impoundment area shall be located at least 15 m from the tanks
draining to it and from any piping or other equipment.
4.
The holdup of the remote impoundment area shall be not less than 25 percent
of the volume of the largest tank draining to it. If the material stored in the
BULK STORAGE
Safety in LPG Design
5.
tank has a vapor pressure less than 690 kPa at 38 °C, the holdup for the
remote impoundment facility shall be not less than 50 percent of the volume
of the largest tank draining to it. Larger holdups shall be provided in the
remote impoundment facility at locations where the expected vaporization is
less than that specified above because of climatic conditions or the physical
properties of the material.
The electrical classification is to be the same as a diked area.
Whether spill containment is provided or not, the ground under and surrounding a tank
used to store LPG shall be graded to drain any spills to a safer area away from the tank.
The drainage system shall be designed to prevent liquid spilled from one tank from
flowing under any other tank. The spill drainage area shall not contain equipment like
pumps or LPG piping not associated with the tank.
3.2.7
Vacuum Conditions
An additional design consideration shall be addressed where commercial Butanes are
stored in cold climates in horizontal “bullet” tanks and spheres. If the temperature of
the stored liquid can fall below boiling point of Butane (approximately –7 °C, for the
typical commercial Butane/Butene mixture and 0 °C for pure normal Butane), the
pressure in the tank can drop below atmospheric pressure. There are several options to
handle this low pressure situation.
1.
The tank can be designed to withstand the maximum degree of partial
vacuum possible by increasing wall thickness or adding stiffening rings. If
an adequate margin of safety is provided, this is a satisfactory solution,
because it requires no operational control.
2.
Pressure can be maintained by bleeding vapor from nearby Propane tanks
or 50 kg cylinders.
3.
Provision can be made to heat the liquid by means of hot gas return from an
external vaporizer, thereby maintaining positive pressure.
4.
The supplying refiner can be asked to raise the vapor pressure of the Butane
during periods when cold weather can be anticipated by increasing the
Propane/Propylene content.
5.
Vacuum breakers may be installed. However, this is discouraged and
should be considered only if no other solution can be implemented since they
involve uncontrolled introduction of air into the system, and may create
potential operating and safety problems.
It is always best to design the system such that it can withstand full vacuum. All other
design options should be evaluated using risk assessment techniques.
In an emergency case, inert gas (Nitrogen) may be injected to maintain pressure. Partial
pressure effects and associated lifting of the pressure relief valves may be encountered.
Therefore, extreme care shall be required when embarking on such activities.
The proposals are ranked by preference. Options 2), 3), 4), and 5) require control and
alarm instrumentation, and are subject to operator error and maintenance deficiencies.
3.3
Refrigerated LPG Storage
Using refrigerated storage (–50 °C for Propane) normally accommodates large capacity
requirements. This type of storage involves a thermally insulated tank operated at
approximately atmospheric pressure. Additionally, a cooling system involving pumps,
compressors and exchangers as well as an emergency blowdown system (flare) are
Safety in LPG Design
BULK STORAGE
3-15
needed. Details on refrigerated storage are not outlined in this Guide. For design,
reference shall be made to EMRE Design Practices, GP 9-6-1 and NFPA 58 Section 9.
3.4
Overpressure Protection for Tanks
This chapter discusses overpressure protection for LPG tanks, the different types of
Pressure Relief Valves (PRV) used, the procedure on how to size them, the principles
of installation and the testing frequency.
Prevention of loss of containment is by far the most important concept during the
design of a pressure system. Overpressuring a system has the potential for loss of
containment. Therefore, it is important to incorporate adequate facilities to prevent
overpressuring LPG systems.
3.4.1
Contingencies to be Expected
A “contingency” is a normal or abnormal event during plant operation that could lead
to overpressuring. The magnitude of the contingency will have a direct impact on sizing
the pressure relief system. Following is a description of common contingencies that
may affect LPG storage:
1.
Tank exposure to external heat in the form of impinging fire or heat
radiation from an adjacent fire will lead to boiling and subsequent pressure
increase.
2.
Overfilling to liquid full can cause overpressure.
3.
Thermal expansion of the liquid can cause overpressure. If a tank is filled
with cold product and filling has exceeded 85% and the tank is exposed to
external heat (sun) its contents will expand and may exceed 100% of tank
volume.
4.
Introduction of Propane into a Butane tank, which has been designed only
for Butane, could cause overpressure.
5.
Improper commissioning (air freeing) of a tank may lead to high pressure
because of presence of non-condensables (inert gas, Nitrogen).
Bullets and spheres exposed to external heat normally will produce the largest relief
requirements. Therefore, this will dominate sizing of the valve. This is not true for
mounded drums.
Avoiding loss of containment by overpressure can be accomplished in two ways. First,
a pressure relief device protects the system. A Pressure Relief Valve (PRV) is the
most cost effective solution to protect the system. Typical PRVs in LPG service are
spring loaded valves that are designed to open automatically if the set pressure (tank
design pressure) is reached. Second, the system could be designed for a pressure
considerably above the operating pressure. The cost for this in most cases would be
considerably higher than that of a normal tank, yet, in some countries such designs are
required for tanks in transportation services. The draw-back here is that in case of truck
overfilling there is no protection against thermal expansion pressure increase. The latter
design method shall only be used where local codes prohibit Pressure Relief Valves.
It should be noted that during fire impingement on a tank, the containment could
ultimately be lost due to excessive metal temperatures causing a Boiling Liquid
Expanding Vapor Explosion (BLEVE). Overpressure protection cannot prevent a
BLEVE since red-hot tank walls will fail at pressures far below the design pressure of
the tank. The best way to prevent this kind of failure is by providing passive fire
protection by mounding the tank. Fireproofing combined with firewater protection
most likely may prevent a BLEVE, however, if the firewater fails during a prolonged
3-16
BULK STORAGE
Safety in LPG Design
fire the tank may fail since fireproofing is only good for a limited duration of fire
exposure.
3.4.2
Refinery and Upstream Pressure Relief
Pressure relief requirements for LPG storage and loading/unloading facilities at
Refineries and Upstream Gas Plants are described in Design Practices Section XV-C.
To prevent liquid discharge and undersizing of PRVs the ERE report EE.28E.90 “Sizing
Pressure Relief Valves in Flashing and Two Phase Service, An Alternative Procedure”
shall be followed. Furthermore, API RP 520 “Design and Installation of Pressure
Relieving Systems,” as well as API RP 521 “Guide for Pressure Relieving and
Depressuring Systems” apply. Refineries and Upstream facilities typically use
conventional pressure relief valves. The typical pressure relief valve in Marketing is the
axial flow valve described below.
Figure 3.4.2: Conventional pressure relief valve
3.4.3
Pressure Relief in Marketing Terminals
Sizing of pressure relief valves is governed by codes. If local codes are more stringent
than those discussed below, they shall apply. Tanks for bulk storage have typically been
Safety in LPG Design
BULK STORAGE
3-17
designed according to the American Society of Mechanical Engineers (ASME) Boiler
and Pressure Vessel Code Section VIII. This code also covers requirements for
overpressure protection and relief devices. In addition, the Compressed Gas
Association (CGA) has applicable provisions in their three pamphlets on Pressure
Relief Device Standards S-1.1, S-1.2, and S-1.3. National Fire Protection Association
(NFPA) has included the CGA provisions in NFPA 58, Appendix E and in NFPA 59,
Chapter 9. Capacity requirements for pressure relief valves on above ground, nonrefrigerated ASME tanks (bullets, spheres) shall be in accordance with the applicable
provisions in these standards. Following are the details on the sizing of PRVs for
bullets, spheres and for mounded drums.
Figure 3.4.3-a: Large pressure relief valves on marketing sphere
The set pressure of a pressure relief valve shall be equal to or less than the design
pressure of the tank. Historically this was not always the case. Tanks built before 1950
3-18
BULK STORAGE
Safety in LPG Design
had different settings, which can be taken from NFPA 58. The CGA pamphlets also
mention the “start to discharge pressure” which is the pressure at which the PRV shows
the first signs of leakage. This is typically at about 90% of the set pressure but is only
relevant for judging the quality of PRV maintenance. The flow rating pressure (at
which the capacity of the pressure relief valve is determined) is 121% of the set pressure
when the valve is sized for the fire contingency only. The reseating pressure (at which
the PRV closes again) shall be 7% below set pressure.
Figure 3.4.3-b: Multi-port pressure relief valve
Flow rating requirements are determined as follows. As already stated earlier for
bullets and spheres the largest relief requirement originates from external heat exposure
caused by fire. For these tanks the heat input will be related to the size of the tank and
its total surface area.
1.
For tanks below 186 m2 total surface area the adequate rate of discharge in
cubic meters per minute of air at 121% of the set pressure is presented in
Table 3.4.3 “Flow Requirements for Pressure Relief Valves for Tanks.”
2.
If the tank surface area is larger than 186 m2 the adequate rate of discharge
in cubic meters per minute of air at 121% of the set pressure is determined by
the formula:
PRV Flow Rate (m3/min., Air) = 10.66 x A 0.82
Where A = Total outside surface area of the tank in m2.
Safety in LPG Design
BULK STORAGE
3-19
A simplified method for calculating the surface of tanks is outlined
below.
The surface area is the total outside surface area of the tank in square meters. The
tank surface area can be calculated by using one of the following simplified calculating
formulas:
1.
Cylindrical tank with hemispherical heads:
Area = Over-all length x outside diameter x π.
2.
Cylindrical tank with other than hemispherical heads:
Area = (Over-all length + 0.3 outside diameter) x outside diameter x
π. Note that this formula is not exact, but results will be within the
limits of practical accuracy for the sole purpose of sizing relief valves.
3.
Spherical tank:
Area = Outside diameter squared x π.
Figure 3.4.3-c: Internal and external pressure relief valves used in marketing facilities
It should be borne in mind that it is important to choose the valve(s) to match the flow
rating as close as possible. The appropriate choice of valve orifice size (or the sum of
orifices if more than one valve is used) will result in slight oversizing. Undersizing
could lead to overpressure; however, grossly oversizing could lead to valve
“chattering” (a rapid succession of opening and closing the valve) and self destruction
of the PRV. If a multi-port manifold is used, one valve shall be spare, the other(s)
good for a total 100% of the required relieving rate.
3.4.3.1
Pressure Relief on Mounded Drums
Mounded drums shall be protected against overpressure as follows. The larger of the
two figures shall be used to determine the flow rate for PRVs on mounded drums.
1.
3-20
Determine the required PRV flow capacity by considering the maximum flow
caused by overfilling, high vapor pressure (if Butane is stored), improper
commissioning (air freeing), or inert accumulation (Nitrogen). Since in all
BULK STORAGE
Safety in LPG Design
of the above cases the filling process will cause overpressure, the pumping
capacity shall be checked when determining the PRV size.
2.
The codes propose the following simplified orifice sizing approach to
estimate fire loads. Calculate the surface area of the mounded drum. If the
mounded drum surface is less than 186 m2 establish 30% of the
corresponding flow rate in Table 3.4.3 “Flow Requirements for Pressure
Relief Valves for Tanks.” If the tank surface exceeds 186 m2, calculate 30%
of PRV Flow Rate (m3/min, Air) = 10.66 x A 0.82. Check whether this
figure is larger than the pumping capacity mentioned above.
Since mounded drums are completely covered by an earth mound they will not be
subject to direct radiation from external fire and PRVs are not sized for this contingency.
If the tank has to be uncovered for maintenance (See LPG Safe Operations Guide) or
other reason, LPG shall first be completely removed from the tank. Otherwise, should
a fire occur during maintenance the tank would not be protected against this
contingency. Likewise, during first commissioning the tank shall be mounded before
filling.
Surface
Area
Air Flow
Rate
3
Surface
Area
Air Flow
Rate
Surface
Area
3
Air Flow
Rate
3
Surface
Area
Air Flow
Rate
3
m2
m /min
m2
m /min
m2
m /min
m2
m /min
>1.86
17.57
12.07
82.12
26.01
154.30
83.61
401.80
2.32
21.27
12.54
84.67
26.94
158.90
88.26
419.90
2.79
24.69
13.00
87.22
27.87
163.10
92.90
438.00
3.25
28.03
13.47
89.77
28.80
167.60
97.55
455.90
3.72
31.15
13.94
92.32
29.73
172.10
102.19
473.50
4.18
34.55
14.40
94.87
30.66
176.40
106.84
491.30
4.65
37.66
14.86
97.42
31.59
180.90
111.48
508.60
5.11
40.49
15.33
99.96
32.52
185.20
116.13
525.80
5.57
43.61
15.79
102.50
33.45
189.40
120.77
543.10
6.04
46.44
16.26
104.80
34.37
193.70
125.42
560.10
6.50
49.56
16.72
107.30
35.30
198.20
130.06
577.10
6.97
52.39
17.19
109.90
36.23
202.50
134.71
594.10
7.43
55.22
17.65
112.10
37.16
206.70
139.35
610.80
7.90
58.05
18.12
114.70
41.80
227.70
144.00
627.50
8.36
60.88
18.58
116.90
46.45
248.00
148.65
643.90
8.83
63.43
19.51
121.80
51.09
268.20
153.29
660.30
9.29
66.26
20.44
126.60
55.74
287.90
157.93
676.80
9.76
69.09
21.37
131.10
60.39
307.50
162.58
692.90
10.22
71.64
22.30
135.90
65.03
327.00
167.23
709.30
10.68
74.48
23.22
140.50
69.68
346.00
171.87
725.50
11.15
77.02
24.15
145.30
74.32
364.70
181.16
757.50
11.61
79.57
25.08
149.80
78.97
383.40
185.81
773.30
Table 3.4.3: Flow requirements for pressure relief valves for tanks (below 186 m2)
NOTE: The required flow capacity is in cubic meters per minute of air at standard
conditions, 15.6 °C and atmospheric pressure. For intermediate values of surface area,
the rate of discharge may be interpolated.
3.4.3.2
Installation of Pressure Relief Valves
All pressure relief valves shall bear a substantial non-corrosive metal identification
plate giving, at a minimum, the manufacturer's name, code under which fabricated,
set pressure and orifice size.
Safety in LPG Design
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3-21
The total required relief valve capacity can be covered by multiple pressure relief valves.
These may be installed with a manifold that includes provision for selectively closing
off any particular relief valve to permit removal for inspection while the remaining relief
valves provide the discharge capacity required for the tank.
Pressure relief valves shall be located on top of the tank in direct flow connection to the
vapor space such that the frictional pressure drop between tank and PRV nozzle
under full flow conditions (at rated capacity of the PRV) does not exceed 3% of the
PRV set pressure. Long or narrow piping connections between the tank and the PRV
are not allowed because this could result in excessive pressure drop and the associated
difficulties (inadequate relief and “chattering”).
Regulations concerning valving between tank and PRVs allow for the following
options:
1.
Install pressure relief valves without block valves.
2.
Provide excess relief capacity with multi-port arrangement valves, interlocked
valves, or sealed (CSO = Car Sealed Open) block valves. Isolating one valve
shall not result in reducing the capacity below required relieving capacity.
3.
Install pressure relief valve with CSO block valve and warehoused spare.
PRV can be removed and replaced under carefully managed procedure which
minimize time without PRV and control operations and pressure during that
time.
In the first case, the tank has to be emptied for testing and maintaining the PRV. The
latter cases permit removal with the tank in operation. CSO valves may not be
permitted by some local codes.
If CSO gate valves are installed upstream or downstream in piping associated with a
PRV their stems shall be oriented horizontally or upside down. This is to prevent the
gate from dropping by gravity, should it detach from the stem.
Pressure relief valve discharges on tanks having volumetric capacity of 10 m3 or larger
shall be discharged via a vertical vent stack extending above the heads of personnel
who could be on the PRV platform (minimum of 2.1 m typical in Marketing and 3 m in
Refining and not less than 3.0 m above ground level.) into open air or closed flare
systems if so required by authorities. The discharge vent or header shall be sized such
that at full flow the pressure drop does not exceed 10% of the PRV set pressure.
Discharges shall not be allowed to enter enclosed spaces. The vent shall be piped to
prevent impingement of escaping gas on the tank, nearby tanks, operating pipelines and
equipment. Vents shall be protected against mechanical impact and be designed to
handle any thrust during PRV discharge. Vent systems where several vents discharge
into a header are not permitted since the discharge velocity at the end of the vent system
may be so low that proper dispersion of the vapors is not guaranteed.
Drilling a 20 mm weep hole in the vent line low point shall prevent accumulation of
liquid or condensate. If vapor exiting the weep hole would impinge on the tank or other
equipment, an elbow or deflector plate may be used to redirect flow. Small diameter
discharge lines (typically Marketing) could have their mechanical integrity
compromised by a 20 mm hole. For these small lines a loose fitting plastic rain cap at
the top of the vent may be combined with a smaller weep hole.
3.4.3.3
Pressure Relief Valve Testing Requirements
Pressure relief valves in LPG service normally operate in a clean, non-corrosive
environment. Pressure relief valves in LPG service have shown a good reliability over
the years. However, since no mechanical device can be expected to remain in operative
condition indefinitely it is recommended to test or replace PRVs on LPG bulk storage
within a 5 year interval. Local regulations, if more stringent, may overrule this
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BULK STORAGE
Safety in LPG Design
interval. Testing of PRVs requires special procedures and equipment, manufacturer’s
advice shall be obtained if testing is to be carried out at plant.
3.5
Emergency Block Valves on Bulk LPG Tanks
Emergency Block Valves (EBV) are provided to stop LPG liquid flow from the tank to
potential downstream emergencies. EBVs permit quick control of hazardous situations
by stopping leaks at pump seals, at hose ruptures, or at fires. EBVs are recommended to
be metal gate or plug valves or high performance ball or butterfly valves with metal
seats (not soft seats). They shall meet the requirements of GP 3-12-1 and Design
Practices Section XV-F. Soft-seated valves NPS 4 and smaller meeting GP 3-12-1 may
be used. Soft-seated valves over NPS 4 shall not be used as EBVs. They may be used
in non-EBV service provided they meet the fire-safe requirements of API 607 and GP 314-1.
EBV shall be of approved make and shall incorporate all the following means of
closing:
1.
Local manual shutoff.
2.
Remote-operated manual shutoff. Remote activation device or pushbutton
shall be accessible during an emergency
Valves which are sandwiched between 2 flanges by long, exposed bolts shall not be
used. EBV's may be either automatic/remote operation or manual operation as defined
below. In Upstream and Refining typically EBV's may be fitted externally between tank
flange and piping. For Marketing EBV's are often installed internally upstream of the
outlet flange. More details on Emergency Block Valves can be found in ERE reports
EE.27E.84, “Guidelines for Selection and Installation of Emergency Block Valves” and
EE.44E.94, “Possible Risk Reduction Design Items for Above Ground Pressurized LPG
Storage.”
3.5.1
Tank EBV's in Liquid Service
All liquid phase inlet and outlet connections on the tank larger than 25 mm diameter
shall have an EBV and an additional manual shutoff valve located as close as practicable
to the tank. The EBV may be installed inside tank (manual or automatic remote
hydraulic operation). Externally installed EBVs to the liquid phase shall either close by
remote operation or automatically. Remote operated valves may be either fail safe or
their actuators and energizing lines fireproofed. Connections to the liquid phase
below 25 mm may only have two manual block valves.
Automatic fail safe valves close upon failure of motive energy or control signal. Their
motive energy supply shall be designed to fail (melt) in the event of a fire. Remote
operation non fail safe valves (gate or quarter turn) usually rely on an electrical or
hydraulic actuator. In order to function, these valves need to be energized during a
potential fire. Therefore, signal and electric cables (or hydraulic ducts) as well as
actuators shall be fireproofed in order to function during the first 15 minutes of an
emergency. The 15 minutes are based on API 2510 and on the assumption that within
15 minutes of a fire developing appropriate action has been taken. Normally refineries
have fireproofed valves and new Marketing installations have fail safe valves. The
control system shall be arranged so that these valves can be closed individually and can
also all be closed simultaneously by an emergency shutdown system that shuts down all
LPG pumps and compressors at the facility as well. See also Chapter 2, “Emergency
Shutdown system.” Means of remote actuation of EBV may be pneumatic, hydraulic,
electrical or mechanical. The actuation point shall be in a safe location outside the
immediate risk area, at least 15 m away from an aboveground tank or the valve
assembly on an underground or mounded tank. Signage shall be installed at the
actuation point to indicate its location and mode of operation.
Safety in LPG Design
BULK STORAGE
3-23
Manual shutoff valves installed close to the tank shall be capable of an adequate seal
under fire conditions as specified in the 30 minutes fire test in API 607 or equivalent.
3.5.1.1
External Shutoff Valve
Remote operated external shutoff valves installed immediately at the first flange of the
tank are the preferred option to provide emergency blocking capability. Spring
actuated quarter turn ball valves (NPS 4 or less) with electric, hydraulic or pneumatic
spring release is an adequate choice to perform fail safe operation. The signal lines to
operate the valve shall be either of plastic or low melting point metal. Should a fire
develop in the vicinity of the tank, these lines should fail and the valve would close
automatically. Tying the valve into the plant Emergency Shutdown System (ESS)
provides remote shutdown capability.
Figure 3.5.1.2-a: Internal excess flow shutoff valve
3.5.1.2
Internal Excess Flow Shutoff Valve
Another option for the remote operated tank emergency block valve is an “internal
excess flow shutoff valve.” It is located inside at the bottom of the tank and kept open
by a hydraulic system. The signal lines to operate the valve shall be either of plastic or
low melting point metal. In case of fire close to the tank these lines fail and the valves
close automatically. Tying a depressuring valve into the plant Emergency Shutdown
System (ESS) provides remote shutdown capability. However, such valves are only
available for sizes up to 100 mm. Larger internal valves without excess flow shutoff
have been installed in the past and may continue to be used.
Selecting the closing flow rate of an excess flow valve involves an analysis of the flow
characteristics of the complete piping system. If the closing flow rate happens to be
considerably above the flow that could be obtained by rupture of downstream
piping/hose the valve will not close automatically. The valve shall be mounted in the
correct direction. Furthermore, installation shall be such that adequate clearance is
provided around the inlet ports of the valve, otherwise the pressure conditions during
normal flow may be equal to excess flow conditions and the valve would close under
normal operating conditions. Excess flow valves shall have a rated closing flow about
50% greater than the expected design flow rate. All liquid and vapor withdrawal
3-24
BULK STORAGE
Safety in LPG Design
connections on the tank (except for the pressure relief valve connection or where the
effective opening into the tank is smaller than 1.4 mm diameter) shall have a positive
shutoff valve located as close as practicable to the tank, in combination with an excess
flow valve installed on the tanks’ withdrawal nozzles.
Figure 3.5.1.2-b: Internal excess flow shutoff valve operation
The effectiveness in the event of a line/hose break of an excess flow valve is limited by
certain conditions. It can only be maintained when the tank is taken out of service. The
valve may not close automatically if:
1.
Piping system restrictions (due to pipe length, branches, reduction in pipe
size or number of valves) decrease the flow rate to less than the valve's
closing flow.
2.
The break or damage to the downstream line is not large enough to create
enough differential pressure across the valve to close it.
3.
A shutoff valve in the line is only partially open and will not allow enough
flow to close the excess flow valve.
4.
LPG pressure upstream of the excess flow valve, particularly due to low
temperature, is not high enough to produce a closing flow rate.
5.
Foreign matter (such as welding slag, scale or sludge) is lodged in the valve
and prevents closing.
Figure 3.5.1.3: Quarter turn ball valves
3.5.1.3
Manual Back-up Valve
All automatic or remote operated EBVs shall be provided with a second, manually
operated isolation block valve. This double block is provided as a backup for
malfunction of the automatic or remote operated valve and to ensure additional shutoff
in case of leak. The best choice is a quick closing quarter turn ball valve, which shall
meet the requirements of GP 3-14-1.
3.5.2
Tank Shutoff Valves in Vapor Service
Vapor return lines and other tank connections to the vapor phase shall have two manual
isolation valves. Any ball valve shall meet the requirements of GP 3-14-1.
Safety in LPG Design
BULK STORAGE
3-25
3.6
Tank Instrumentation
The following LPG tank instrumentation is needed to satisfy the basic needs for safe
operation of LPG bulk storage:
1.
Tank level measurement shall be based on risk of overfill considerations.
Tanks receiving rundown streams from processes or pipelines or ship
unloading shall be equipped with two independent level gauges and a high
level alarm. Tanks receiving smaller parcels like road bulk trucks or single
rail cars may need only one level gauge.
2.
Independent level high-high alarm (LHHA) for continuous rundown and large
parcel receipt. If tank is filled by ship or pipeline the LHHA may also act as a
Cut Off (LHHA(CO)) on the tank inlet valve.
3.
Pressure indicator.
4.
Temperature indicator.
Figure 3.6: Typical connections for LPG sphere tank
Legend:
1.
2.
3.
4.
5.
6.
3-26
Inlet Nozzle
Outlet Nozzle
Pressure Relief Valve Nozzle
Atmospheric Vent
Drain Nozzle
Top Manhole
BULK STORAGE
7. Bottom Manhole
8. Top Pressure Gauge Connection
9. Top Level Indicator Connection
10. Bottom Level Indicator Connection (DP Cell)
11. Top and Bottom Temp. Indicator Connect.
12. Level Gauge Connection
Safety in LPG Design
The level high alarm and the independent level high-high alarm need an audible alarm at
the control room and, if the control room is not permanently manned, also a local
audible alarm. There shall be a local read-out for the tank level gauge, the pressure
gauge, and the temperature indicator, however, the design engineer may decide to have
additional remote read-outs at the control room. The alarm instrumentation shall be
designed and installed so that the alarms can be tested without taking the tank out of
service. In addition, an appraisal of the measurement needs of the particular location
may reveal the need for more instrumentation depending on automatic stock control,
custody transfer etc. Instrumentation shall be replaceable without taking the tank out of
service.
Level and flow indicators with glass components shall not be used, because the glass is
subject to breakage from fire, mechanical damage, or improper assembly.
Figure 3.6.1: Radar gauge (Saab)
3.6.1
Tank Level Measurement
New or renovated installations may be equipped with radar type tank level
measurement. This technology developed recently and has proven to be reliable. It is
suitable for complete custody transfer automation with control from remote locations. It
has no moving parts and is easy to be repaired with the tank in operation.
The previous type of tank level measurement was the servo gauge. Servo-gauge
technology also offers high measurement precision but has moving parts and therefore
needs more maintenance Servo gauges employ a small displacer, rather than a float,
attached to a tape that passes over a measuring drum. A shaft to a servo motor (or
“stepping motor” that continuously balances the downward force of the barely immersed
displacer and its tape against a stress transducer connects the measuring drum. This
Safety in LPG Design
BULK STORAGE
3-27
balanced system completely eliminates measurement error due to tape weight variations
or tape system friction.
The Magnetic Gauge may often be a solution to low cost installations in bullet tanks. It
consists of a gauge constructed with a float inside the tank resting on the liquid surface
which transmits its position through suitable leverage to a pointer and dial outside the
tank indicating the liquid level. The motion is transmitted magnetically through a
nonmagnetic plate and, since no venting of LPG is required, the magnetic gauge is a
recommended type. However, it cannot be used for Custody Transfer of product.
Early installations have used dip-tubes, fixed level gauges or rotary gauges for level
indication. The fixed maximum liquid level gauge shall have the liquid level
determined on the maximum permitted filling limit when the liquid is at 4 °C for
aboveground tanks or at 10 °C for underground or mounded tanks. Though outdated,
they may still serve their purpose especially as a back-up indication for the radar or
servo gauges. If such a vent type gauge is the only level control, a more modern level
measurement system may be warranted.
Float and tape gauges have been frequently installed in LPG sphere tanks. However,
reliability has not been satisfactory. Their precision is limited by two factors inherent
in their design: friction in the tape guides and pulleys, and varying weight of the tape
itself as product level changes in the tank. Therefore, installation is no longer
recommended.
3.6.1.1
Level High Alarms
Up to two level high alarms may be provided for LPG bulk tanks. The first alarm
(Level High Alarm, LHA) can be integrated into the level gauge system. If there is a
computerized level measurement system the first alarm may be an integrated software
alarm. The second alarm (Level High-High Alarm, LHHA) shall be entirely
independent. It shall be installed for continuous rundown or large parcel receipt. It
shall be hard wired, fail safe (i.e. self checking with failure indication) and shall
continue working when the level gauge and/or the first alarm fails. If the alarms are not
fail safe the system shall be tested frequently. The first alarm shall be set just above the
85% level, the second shall be set as close as possible above the first. If the plant is
supplied by pipeline or tanker large volumes come into the tanks at high velocity.
Therefore, the level high-high alarm shall also act as a cut off (LHHA(CO)) on the
supply line.
Alarm systems shall be tested quarterly with the option to change if there is a
documented good history, but not less than annual. Therefore, installation of a high
level alarm system shall provide for complete testing of all mechanical and electronic
system components without depressuring the LPG tank. The system shall be configured
to actuate the alarm when the electrical power circuit is de-energized, thus assuring
protection in the event of an undetected power supply failure. This latter feature shall
necessitate battery back-up for the alarm system. For horizontal pressure tanks a
packless ball float in an external chamber, actuating a micro switch outside the chamber,
is the preferred option. This typical design, explained in Section XII-C of Design
Practices, permits maintenance and testing without depressuring the LPG tank, but
requires two 38 mm valved connections on 609 mm centers for installation. A similar,
but more complex installation, also described in Section XII-C, could be made using a
refinery type displacer level measuring device.
3.6.2
Pressure and Temperature Indicators
A pressure indicator shall be installed on a 13 mm valved fitting to the 50 mm
connection at the top of the tank. The indicator shall be a high quality instrument
designed to permit test bench re-calibration, and shall be capable of accurately reading
3-28
BULK STORAGE
Safety in LPG Design
pressures up to 120 percent of the pressure relief valve setting. There shall be a 50 mm
block valve on the tank nozzle and a 13 mm quarter turn valve as a second block valve.
The tank shall be equipped with a stainless steel thermowell, and a precision liquid
filled dial thermometer capable of accurately reading all expected tank operating
temperatures. This requirement is not eliminated by provision of resistance temperature
detectors or thermocouples. The independent thermowell and thermometer are still
required as a quick verification check on accuracy of the electrical/electronic devices.
The liquid filled dial thermometer is recommended as it can easily be removed for
check-calibration or for storage in a secure place when not in use. Flanges shall attach
thermowells since this arrangement cannot result in erroneous unscrewing of the whole
attachment.
Figure 3.6.2-a: Pressure and temperature indicators
Pressure
Indicator
Temperature
Indicator
Double
Block
Valves
Tank Shell
Thermowell
Figure 3.6.2-b: Pressure indicator (PI) and temperature indicator (TI)
3.6.3
Grounding Connections for Tanks
Each tank having electric instrumentation shall be grounded to earth from at least two
points. It shall be designed to dissipate lightning strokes, which may affect the
instrumentation. Copper tape, 25 mm by 3 mm shall be used for this connection and
each grounding point shall be taken to a separate electrode. The ground electrode
(ground rod) may be a 16 mm diameter extensible type copper rod with a minimum
Safety in LPG Design
BULK STORAGE
3-29
length of 2.4 m. These electrical connections shall be checked and tested on an
annual basis. Therefore, the connections shall be accessible. Some local regulations
require grounding on all tanks above a minimum size. Equipment using cathodic
protection shall not be grounded. Cathodic protection shall comply with GP 19-5-1.
3.6.4
Product Odorization
LPG product, which is provided for sale for combustion use, shall be odorized.
Odorization systems are offered as package units by companies like Williams,
(Valencia, Ca. USA) and Lewa (Leonberg, Germany). They are fully integrated,
electro-pneumatically operated and may have one or two injection pumps. They may be
controlled from either the digital or analog output signal from measuring equipment in
the gas line. A standby solid state repeat cycle timer may permit manual operation and
stroke rate adjustment in the event the flow signal is temporarily lost or disconnected for
maintenance. The odorant may be delivered in ordinary 200 liter drums and pumped
over into a storage tank, or a dedicated stainless steel drum may be filled at the
manufacturer site and transported to the plant.
Figure 3.6.4: Odorization systems by Williams and Lewa
3-30
BULK STORAGE
Safety in LPG Design
4
PUMPS & COMPRESSORS
4.1
Pumps
4.1.1
Pump Types Commonly Used
There are two fundamental types of pumps used for any service including LPG.
These are the positive displacement pump or “PD” pump and the centrifugal
pump. Positive displacement pumps have internal gears, vanes or other
geometry's which enclose discrete volumes of liquid at the suction and transport
them to the discharge. Positive displacement pumps do not in themselves generate
pressure but simply move the fluid to the discharge where it assumes the back
pressure in the piping.
In a centrifugal pump, the liquid enters an impeller and is accelerated to a high
velocity. Upon exiting the impeller the fluid is slowed down in the pump diffuser
and the energy imparted in the form of velocity is changed to pressure.
Pump Head
Centrifugal Pump
Curve
Positive
Displacement
Pump
Curve
Pump Flow
Figure 4.1.1: Pump characteristic curves
Safety in LPG Design
PUMPS & COMPRESSORS
4-1
Positive displacement pumps are virtually insensitive to the type of fluid being
pumped, the fluid viscosity and fluid density. They deliver essentially the same
flow rate regardless of the fluid or pressure in the discharge. Centrifugal pumps
on the other hand are very sensitive to the type of fluid (density) and the pressure
developed is a function of both the density and flow rate. The figure above shows
the characteristic flow curves for both types of pumps.
4.1.1.1
Rotary Shaft Positive Displacement Pumps
The type of positive displacement pump normally used in LPG service is a rotary
shaft positive displacement pump (see the Design Practices, Section X-F,
“Positive Displacement Pumps” for a complete description of all positive
displacement pump types).
Figure 4.1.1.1-a: Multiple gear pump
Figure 4.1.1.1-b: Internal gear "Crescent" pump
The design and installation of positive displacement pumps for most operating
plants are governed by American Petroleum Institute API 676, “Positive
Displacement Pumps, Rotary Shaft;” GP 10-2-2; and Design Practices Section X,
“Pumps.” The reader may wish to review these documents to determine
applicable features dependent on the particular installation. See also GP 10-1-1
4-2
PUMPS & COMPRESSORS
Safety in LPG Design
“Centrifugal Pumps.” The following API codes may apply: API 610 “Centrifugal
Pumps for Petroleum, Heavy Duty Chemical and Gas Industry Series.” ASME
B73.1M “Horizontal End Suction Centrifugal Pumps,” ANSI B73.2M “Vertical
In-Line Centrifugal Pumps.”
The figures here show two different types of internal gear pumps, one with several
gears and one with only two gears, also called a Crescent pump. In these pumps
fluid is trapped between the teeth of the gear and casing and transported to the
discharge.
The figure below shows a sliding vane pump. Fluid is trapped between the vanes
and the casing and transported to the discharge. The benefit of the sliding vane
pump is that as wear occurs, the vanes simply move out automatically
compensating. In both of these pump types, a single shaft penetrates the pressure
casing and shall be sealed to keep the LPG in the pump. See the section on “Shaft
Sealing” below, for additional details.
Figure 4.1.1.1-c: Vane pump
The advantages of positive displacement or “PD” pumps include:
1.
PD pumps are capable of handling some vaporization in the suction.
2.
PD pumps can tolerate more change in the net positive suction head
available (NPSHa) and are self priming.
3.
PD pumps are generally smaller and operate at lower speed than
centrifugal pumps making them ideal for mounting on road transport
and taking power from an engine power take off (PTO).
4.
PD pumps have higher efficiency than centrifugal pumps (dependent on
the viscosity of the fluid).
5.
The pumping direction is reversible by reversing the direction of
rotation.
The disadvantages of PD pumps include:
1.
Safety in LPG Design
Higher maintenance costs due to actual rubbing of the internal parts
causing wear.
PUMPS & COMPRESSORS
4-3
2.
Potentially higher cost due to the need for special low speed motor or
gear.
3.
Not suitable for continuous operation over many weeks of duration.
4.
Requires a pressure relief valve on the discharge.
Pressure relief valves are always required in the discharge of PD pumps because
the pump simply pushes the fluid into the discharge line. If the discharge is
blocked, the pump will continue to push the fluid until either there is insufficient
power to turn the pump or the pump casing or piping breaks. Pressure relief
valves shall be sized for the maximum pump capacity and there shall not be any
valves in the line between the pump discharge and relief valve. See Design
Practices Section XV-C, Safety in Plant Design, “Pressure Relief” for a complete
description of relief valve sizing and installation.
Figure 4.1.1.2-a: Centrifugal pump operation
Figure 4.1.1.2-b: Multi-stage centrifugal pump
4.1.1.2
Centrifugal Pumps
The figure below shows a cross section of a centrifugal pump. Inside the
stationary casing (C), turns the shaft (A) to which the rotating impeller (B) is
attached. The blades of the impeller accelerate the liquid inside the impeller and
push it to the impeller outside diameter at high speed. This action creates a lower
pressure at the impeller eye or inlet (D) thus drawing more fluid into the impeller.
The high velocity fluid at the impeller exit is slowed within the casing that
converts the velocity energy to head or pressure before the fluid exits the casing at
(E). More information about the many styles of centrifugal pumps can be found in
the Design Practices, Section X: “Pumps.”
4-4
PUMPS & COMPRESSORS
Safety in LPG Design
As previously stated, the head or pressure developed by a centrifugal pump varies
with flow. To match the required flow and pressure, centrifugal pumps can be
designed with different width and diameter impellers, different speeds, and by
providing multiple impellers on one shaft as shown on the figure below.
Figure 4.1.1.2-c: Tank mounted submersible pump
The design and installation of centrifugal pumps for typical operating plants are
governed by API 610, “Centrifugal Pumps for Petroleum, Heavy Duty Chemical
and Gas Industry Service,” GP 10-1-1, “Centrifugal Pumps” and for less severe
duties, ANSI B.73. Unlike PD pumps, centrifugal pumps develop their own
pressure and the maximum pressure they can develop is at zero flow. Therefore,
the pump casing and downstream equipment are usually designed accounting for
the maximum pressure and a relief valve is not normally provided in the
discharge.
The advantages of centrifugal pumps include:
1.
Can provide very high flow rates economically.
2.
Lower maintenance cost (no rubbing parts).
3.
Lower installation cost on a cost per unit capacity basis.
4.
Wide discharge pressure and flow range.
The disadvantages of centrifugal pumps include:
Safety in LPG Design
1.
Lower efficiency, especially at lower flow rates.
2.
Need to be primed.
3.
Not tolerant of vapor in the suction.
4.
More sensitive to inlet system problems and NPSH.
PUMPS & COMPRESSORS
4-5
4.1.1.3
Minimizing Pump Cavitation and Vaporization
Pumping systems and the choice and location of pumps shall be based on an
assessment of the Net Positive Suction Head (NPSH) required at the pump with
the aim of minimizing the likelihood of pump cavitation and vaporization
occurring in the pump's suction system.
NPSH or Net Positive Suction Head, is the amount of energy available at the
inlet of a pump above the total energy at which the fluid will vaporize. All pumps
have a Net Positive Suction Head requirement or NPSHr expressed in feet or
meters of water.
The suction system that the pump is connected to has an available NPSH or
NPSHa. The available NPSH must always exceed the required NPSH or vapor
bubbles will form at the inlet to the pump. These bubbles form at the impeller eye
and then implode within the impeller in a process called cavitation. Cavitation is
damaging to the pump and will eventually cause failure.
In addition to adequate NPSH, the inlet line to the centrifugal pump shall be free
of vapor bubbles and shall be completely filled with liquid: There shall not be any
high point in the suction line where vapor can collect and restrict the flow. A
vapor return line to the supplying LPG tank will help to reduce vaporization in the
suction line. Heat reflective paint, shading from the sun or similar arrangements
to reduce the temperature of the product in the suction system may be helpful to
prevent vaporization.
Vaporization of liquid LPG can also be reduced by providing sufficient static head
on each restrictive piping device to cancel out the pressure drop of each device. A
1370 mm column of LPG creates about 6.9 kPa (0.069 bar g) of static pressure.
Suction line and NPSH problems usually occur in centrifugal pumps for the
following reasons:
1.
2.
Suction line is too long or too small or both. Suction lines are typically
at least one size larger than the flange of the pump to which they are
connected.
Too many bends, valves or fittings in the suction line.
3.
Too high a flow rate causing increased pipe friction and inability to
vaporize fluid in the suction tank to accommodate the drop in liquid
level. A guideline to help control liquid LPG boiling is that no more
than 2 - 3 percent of the total tank volume shall be removed per minute.
For underground tanks, it is even less: 1 - 2 percent..
4.
Pump installed on a level that is higher than the tank minimum
operating level.
5.
Suction line has high points between the pump and tank. Typically the
suction line shall slope upwards continuously from the pump back to
the tank.
6.
LPG temperature higher than normal.
7.
Debris or obstructions (such as valves not fully open) in suction line.
This may include strainers or filters.
Taking these factors into account leads to the design criteria that the pressure drop
in the suction line shall not exceed 0.07 to 0.14 bar. Complete information on
pump NPSHa calculation can be found in the Design Practices Section X-D,
“NPSH” and other Design Practice Sections on sizing lines for pressure drop,
Section XIV, “Fluid Flow.”
One method to resolve NPSH and suction line concerns is to use a tank mounted
submersible pump as shown in the figure above. Here, the pump is installed in a
4-6
PUMPS & COMPRESSORS
Safety in LPG Design
pipe or barrel inside the tank with a ball valve on the barrel bottom.
maintenance, the ball valve is closed and the barrel is de-pressurized.
4.1.1.4
For
Selecting Pump Type and Pump Features
Selection of the correct pump type is the key to a reliable, low cost installation.
The following guides shall be used when selecting the pump type.
For most LPG transfer applications which are not in continuous service and where
highly variable suction conditions exist, where maximum evacuation of the tank is
desired, where pumps are motor transport mounted and power take-off driven, a
positive displacement pump shall be used. The sliding vane pump is preferred
over a gear pump since it operates at higher speed (more compatible with available
drivers) and is self compensating for wear when the correct vane and casing
materials are chosen. The sliding vane type pumps are mostly used in cylinder
filling and loading/unloading applications with capacities up to 80 m3/h and
pressure differential of 1.4 – 5.5 bar. The regenerative turbine type pump is very
reliable in cylinder filling applications where filling rates are up to 8 m3/h and
pressure differentials up to 10 bar. The low NPSH vertical turbine canned type
centrifugal pumps are used in loading/unloading marine tanks or in applications
where the flow rates are higher than 80 m3/h.
Figure 4.1.1.4: Regenerative turbine pump
For LPG services, which are continuous, 24 hour a day, seven days a week,
involve large flow rates (typically over 12 m3/h), have relatively constant inlet
conditions or are outside the range of commercially offered vane pumps, a
centrifugal pump shall be selected.
For additional information on selecting pumps for any general service, see Design
Practices Section X, “Pumps.”
Safety in LPG Design
PUMPS & COMPRESSORS
4-7
Pumps used in LPG service shall have the following minimum features:
1.
Pumps shall be designed for LPG service. The characteristics of LPG
require that all shaft and stem seals, set disks and other resilient parts be
of materials that are impervious to the action of both LPG vapor and
liquid.
2.
Steel is the best choice of material for pressure containing parts of LPG
pumps. However, some manufacturers produce pumps in ductile iron
due to limited market demand on steel. Ductile iron would be the
second choice. Cast iron is not allowed.
3.
All pumps shall have proven experience in LPG service at or near the
conditions of the service they will be used in. See GP 10-1-1 for the
experience clause required.
4.
Positive displacement pumps shall have a relief valve in the discharge
piping. Integral relief valves in the pump are not considered a
replacement for PRVs.
5.
Centrifugal pumps shall be self venting or incorporate a vent
connection.
6.
All pumps shall have a mechanical seal (proven LPG service), not
packing. More information DP X- G.
7.
O-rings shall be of a fluorelastomer material such as viton.
8.
Consideration shall be given to the non-lubricating characteristic of
LPG product when selecting pump for LPG service.
9.
Pumps shall be selected and installed with sufficient net positive suction
head (NPSH) to avoid cavitation under both normal operating
conditions.
10. Suction and discharge piping systems shall be sized to accommodate
the maximum design flow rate of the pump and the related pressure
losses.
11. Check valve shall be installed on the discharge side of all centrifugal
pumps.
12. Where positive displacement pumps are used, a suitably sized pressure
bypass system shall be installed. The product shall be re-circulated to
the pump suction line at least 5 m ahead of the pump suction.
13. Pumps capable of producing pressure high enough to damage any
component on the discharge side shall be equipped with a suitable relief
device that discharges to a safe location. This device shall be located
upstream of the of the first block valve.
14. Electric motors shall be Class I, Division 2, Group D or equivalent.
Motor enclosure shall be flameproof. Other electrical equipment shall
be approved for operation in the hazardous area in which they are
located. All equipment located in the open shall be weatherproof.
4.1.1.5
Pump Sizing
Pumps shall be sized to suit the maximum operational flow rates and pressures
required. Pumps, compressors and piping systems shall have a loading rate sized
according to the size of the tank being filled. Adequate controls shall be provided
to prevent tanks from being filled beyond their maximum liquid level.
A loading rate of 11.5.- 23 m3/h shall prove satisfactory for tank truck units up to
a capacity of 9500 liters. A loading rate of 23 - 46 m3/h shall be required for
38,000 liter units. In larger terminals, consideration shall be given to loading rates
of 68 - 114 m3/h to reduce the number of loading positions required.
See Table 4.1.1.4 for typical pump flow rates and pressure differentials.
4-8
PUMPS & COMPRESSORS
Safety in LPG Design
Flow Rate
Pressure Differential
m3/h
kPa
With vapor return
23–34
340–520
Without vapor return
7–11
520–860
Pump Sizing
Rail car or truck unloading
Loading delivery tank truck
With vapor return
23–34
340–520
Without vapor return
7–11
520–860
Cylinder filling
3–11
410–860
Table 4.1.1.4: Typical Pump Flow Rates and Pressure Differentials
4.1.1.6
Shaft Sealing
Whether the pump is a rotary shaft PD pump or centrifugal, the shaft must
incorporate some kind of device to keep the LPG in the pump. The characteristics
of LPG require that all shaft and stem seals, set disks and other resilient parts be of
materials that are impervious to the action of both LPG vapor and liquid. Most
pumps of current design will use a face contact mechanical seal consisting of a
carbon ring and a silicon carbide or tungsten carbide ring. Either of the rings may
rotate or is stationary depending on the pump vendor's design.
The primary concern in sealing LPG is to ensure that there is a back-up device,
which will limit the release of product in the event of primary seal failure. Current
practice for onsite units and operating plants is to utilize a single mechanical seal
with a contacting, dry running back up seal, which takes over in the event of
primary seal failure. There is also a pressure switch connected to the area between
the primary and back up seal to alarm a rise in pressure. Note: Due to a large
population of high pressure pump applications, Upstream requires at least tandem
seals in all hydrocarbon applications, including LPG.
Complete details for selection of seals, seal types and seal arrangements can be
found in the “ERE Pump Sealing Technology Manual,” No. TMEE-23 and in the
Design Practices section X - G.
4.1.1.7
Pump Pressure Control Valve
Where positive displacement pumps are used or pumps run at intermittent service
for long intervals at shutoff conditions (e.g., topping off transports, filling
cylinders), a suitably sized pressure bypass system shall be installed. In such
conditions a pressure control valve shall return the product to the tank. This is
important since just returning the product to the suction side or relying on the
internal pressure relief valve will result in rapid heat up, loss of suction and
potentially in damage to the pump. Spring loaded valves can be used to control
the pressure, however they are not as good as the “Camflex” valves, which have
smooth flow characteristics. The Pressure Relief Valve shall not serve as a
pressure control valve since this will result in chattering, vibration and destruction
of the pressure relief valve.
Safety in LPG Design
PUMPS & COMPRESSORS
4-9
4.1.1.8
Strainers
Temporary strainers are used to protect a pump or compressor during the startup
of new equipment. The piping layout shall permit insertion and removal of the
strainer without disturbing equipment alignment. Fittings such as tees, Ys, or
spool pieces may be used for this purpose. The manufacturer of the equipment
being protected should be consulted concerning the size and type of strainer or
filter required.
A permanent strainer or filter may be needed to protect a meter or other sensitive
equipment. Its design and location shall permit cleaning without removing the
strainer body or draining long sections of line. Block valves and bypasses shall be
provided for this purpose. Connections larger than 50 mm diameter shall be
flanged.
4.1.1.9
Pump Installation Requirements
Regardless of the pump type selected, the pump installation shall incorporate the
following items to help ensure safety and reliability:
1.
The pump shall be located outside the LPG tank drainage and impound
area in a freely ventilated space. Pumps shall not be positioned
underneath LPG tanks or containers. Drainage shall be provided to
prevent liquid accumulation around a pump and to drain a spill to a
safer area to minimize exposure to other pumps and piping.
2.
The pump suction line shall be as short and large as possible with a
minimum of bends, fittings and other obstructions. The suction line
shall slope up continuously from the pump to the tank with no high
points or vertical U-bends than can trap vapor. Typical installation
details for piping at a pump can be found in GP 3-3-2, “Suction and
Discharge Piping for Centrifugal Pumps.”
3.
Suction and discharge piping shall have flexibility and shall be properly
aligned to the pump in order not to exert excess forces on the pump.
Incorporation of one or two horizontal ninety degree bends in the
suction and discharge near the pump will impart flexibility to minimize
thermal stresses as ambient temperature changes. However, the best
method is to have a piping flexibility analysis done and compare the
results to the allowable flange stresses from the pump vendor.
Additional requirements and information on piping installation can be
found in GP 3-7-1, “Piping Flexibility and Support;” GP 3-18-1,
“Piping Fabrication;” and GP 3-19-1, “Piping Erection and Testing.”
4.
When a stationary pump is installed it shall be bolted and grouted to a
substantial concrete foundation and steel base plate. The pump shall
also be level and the driver aligned to the pump within the
manufacturer's tolerances. Pump installation information can be found
in the pump and driver's manual from the vendor and in the API 686
“Recommended Practices for Machinery Installation and Installation
Design.”
Standard valving around the pump shall be as follows:
5.
4-10
+
Remote and automatic shutdown valve at the tank outlet.
+
Manual block valve at the pump suction.
+
Manual block valve at the pump discharge.
+
Check valve in the pump discharge just inside the block valve.
+
A valved connection on the discharge within the check valve for
a pressure gauge. The connection shall be seal welded and two
plane gusseted.
PUMPS & COMPRESSORS
Safety in LPG Design
6.
If the pump will be operated for any period of time below the minimum
allowable flow from the vendor or at shutoff (such as in topping off
transports, filling cylinders, etc.) then a pressure controlled recycle
system shall be installed (see Pump Pressure Control Valve). This
would be in addition to the relief valve on a positive displacement
pump.
7.
Suction and discharge piping systems shall be sized to accommodate
the maximum design flow rate of the pump and the related pressure
losses. If high transfer rates (above approximately 2.5% of tank volume
per minute) must be achieved, provision shall be made to supply
additional vapor to the tank. The vapor pressure within the tank being
emptied may become sufficiently depressed during high flow rates to
cause loss of suction to the pump. A vapor return line from the tank or
vehicle being filled automatically eliminates this problem.
Withdrawing liquid from two or more tanks at the same time may also
eliminate this situation. A tank with fireproofing, which retards heat
transfer from the surroundings, may also need supplemental vapor at
medium rates.
8.
A pressure relief valve shall be installed on the discharge of all positive
displacement pumps. The valve shall be located in the piping before
the check valve and discharge block valve. The relief valve shall be
sized for the maximum pump flow rate. Since such relief valves release
LPG in the liquid phase, the discharge piping of the valve shall be
directed back to the LPG tank.
9.
If the LPG is stored below the freezing point of water, special
provisions for the pump, seal, piping and installation are required.
These include such items at special pump materials to withstand the
Critical Exposure Temperature, special seal designs to avoid hang up
from icing, and special connections to allow chill down of the pump
prior to starting. For such services contact EMRE for special
requirements.
10. Pump discharge piping shall be securely anchored as close as practical
to prevent system vibrations from acting directly on the pump.
11. The pumps shall be provided with the following operational and safety
feature: A local start-stop pushbutton shall be installed in the vicinity of
the pump. A remote Emergency Shutdown System (ESS) pushbutton
shall be provided in a safe location in case the local start-stop
pushbutton is not accessible because of fire or vapor cloud. Pump shall
be interlocked to stop when any Emergency Shutdown (ESS)
pushbutton is activated if such a system is provided.
12. LPG pump shall be provided with block valves to isolate the pump from
LPG source. The block valves shall be installed on both suction and
discharge piping lines. They shall be located within 3 m from the pump
flanges.
4.2
Compressors
In transfer operations, compressors are used to create a pressure differential
between two drums causing a flow of liquid from the drum at higher pressure.
Compressors are also employed to transfer residual vapor from supplying drums
and other facilities in transport and maintenance operations. When designing a
compressor system, consideration shall be given to the principal safety requirements of
selecting a compressor that is fit for the purpose of handling LPG vapor in the
specified conditions of service and that the system prevents liquid entering the
compressor.
Safety in LPG Design
PUMPS & COMPRESSORS
4-11
Transfer of product by compressor is usually less economic compared to pumping.
However, sometimes it is the best or only available method. Compressors are
used for the following two purposes:
1.
To transfer liquid product by aspirating LPG (taking compressor
suction) from the tank to be filled and discharging into the tank to be
emptied. The pressure differential thus created will cause liquid to flow
into the tank to be filled. This transfer method is used to unload rail
cars, trucks and marine vessels since it will completely empty the tank
without harming the equipment making the transfer.
2.
To evacuate tanks. This may be done for economic reasons when
receiving product by rail or for maintenance when a tank (or container)
or cylinder has to be taken out of service.
.
Figure 4.2.1-a: Reciprocating compressor (Corken)
4.2.1
Compressor Types Used
Reciprocating, single stage, air cooled piston type compressors are most
frequently used in LPG plants. These operate at low compression ratio. They are
used to create pressure differentials between tanks to transfer liquids. Generally
they are small skid-mounted units. The exception may be larger reciprocating
compressors used to off-load large ships. Various cylinder configurations
(vertical, horizontal, 2 stage etc.) are used, depending on the flow rate and
differential pressure required. There are also several different methods for power
transmission but belt drive is the most common. All belts drives shall comply
with GP 10-11-1 para. 5.9 (static conductive belts). A typical reciprocating
compressor is shown below. In Marketing operations centrifugal compressors are
4-12
PUMPS & COMPRESSORS
Safety in LPG Design
not normally justified unless very large amounts of gas and liquid must be
transferred and they are not covered in the requirements below.
4.2.1.1
Design Requirements for Compressors
Compressor shall be designed for LPG service. The characteristics of LPG require
that all parts be made of materials that are impervious to the action of both LPG
vapor and liquid. Compressors shall be equipped with a device to prevent
excessive pressure in the delivery system. The design of the compressor shall
limit or exclude lubricating oil contamination of LPG vapor. Compressor shall be
sized within the maximum flow rating of the excess flow valves, if available, on
the incoming supply tanks.
Size, configuration and speed of compressors shall be selected on the basis of the
maximum operational flow rate and pressure required. Electric motors shall be
Class I, Division 2, Group D or equivalent. Motor enclosure shall be flameproof.
Other electrical equipment shall be approved for operation in the hazardous area in
which they are located. In addition to the electrical area classification, all
equipment located in the open shall be weatherproof
Figure 4.2.1-b: Typical reciprocating compressor package (Corken)
For reference, larger reciprocating compressors intended for services in refineries
and chemical plants are governed by API 618, “Reciprocating Compressors for
General Refinery Services” GP 10-4-1, “Reciprocating Process Compressors; and
for less severe installations,” API 11P, “Specification for Packaged Reciprocating
Compressors for Oil and Gas Production Services.”
Small compressors intended for LPG transfer service shall have the features listed
below as a minimum:
Safety in LPG Design
1.
Compressors shall have proven experience in LPG at or near the
conditions for which they will be installed. See GP 10-4-1 for
experience requirements.
2.
Reciprocating compressors may be of the lubricated (where some oil is
injected into the cylinder to lubricate and seal the piston rings) or nonlubricated type. When the compressor is lubricated, some means shall
be provided in the discharge to remove the excess oil from the vapor
stream. Filters and/or centrifugal traps may do this. Non-lubricated
compressors and compressors specially designed for LPG applications
do not require oil removal facilities.
PUMPS & COMPRESSORS
4-13
4.2.2
3.
Some means (control) shall be provided to limit the suction pressure to
the maximum for which the compressor is designed, otherwise, the
driver will be overloaded. In addition, compressors shall have at least
one high pressure cut-off switch on the discharge side or similar device
to prevent excessive pressure in the delivery system.
4.
The compressor shall have a high temperature alarm to signal that the
pressure differential becomes too high. As an alternate, temperature
indication on the discharge or differential pressure alarm may be used.
As the compression ratio of a compressor increases, the discharge
temperature also increases which can then damage the compressor.
5.
Traditionally, most reciprocating compressors (for LPG and other
hydrocarbons) have been manufactured with cast iron cylinders. In
fact, current standards by API allow cast iron up to 69 bar gauge.
However, if steel or ductile iron equipment can be obtained, it is the
preferable material choice.
Compressor Sizing
Size, configuration and speed of compressors shall be selected on the basis of the
maximum operational flow rates and pressures required. Compressors shall be
sized within the maximum flow rating of the excess-flow valves installed on the
incoming supply tanks when used for unloading shipments and on the plant's LPG
tanks when used for loading shipments. When a compressor is used for
transferring liquid LPG in this manner, the liquid transfer rate is less than the
vapor displacement of the compressor. The compressor manufacturer shall be
consulted to obtain the expected transfer rate under the plant operating conditions.
Typical compressor volume flows and pressure differentials for operations in an
LPG marketing terminal are as follows:
Volume
Differential Pressure
25–340 std m3/h
70–140 kPa
3
Table 4.2.2: Compressor Volume Flows and Pressure Differentials Flow Rate: std m /h
at 15.6 °C and 101.325 kPa
4.2.2.1
Compressor Installation Requirements
Compressors shall be installed in a freely ventilated location. The compressor
shall be securely mounted on a suitable foundation or base plate in accordance
with the compressor manufacturer’s recommendations. The compressor casing
shall not be subjected to excessive strains transmitted to it by the suction and
discharge piping. Compressor piping shall be suitably braced to minimize
vibration Properly sized knock out pots, fabricated to appropriate pressure vessel
code, shall be installed on the inlet of the compressor to prevent liquid from
entering the compressor.
On each knock out drum, a high level shutdown system is required so that if too
much liquid accumulates in the drum, the compressor will automatically shutdown
before liquid carry over to the compressor occurs. Check valve shall be installed
on the discharge side of all centrifugal compressors. Strainers shall be installed on
the suction piping. The strainer element shall be designed and installed so that it
can be serviced.
The compressor shall be provided with the following operational and safety
features:
4-14
PUMPS & COMPRESSORS
Safety in LPG Design
1.
A local start-stop pushbutton shall be installed in the vicinity of the
compressor.
2.
A remote Emergency Shutdown (ESS) pushbutton shall be provided in
a safe location in case the local start-stop pushbutton is not accessible
because of fire or vapor cloud.
Compressor shall be interlock to stop when any Emergency Shutdown System
(ESS) pushbutton is activated if such a system is provided.
Figure 4.2.2: Knock-out drums (liquid traps)
Positive displacement compressors shall be equipped with suction and discharge
shutoff valves. A blowdown valve which relieves the trapped pressure when the
compressor is shut down shall be provided. Automatic blowdown valve is
acceptable. Each positive displacement compressor shall be equipped with a
pressure-relieving device on the discharge side discharging to a safe location.
Following are installation requirements for compressors in LPG bulk plant
service:
Safety in LPG Design
1.
Compressors shall not be positioned underneath LPG tanks. They shall
be located outside LPG drainage and impound areas and in accordance
with the spacing requirements defined in the section of this guide
entitled “Equipment Spacing to Maximize Separation” in Chapter 2.
2.
The compressor shall be bolted and grouted to a substantial concrete
foundation in accordance with compressor manufacturer's
recommendations. Often the compressor and auxiliary equipment are
supplied on a steel frame, which is then bolted and grouted to the
foundation. The proper procedures to mount, level, install and shim the
compressor can be found in the API 686 “Recommended Practices for
Machinery Installation and Installation Design.” Pipework to the
compressor shall be fitted and supported in accordance with the
manufacturer's recommendations so as not to subject compressor
components to excessive stress.
3.
On the suction side of the compressor appropriate means shall be
provided to prevent liquid phase of LPG from entering the compressor.
PUMPS & COMPRESSORS
4-15
This is typically done by installing a suction knock out drum (liquid
trap) with a high level shut down to stop the compressor driver. Traps
shall be fabricated to an approved pressure vessel code and in addition,
equipped with an independent liquid level alarm switch and a liquid drain
connection. For small compressors a knock out drum with a float to
close the inlet line to the compressor may be provided. When the inlet
valve is closed the compressor should trip, e.g. on high current or low
suction pressure, prior to pulling vacuum.
4.
5.
6.
7.
4-16
Reciprocating compressors, since they are positive displacement
machines, require a pressure relief valve on the discharge line. There
shall be no valves in the line between the compressor outlet and relief
valve. The pressure relief valve shall be sized for the maximum flow of
the compressor. The valve may be routed to an appropriate, safe
location.
Standard valving on the inlet and outlet of a compressor is as follows:
+
Remote and automatic shutdown valve at the tank inlet and
outlet.
+
Manual block valve at the compressor suction.
+
Manual block valve at the compressor discharge.
+
A valved connection on the discharge for a pressure gauge. This
connection shall be two plane gusseted and seal welded in
accordance with GP 3-18-1.
+
An optional four way valve to automatically reverse the suction
and discharge when the compressor is to be used for both
loading and unloading.
Suction and discharge piping shall be fabricated so that they exert
minimum stress on the compressor. Guidelines for piping fit up can be
found in GP 3-19-1. Incorporation of one or two horizontal ninety
degree bends near the compressor may provide adequate flexibility but
a piping stress analysis will reveal any significant problems. Suction
piping shall also be designed without low points and pockets to trap
liquids.
For reference, GP 3-3-4 provides requirements for
reciprocating compressor suction and discharge piping. Pipework for
the transfer of liquid LPG shall be sized so the overall pressure drop in
the system does not result in excessive condensation. Experience in
typical LPG installations indicates that this pressure drop is generally
not in excess of 200 kPa. Install suction and discharge lines so that any
condensate that may form in the piping system does not drain into the
compressor. Isolating valves shall be installed on either side of the
compressor to permit its removal or maintenance while minimizing the
volume of LPG vented to atmosphere.
In cold climates compressor suction lines in Butane service shall be
specially reviewed to ensure condensed vapor cannot collect in the
suction line. Sloping the line continuously back to the tank and
avoiding low points may do this.
PUMPS & COMPRESSORS
Safety in LPG Design
5
PIPING AND VALVES
5.1
Piping in Plants
This chapter discusses piping in plants. It highlights that piping shall run above ground
whenever possible, clarifies choice of piping materials and briefly explains pressure and
temperature ratings for piping. All metallic LPG piping shall be designed according to
ASME B31.3 “Chemical Plant and Refinery Piping” and shall meet the requirements of
the appropriate Global Practices (GPs) noted in this chapter. All Pipelines shall be
labeled to indicate contents and function. Country language must be used. Piping
arrangements for customer installations and automotive LPG can be found in the
respective Chapters 10 and 11.
5.1.1
Piping Arrangements
5.1.1.1
Small LPG Plant
Figure 5.1.1.1 shows a small LPG plant with a truck unloading facility and cylinder
filling. This figure shows the pump installation in the plant for unloading trucks. A
compressor is preferred for unloading a tank car when a flooded suction is not available
or when vapor recovery from the tank car is required. The pump supplying the cylinder
filling operation requires a liquid bypass line with a back-pressure regulator because of
the intermittent duty of the filling operations. Internal excess-flow valves and internal
back-flow check valves are preferred on horizontal LPG tanks to prevent a major release on
accidental breakage of external piping or fittings. When external valves are used, they shall
be installed so that any undue strain beyond their limits will not cause breakage between the
tank and the valves. The liquid volume between the block valve at the unloading vehicle and
the block valve of the plant must be minimized.
Figure 5.1.1.1: Small LPG Plant–Truck Unloading and Cylinder Filling Using a Pump
Safety in LPG Design
PIPING AND VALVES
5-1
Description
(A) Use self-sealing couplings and rigid steel pipe/swing joint connections to
truck or tank car.
(B) Equip vent line valves at unloading connections with spring-loaded
actuators that must be manually held open.
(C) The emergency fail-safe shutdown system that closes valves must also stop
the pumps. Valves are not required at unloading point when receipt lines
hold less than 0.05 m3 (50 l) of product.
(D) If pump bypass is not required, valve after pump may be eliminated for
short discharge lines.
(E) Piping at unloading point must be securely anchored to prevent piping
damage from vehicles that pull away while connected.
Notes:
5.1.1.2
1.
ASME tanks of 8 m3 or less capacity shall have no more than two plugged
openings (typical in all installations).
2.
Only liquid lines shall have thermal relief valves between all shutoff valves.
Small Plant with Rail Supply
A small LPG plant with truck or rail car supply and cylinder and truck filling facilities is
shown in Figure: 5.1.1.2. This figure illustrates the piping arrangement required when a
compressor is used for unloading tank trucks or tank cars. Although a compressor can
be used for truck loading, a pump is more common because it permits loading without
the use of vapor return from the truck. . The liquid volume between the block valve at the
unloading vehicle and the block valve of the plant must be minimized.
Figure 5.1.1.2: Small LPG Plant–Truck or Rail Supply, Compressor Unloading, Cylinder and Truck
Filling
5-2
PIPING AND VALVES
Safety in LPG Design
Description
(A) Use self-sealing couplings and rigid steel pipe/swing joint connections to truck
or tank car.
(B) Equip vent line valves at unloading connections with spring-loaded actuators
that must be manually held open.
(C) The emergency fail-safe shutdown system that closes valves must also stop the
pumps and compressor. Valves are not required at loading point when receipt
lines hold less than 0.05 m3 (50 l) of product.
(D) Flow reversal four-way valve.
Piping at loading/unloading points must be securely anchored to prevent damage from
vehicles that pull away while connected.
NOTE: Only liquid lines shall have thermal relief valves between all shutoff valves.
5.1.1.3
Multi-tank Installation
Figure 5.1.1.3 a shows a multi-bullet installation for cylinder and truck filling. When
several LPG tanks are installed to meet the storage requirements, they shall be
manifolded in groups to provide operating flexibility. The maximum number of tanks
allowed in any one group shall comply with the limitations as stated in NFPA 58. . The
liquid volume between the block valve at the unloading vehicle and the block valve of the
plant must be minimized.
Figure 5.1.1.3: Multi-tank Installation
Safety in LPG Design
PIPING AND VALVES
5-3
Description
(A) Use self-sealing couplings and rigid steel pipe/swing joint connections to truck
or tank car.
(B) Equip vent line valves at unloading connections with spring-loaded actuators
that must be manually held open.
(C) The emergency fail-safe shutdown system that closes valves must also stop the
pumps. Valves are not required at loading point when receipt lines hold less
than 0.05 m3 (50 l) of product.
(D) Piping at loading/unloading points must be securely anchored to prevent
damage from vehicles that pull away while connected.
NOTE: Only liquid lines shall have thermal relief valves between all shutoff valves.
5.1.2
Piping Location
Piping shall be located above ground and adequately supported and secured.
Installations shall incorporate sufficient flexibility to withstand thermal expansion or
contraction, movement or settling of tanks, pumps, compressors, etc. Suction lines from
tanks or unloading racks to pumps shall slope continuously down from the tank outlet
or rack to the pump suction. For reference also see GP 3-7-1, “Piping Layout,
Flexibility and Supports.”
Where piping is installed under a driveway, underground installation shall be within a
conduit of sufficient size to accommodate the pipeline and the pipe shall have a
protective coating. The protective coating shall be suitable to withstand corrosion and
shall extend at least 150 mm beyond the ends of the conduit. Both ends of the conduit
shall be open to atmosphere.
5.1.3
Piping Integrity
LPG poses a higher risk than many liquid hydrocarbons because at atmospheric pressure
it vaporizes readily and may develop large vapor clouds. All components of the piping
system shall be able to withstand internal temperatures and pressures, plus external
corrosion and mechanical stresses. They also need a high degree of fire resistance. For
reference, also see GP 3-18-1 “Piping Fabrication” and GP 3-19-1 “Piping Erection and
Testing.”
Piping shall conform to the provisions of ASME B31.3 or other appropriate national
standard. Piping shall also meet the provisions of GP 3-10-1 “Piping Selection and
Design Criteria”. Although ASME B31.3 allows construction of pipelines to meet only
B31.4, within the plant site any pipeline shall meet the more stringent requirements of
B31.3. All welding at metallic piping shall be in accordance with the ASME Boiler and
Pressure Vessel Code Section IX.
Piping LPG service shall be selected from the following options:
Seamless pipe meeting the requirements of ASTM A106, API 5L or approved equal.
The use of non seamless pipes shall be approved by the LPG Technical Advisor.
Electric fusion welded pipe (submerged arc or gas metal arc) meeting the requirements
of API 5L or approved equal.
Electric resistance welded pipe meeting the requirements of API 5L and the following
supplementary limitations and/or requirements:
5-4
PIPING AND VALVES
Safety in LPG Design
1.
Grade A25 shall not be used.
2.
Minimum pipe size shall be 19 mm.
3.
If the percent carbon exceeds 0.23, the Carbon Equivalent (per API 5L) shall
not exceed 0.43%.
4.
Pipe shall be normalized and tempered.
5.
The pipe mill shall be audited or shall provide quality control data that is
acceptable to the owner’s engineer.
Note: Upstream allows only seamless pipe in all applications, including LPG.
Pressure, psig
Carbon steel and Stainless steel are satisfactory for LPG piping components; but cast
iron, wrought iron, brass and copper shall not be used. Aluminum has limited
application for refrigerated LPG in low pressure services.
800
700
600
500
400
300
200
100
0
class 300 #
class 150 #
-50
0
50 100 150 200 250 300 350 400
Temperature, C
Figure 5.1.4: Graph of Pressure Limits Vs Temperature for A106 Carbon Steel
5.1.4
Pressure Ratings
It is suggested that all piping be designed for Propane, with a minimum working
pressure of 17 bar gauge, even where only Butane or a Propane/Butane mixture is
expected to be handled. This provides for flexibility if Propane operation is required at
a later date. This requires a minimum flange rating of ASME Class 150. For piping
within a plant, minimum wall thickness is as follows:
50 mm and smaller:
above 50 mm:
Schd. 80 (Extra strong)
Schd. 40 (Standard)
Higher flange and pipe ratings may be required for piping connected to high pressure
sources, such as reciprocating pumps or cross-country pipelines. In such cases, unless a
pressure relief valve sufficient in size to relieve the maximum flow from the supply
source provides protection, the rating shall be at least equal to that of the supply piping.
Where fluid surges may occur due to rapid valve closing, pump starting etc., a surge
analysis shall be performed.
5.1.5
Pipe Sizing
Piping system shall be designed to meet the highest operating pressure at the expected
operating temperature. Pipe sizing for lines shorter than 30 m can usually be determined
by the sizes of connections on transfer pumps and compressors. There is little economic
incentive for minimizing diameter at these sizes.
Safety in LPG Design
PIPING AND VALVES
5-5
Pressure drop (Friction Loss) calculations are needed for longer lines, where pump or
compressor suction conditions are suspected to be marginal, and where flow from
several sources joins in a manifold. The designer shall refer to Design Practices Section
XIV for liquid and vapor flow, and Section XV-C for pressure relief valve inlet and
discharge lines. Some valve and fitting manufacturers also publish guides for fluid flow
calculations.
Examples of situations where pressure drop shall be calculated include:
1.
Pump suction lines (including internal isolation valve, when present). Where
a line size larger than the pump connection is found to be necessary, an
eccentric reducer shall be installed, flat side up to prevent collection of vapor
in the suction line.
2.
Lines containing excess flow valves, to assure that cumulative pressure drop
would not limit flow in the event of a break to a rate, which prevents the
excess flow device from functioning.
3.
Liquid or vapor manifolds or headers.
4.
Long lines, such as to or from marine piers.
5.
Pressure relief valve inlet and discharge lines.
10
25
32
38
51
64
76
102
mm pipe
diameter
1
152
0.1
100
1000
10000
Flow, l/min
Figure 5.1.5: Pipe friction loss for Propane (for Butane multiply by 1.15)
5.1.6
Pipe Connections
Threaded fittings shall be forged steel while butt-welding fittings shall be seamless steel
or equivalent material. Cast iron fittings (elbows, tees, couplings, unions, flanges, and
plugs) shall not be used. Pipe joints in steel shall be screwed or welded.
Following are requirements for joints and connections used for piping in bulk storage:
1.
5-6
Threaded and socket welded joints are limited to pipe sizes 50 mm and
smaller per GP 3-10-1. (Note: Upstream allows threaded and socket welded
joints only up to 37 mm). Larger piping can be butt welded or flanged;
welded joints shall be used whenever possible, and flanged or threaded
PIPING AND VALVES
Safety in LPG Design
joints shall be minimized.
shall not be used.
Packed-sleeve and resilient-sealed couplings
2.
Flanges shall be weld neck, raised face, per ASME B16.5 or equivalent.
Gaskets shall be the self-centering or confined type. Stud bolts shall be used,
threaded full length with continuous threads. Flanges, gaskets, bolting and
fittings shall meet the requirements of GP 3-16-1, except that slip-on flanges
shall not be used for LPG.
3.
Pipe bends may be used instead of welding elbows. The centerline bend
radius shall be at least 3 times nominal pipe diameter. Bends shall meet the
requirements of GP 3-18-1.
4.
Flexible bellows type connectors are more vulnerable to mechanical damage
than steel pipe and are very difficult to inspect. Therefore, all efforts shall be
made to avoid installation of flexible bellows connections. In most situations,
multiple welded bends can provide required flexibility without loss of
structural integrity. Where flexible connectors (bellows) are used, they shall
be stainless steel Type 321 or 316L, with the flexible inner hose covered by a
braided wire jacket. Working pressure shall be at least 17.25 bar gauge and
burst pressure no less than 69 bar gauge. The connector shall be fabricated as
a unit with end fittings for attachment to the piping system. If the
connections are flanged, one end shall have a floating flange to avoid
twisting. Overall length of a flexible connector shall not exceed 1 m; this is
sufficient for an offset up to 50 mm. This type of flexible connector shall not
be used in locations where it can come in contact with water, which contains
Chlorides.
5.
Gaskets shall be suitable to protect against leakage during fire. See
GP 3-16-1, “Flanges, Gaskets, Bolting, and Fittings” for specific details.
Pipe unions shall not be used except on cylinder filling carrousels, pump seal
connections or similar connections that need dismantling for maintenance and which can
be reliably blocked off. Unions shall be of forged steel with a working pressure of at
least 207 bar gauge, and shall have ground metal-to-metal seats. Gasket unions shall not
be used.
The designer may consider upgrading to stainless bolts on carbon steel flanges at
locations with severely corrosive atmospheres. This will limit the spread of corrosion
into the flange itself.
5.1.7
Small Piping Connections
Piping 50 mm and smaller has less mechanical strength than larger sizes, and less wall
thickness to withstand external corrosion, so additional reinforcement of connections is
needed.
Safety in LPG Design
1.
Branch connection 50 mm and smaller shall meet the requirements of
GP 3-18-1.
2.
Threaded connections shall be seal welded in accordance with GP 3-18-1.
3.
In addition, small connections in vibrating service and where vulnerable to
mechanical damage shall be gussetted as required by GP 3-18-1.
4.
An emergency block valve (manual) shall be installed in each small piping
take-off connection (instrument lines etc.), located as close to the tank or line
as possible, with no elbows between the connection and valve. Connections
to piping shall be 13 mm or larger. Connections to the bottom of tanks shall
be minimized. Tubing downstream of the emergency block valve shall be
stainless steel, and in accordance with GP 3-6-1.
5.
Liquid drawoff piping (sampling piping) leading to the atmosphere, or to an
open tank, shall be double valved if the tank contains stocks which can autorefrigerate. The valve next to the shell shall be a quick action type, such as a
PIPING AND VALVES
5-7
metal-seated plug valve. The second valve, for flow control, shall be of a
type suitable for partially open operation.
6.
5.1.8
Water drawoff piping shall discharge at a point not less than 4.5 m from the
peripheral boundary of the LPG tank. The discharge shall be so located that
any flammable spillage will drain away from the tank.
Installation
All metallic LPG piping shall be installed in accordance with ASME B31.3. All
welding of metallic piping shall be in accordance with ASME Boiler & Pressure Vessel
Code, Section IX.
Aboveground piping shall be supported and protected against physical damage. To
prevent corrosion under supported un-insulated pipes, pipe supports and pipe sleepers
shall be designed with steel standoffs (19 mm diameter minimum), such as steel rods
welded to the top of support to raise the pipes above possible water/liquid pool that may
encourage corrosion (Figure 5.1.8).
Underground piping shall be avoided, however, when there is no other alternative and
piping is beneath driveways, roads, or streets, possible damage by vehicles shall be
taken into account. Underground metallic piping shall be protected against corrosion as
warranted by soil conditions. Depending on conditions and risk involved some
country/clusters have chosen to install double wall stainless steel flexible piping with
leak detection. LPG piping shall not be used as a grounding electrode. LPG piping that
crosses an open drain where it may contain flammable product (spill in a fuels terminal)
shall be protected or fireproofed against any flash fire. Interconnecting piping between
tanks and tanks accessories shall be installed to permit flexibility (possible vertical and
horizontal movement) due to tanks foundation settlement and tanks expansion. Flexible
connectors between tanks and piping system are prohibited, however, they are permitted
where local codes require them for earthquake protection.
Figure 5.1.8: Typical pipe sleeper with steel rod standoff
Metallic pipe joints shall be threaded, flanged or welded using pipe and fittings. When
joints are threaded or threaded and back welded the following applies:
5-8
1.
For LPG at pressures in excess of 865 kPa or for LPG liquid the pipe and
nipples shall be schedule 80.
2.
For LPG vapor at pressures of 865 kPa or less, the pipe and nipples shall be
schedule 40 or heavier.
PIPING AND VALVES
Safety in LPG Design
The fittings or flanges shall be suitable for the service for which they are to be used.
Gaskets used to retain LPG in flanged connections in piping shall be made of metal or
other suitable material confined in metal having melting point over 816 °C.
The outlet of a differential pressure valve in a pumping system shall be piped back to the
pump suction line at a point at least 5 m ahead of pump suction. All drain sets or
isolation leading directly to atmosphere shall be double block using one ball and one
globe valve. The globe valve shall be located at the end closer to the atmosphere. The
minimum distance between the valves shall be 600 mm. All drains and vents shall be
closed and capped or plugged when not in use.
After assembly, piping systems (including hoses) shall be tested and proven free of
leaks at not less than 1.3 times the design pressure or maximum operating pressure
whichever is higher. The results of the tests shall be documented. LPG leak tests shall
never be made with an open flame.
5.2
Valves in Piping
5.2.1
Valve Integrity
All pressure containing metal parts of valves in plants or terminals shall be
manufactured in forged or cast carbon steel or stainless steel. All materials use,
including valve seat discs, packing, seals, and diaphragms, shall be resistant to the action
of LPG under service conditions. Soft-seated valves shall meet API-607 conditions as
“fire-safe.” Cast iron or brass valves shall not be used, with the exception that that
valves and instruments on LPG cylinders and containers (small tanks) may be made of
brass.
For reference also see GP 3-12-1 “Valve Selection Criteria.” The following valve types
shall be provided in accordance with API STD 2510. Valve used at pressure higher than
LPG tank pressure shall be suitable for working pressure of at least 4,830 kPa. Valves
to be used with liquid LPG, or with vapor LPG at pressures in excess of 865 kPa, shall
be suitable for a working pressure of at least 1,725 kPa.
Shut-off valves in LPG service shall provide positive bubble-tight shutoff. To
accomplish this, it is usually necessary to employ resilient seating materials. However,
resilient materials are inherently susceptible to damage during a fire. All resilient seated
valves, other than modified API STD 600 and API STD 602 gate valves shall meet the
30 minutes fire test specified in API STD 607 or equivalent standard. Ball valves with
double-sealing feature shall be used. Valve bodies shall be forged or cast carbon steel.
Gate and globe valves shall be made to API valve standards or equivalent.
5.2.2
Shutoff Valves
The shutoff valve considered most dependable in LPG service is the valve manufactured
by the Orbit Valve Company. If a quick-operating valve is needed, the Maxon-Okadee
and Everlasting metal-to-metal seated disk valves are acceptable.
Other suitable shutoff valves are Stockham Wedgeplug (with a resilient insert in the
plug), General Twin Seal, fire-safe ball valves by Neles-Jamesbury and McCanna, firesafe butterfly valves by Neles-Jamesbury, McCanna, Posi-Seal and Flowseal.
Ball valves with a double-sealing feature are recommended for LPG service. This
feature provides a leakproof shutoff when pressure is applied in either direction. The
type of ball valves that effect positive shutoff in only one direction are not suitable.
Safety in LPG Design
PIPING AND VALVES
5-9
Gate and globe valves shall be made to API valve standards. Most NPS 3 (NPS =
Nominal Pipe Size, inches) and larger gate and globe valves shall be purchased to
include resilient seating materials in the disk or gate, thereby providing tight shutoff.
5.2.3
Backflow Check Valves
Backflow check valves perform an extremely important safety function in tank openings
and in pipelines intended for flow in one direction only. All filling or liquid-return tank
connections designed for flow into the tank only shall be fitted with backflow check
valves. Some of these valves are for tank installation only; others can be used either in
tanks or in pipelines. The type most commonly used in larger transfer piping is the
swing check, which has relatively little pressure drop. Poppet, lift, and ball type check
valves in smaller sizes are used for truck, customer tank, and cylinder filling
applications. They are smaller and less expensive, but have more pressure drop. Valves
shall meet the requirements of GP 3-14-2. Backflow check valves are required in
discharge lines of pumps and compressors. The check valves shall be installed with a
positive shut off valve immediately adjacent to the anchor point and on the plant side of
the anchor point. They are also recommended in dedicated loading or unloading lines
for vapor and liquid, especially in piping from marine berths to shore tankage, where
piping is designed for flow only in one direction. In marine applications, check valves
shall be positioned in such that they will not present problems when the Cargo Transfer
Equipment is being de-pressurized under normal operating conditions. Soft-seated
check valves have the advantage of a relatively tight shutoff, and would minimize
release of product in case of a hose or connection failure.
Figure 5.2.3: Double back check valve and swing check valve
Backflow check valves restrict flow but may not provide positive shutoff. Emergency
Block Valves are required if it is necessary to stop flow reliably. For maintenance
purposes blinds shall be provided for leak tight shutoff.
5.2.4
Thermal Relief Valves
Thermal relief valves (hydrostatic relief valves) are designed for installation in any
portion of a piping system, or in any equipment in which liquid may become entrapped
by shutoff valves at both ends, become heated, expand and develop overpressure. The
liquid expansion factor of LPG is twelve times higher than that of water and twice that
of gasoline. To protect piping, the set pressure shall be no higher than the lower of
120% of the piping design pressure or system test pressure. This takes advantage of
ASME exclusion for thermal relief valves, which protect only blocked in piping.
Thermal relief valves are attached directly to piping and manifolds and may be exposed
to mechanical damage. Their location shall therefore be carefully selected and adequate
5-10
PIPING AND VALVES
Safety in LPG Design
protection shall be provided where necessary. The thermal relief valves shall be
installed as close as possible to the piping being protected. The piping connection shall
be 19 mm along with a quarter turn 19 mm Car Sealed Open (CSO) valve. This will
permit maintenance of the PRV without taking the piping system out of service. Where
permitted, the discharge of the thermal relief valves may be directed to atmosphere but
shall be covered by a plastic cap to prevent ingress of rain. Discharge shall be directed
so that it does not impinge on adjacent pipework or equipment. When the discharge of a
thermal relief valve goes to atmosphere and might cause a hazardous condition, the vent
outlet may be fitted with pipe, extended to a safe area and sized so that the free vent area
of the valve is not reduced. The vent outlet shall be at least 15 m away from all fired
equipment. A daily walk-by tightness check by the operator is recommended. At
marketing terminals, odorized LPG is likely to aid in leak detection during this
inspection.
Internal valves specified for LPG tanks will relieve pressure from thermal expansion in
the attached line into the tank. If the auxiliary manual shutoff valve downstream of the
internal tank valve is Car Sealed Open (CSO) in normal operation, lines connecting
directly with an internal valve do not require a thermal expansion relief valve.
It is recommended to test or replace PRVs in thermal relief service within a 5 year
interval.
5.2.5
Emergency Block Valves for Piping
For automatic shutoff design of the Emergency Block Valves (EBV), the actuating
system shall close valves upon failure of any system component (i.e. also known as failclose FC) Emergency Block Valves, described in Chapter 3, are provided at strategic
locations to stop LPG flow to potential downstream emergencies.
EBV's in Marketing plants are required for the following services:
1.
Tank connections.
2.
Transfer Points.
3.
Rotating Equipment.
4.
Cylinder Filling Shed.
The preferred choice for automated Emergency Block Valves in Marketing plants are
spring actuated quarter turn ball valves with electric, hydraulic or pneumatic spring
release as described under “Emergency Block Valves on Bulk LPG Tanks” in Chapter 3.
5.2.6
Valve Packings
The valve stem packings used in LPG service shall be capable of meeting the following
requirements:
5.2.6.1
1.
Minimize LPG emissions to the atmosphere during normal operations for
environmental and safety (fire) reasons, and
2.
Be fire-safe, especially when used in block valves, to hinder LPG escaping
from the valve stem during a fire.
Block Valve Packings
Global Practice GP 3-12-8 specifies the packing systems that shall be used in block
valves to meet the above requirements. For LPG service all-graphite ring packings
(Type 1) shall be used. The individual packing rings, that will perform the sealing
function, shall consist of flexible graphite with 1120 kg/m3 nominal density ( Style B2).
Safety in LPG Design
PIPING AND VALVES
5-11
The end wiper rings shall consist of interlaced braided graphite filament or solid 1600
kg/m3 minimum density compound carbon rings (Style A).
Graphite die-formed rings, braided rings, and bushings shall have a minimum carbon
content of 95%. Bushings shall be used only to take up excess packing chamber depth.
The packing rings shall have an active zinc, or a passive barium molybdate, or a passive
phosphorus based corrosion inhibitor. Separate zinc washers between packing rings
shall not be used.
Five-ring packing sets shall be used on API 600 and ASME B16.34 gate valves and
other rising stem valves. Four-ring packings shall be used with API 602 valves and on
all quarter turn valves (ball, plug and butterfly types) except that for quarter turn valves
NPS 1 and smaller, the wiper rings may be eliminated.
Live-loaded glands (using disk or coil springs installed on the gland stud bolts, on the
gland itself, or in the stuffing box) are not recommended. Stuffing box clearances,
procedures for environmental emissions testing of the valves, and other block valve
packing information may be found in GP 3-12-8. Additional pertinent information may
be found in the ERE report EE.34E.93 “Low Emission Block Valve Packing Guidelines
Updated” and report EE.58E.94 “Valve Packing Fire Tests Reinforce Recommendations
Not to Use All-Braided Packings.”
5.2.6.2
Control Valve Packings
Graphite packing may also be used for control valves but is not necessary, since control
valves do not have to be fire-safe in most cases. Graphite may result in higher stem
friction, which in turn may create problems for the control valve actuator motor. This
generally happens when graphite is used in a control valve that originally had an
elastomer/plastic packing. In such cases, the valve stem actuator shall be tested with the
graphite packing installed before the valve is placed in service.
For new valves, the actuators are sized for the appropriate friction loads, so there shall
not be any problems using graphite packing. Most major control valve manufacturers
are providing their own proprietary packings, which have already been tested, for
emissions and friction. See ERE report EE.84E.93 “New ‘Low-Emission’ Control
Valve Packings Successfully Demonstrated In Plant Performance” for additional details
on control valve packings.
5-12
PIPING AND VALVES
Safety in LPG Design
6
PRODUCT TRANSFER
6.1
Principles of Product Transfer
This section describes design requirements for equipment used to receive bulk product
into an LPG plant and ship it to customers. Components of product transfer within the
plant is described in Chapters: PUMPS AND COMPRESSORS as well as PIPING AND
VALVES. This Chapter discusses: principles of product transfer, static electricity in
loading and unloading operations, hoses in product transfer, loading and unloading for
trucks, rail cars and marine vessels. Also pipeline dispatch and receipt facilities are
discussed.
New piping transfer systems shall be designed for Propane vapor pressure. The
difference in cost, compared with Butane conditions, is not significant. However, if at a
later date a Butane system would be changed to Propane service this could only be done
at high cost.
Unloading systems can use compressors, pumps or a combination of the two. Pump
design ratings shall depend on type of product, desired pumping volume, resistance in
liquid line and availability of vapor return line. When a pump alone is used, vapors
cannot be recovered from shipment tanks.
Vapor
Pump
Flow Indicator
Liquid
Figure 6.1-a: Pumping liquid from one tank to another
In most design cases, two options are available to provide equipment for the tank filling
operations, plant receiving and transport loading.
1.
Safety in LPG Design
Product can be pumped into the bottom of the receiving tank with vapor
returned to the source tank via a vapor balance line, or
PRODUCT TRANSFER
6-1
2.
Product can be sprayed into the receiving tank vapor space (usually through a
longitudinal perforated pipe) and depend on cooling and absorption of the
vapor to accommodate the liquid volume transferred.
Both methods have advantages and disadvantages:
1.
Vapor balanced bottom loading can be accomplished with lower pump head
pressure, and consequently, consumes less energy. It provides positive
assurance that the receiving tank will not be overpressured as long as its
volumetric liquid capacity is not exceeded. However, it requires a vapor
balance line with valving and connection fittings and exposes the operator to
making and breaking twice as many temporary connections to the
transportation tank as is needed for spray filling. Another disadvantage of
vapor balanced bottom loading is for custody transfer using meter. On
average a quantity (2 - 3%) of vapor is returned to the delivering tank which
creates stock accountability problems.
2.
Spray filling makes use of the condensation of the LPG vapors and requires
only a single discharge piping system, however, it usually requires a higher
head transfer pump (consuming more energy) to compress the receiving tank
vapor space. Due to higher pressures, loading rates are reduced compared
with using a vapor return line. Depending on ambient temperature, product
vapor pressure, and receiving tank pressure rating, it may be necessary to
reduce the transfer rate to avoid lifting pressure relief valves on the receiving
tank. It is difficult to predict precisely the results in limiting pressure. In
general, the pressure increase using spray filling is approximately 25 percent
of the increase caused by filling into the liquid space without vapor return.
For example, if a product has a vapor pressure of approximately 1000 kPa and
is filling into the liquid space without vapor return, the terminal pressure may
increase by approximately 700 kPa. However, if the spray filling method is
used, the terminal pressure increase would be about 175 kPa.
Flow Indicator
Liquid
Compressor
Vapor
Figure 6.1-b: Transfer by displacement through compressed vapors
In order to select the most desirable mode of tank filling, the designer shall consider all
of the factors listed above and weigh their relative importance in each individual case.
The only normal exceptions shall be:
6-2
1.
Receiving from a product pipeline, where there is no place to return vapor,
and spray filling into the vapor space is the only option available, and
2.
Receiving into a refrigerated tank, which has an integral vapor handling
system. The tank shall still be equipped with a jet mixing nozzle to avoid
potentially hazardous product “rollover” when the receipt temperature is
significantly above inventory temperature.
PRODUCT TRANSFER
Safety in LPG Design
6.1.1
Loading or Unloading with Pumps
Pumping systems designed for the discharge of LPG from plant storage into another
tank are closed systems with no venting to the atmosphere.
1.
If the two tanks contain liquid product before pumping is started, the pressure
shall be approximately the same in both tanks.
2.
If there is no vapor line connection between the two tanks, the pressure in the
receiving tank increases during the product transfer.
3.
With an increase in pressure, some vapors in the tank change to liquid by
condensation.
Figure 6.1-c: Pumping with vapor balance line
Figure 6.1-d: Pumping without vapor balance line
Safety in LPG Design
PRODUCT TRANSFER
6-3
The rate at which vapor changes to liquid is determined by the following:
1.
The differential between the pump discharge pressure and the normal vapor
pressure of the product.
2.
The temperature of the liquid in the tank.
3.
The surface area of the liquid.
When the liquid enters from the top of the tank and is sprayed into the vapor space, the
total surface area of the liquid is increased and less differential pressure is needed.
The most important factor in the design and installation of LPG pumping transfer
systems is to keep LPG from vaporizing in the suction system.
1.
LPG is usually stored at the product vapor pressure and this makes product
transferring more difficult.
2.
When internal pressure is reduced due to product volume removal by the
pump, LPG vaporizes to maintain its temperature-pressure balance. The
vapor bubbles are formed at the tank wall because it is warmer and therefore
bubbles at the bottom of the tank that are in close vicinity of the pump suction
travel easily to the pump. In addition, vapor bubbles may be formed in the
suction line due to higher velocity and lower pressure.
3.
Excessive entrained vapor at the suction of the pump could eventually
damage it.
Design features for avoiding low pressure and vapor bubbles in pump suction systems
are discussed in section “Minimizing Pump Cavitation and Vaporization” in Chapter 4.
A vapor-equalizing line installed between the two tanks will greatly reduce the
differential pressure requirements for the pump and, therefore, shall be used wherever
possible. With such a line, the differential pressure will be the total flow resistance
through the pipes and fittings of the liquid line, plus the resistance in the vapor lines.
This shall not exceed 275 kPa. When the equalizing line is used, a quantity of vapor is
transferred from the receiving tank during the pumping operation.
6.1.2
Loading or Unloading with Compressors
Vapor compressors are often used to transfer liquid LPG by withdrawing vapor from a
container being filled, increasing the pressure through the compressor and discharging
the vapor into the supply tank.
1.
2.
The increased pressure in the supply tank and the decreased pressure in the
receiving container provide the differential pressure needed to force liquid
through a pipeline from one tank to the other.
The differential pressure is normally 70–140 kPa.
Vapor compressors offer an advantage in that the remaining vapor can be removed after
the liquid transfer has been completed. If the product is Propane, the savings can be
substantial, since 900 kg of product is contained in vapor in a 38 m3 tank at a pressure of
1000 kPa. Approximately 75 percent of it is economically recoverable.
6.1.2.1
Removing Vapor
To remove the vapor, the liquid line is closed and the pipe connections at the
compressor are reversed, so the vapor is drawn from the supply tank and discharged into
the receiving tank. This is usually accomplished automatically through a four-way valve
supplied by the compressor manufacturer. As previously described, vapors will be more
easily converted to liquid if the liquid surface area can be increased. This is
accomplished by discharging vapors into the bottom of the receiving tank so they will
6-4
PRODUCT TRANSFER
Safety in LPG Design
bubble up through the liquid and get cooled to minimize pressure increase. See Figure
6.1.2.1 for an illustration of a compressor transfer with a four way valve arrangement.
Figure 6.1.2.1: Transferring with a Compressor-Liquid Transfer (Top) and Vapor Recovery (Bottom)
6.1.3
Using Pumps Versus Compressors
The major criteria for choosing between pumps and compressors for loading/unloading
operations are as follows:
Use compressors when:
1.
Liquid cannot be gravity fed to an unloading pump (negative suction head).
2.
Vapor recovery is required.
3.
A plant has only one transfer device.
Use pumps when:
Safety in LPG Design
1.
Flooded gravity fed suction is available.
2.
Vapor recovery is not required.
3.
Differential pressures above 2 bar gauge (200 kPa) are required.
PRODUCT TRANSFER
6-5
4.
Liquid is to be metered.
5.
A lower initial the cost is desired.
There are cases where a pump/compressor combination is required. This case is very
common where distances between shore LPG tanks and marine LPG tankers are very
long. This combination helps to reduce the total head of a tanker pumping system by
eliminating the pressure drop of the return vapor line from the LPG tank to the ship.
The pressure drop is eliminated from the ship's pumps by installing a compressor to do
only the vapor transfer from one tank to another. This way the pumps only need enough
head to overcome the pressure drop through the liquid line.
6.1.4
Static Electricity in Unloading and Loading
Static electricity can be a hazard during loading/unloading procedures since, when
breaking connections, a spark could lead to LPG vapor ignition and fire.
Loading/unloading rack structures shall be grounded to earth from at least two points.
In addition, there shall be bonds between piping and the rack supports. The objective
is to keep all metal parts at the same electrical potential. Copper tape, 25 mm by 3 mm
shall be used for this connection and each grounding point shall be taken to a separate
electrode. One ground electrode (ground rod) example is a 16 mm diameter extensible
type copper rod with a minimum length of 2.4 m. The resistance of these ground
circuits shall be tested on an annual basis.
All flanged connections on piping, valves, etc. can be considered to be conductive
connections. In some countries an additional copper tape link across the flange is
required. However, this does not increase conductivity above the flange/bolt/flange
connection, in fact it creates the risk of element corrosion since copper and iron have
different electrolytic potentials. In addition, copper bonding straps can also cause
uneven bolt tensioning. Therefore, such copper tape connections are not recommended.
For road delivery vehicles normally the grounding wire is permanently attached to the
truck and for loading/unloading a spring-loaded “alligator” clamp at the end of the
grounding wire shall be connected to a dedicated bare steel grounding lug at the
loading rack or the customer tank respectively. It is preferred that the loading rack
grounding system is permissive i.e. the loading pump will only start if the grounding
connection is made and properly working.
Grounding and bonding to discharge static electricity and stray currents shall be
provided at the loading rack. Grounding systems composed of a Scully Biclops with
Ground HOG is acceptable. Lightning protection or grounding rods shall be provided at
the loading rack structure to protect personnel, piping and equipment on non-conductive
foundations. All electrical installations and equipment shall conform to the provisions
of NFPA 70. The area classification for the loading rack shall be Class 1 Division 2 0.9
m from point of connection of filling in all direction and up to 0.45 m above ground
within 3.3 m radius from point of connection. The electrical fittings shall be explosion
proof. In addition to the electrical area classification, all equipment located in the open
shall be weatherproof. The electrical and instrumentation of the weighing bridge shall
comply with area classification of the installed area. If installed at loading and
unloading rack, the area classification shall be of Class 1 Division 2.
6.1.5
Hard Arms
A hard arm (also flexible arm, rigid steel pipe with swivel joints) is required to connect a
rail car or tank truck to the plant storage system. Hard arms are preferred to hoses for
this service. When a dry break connector is used, the bleeder vent is unnecessary for
daily operations but shall be installed for maintenance purposes. For sizes 50 mm and
larger, rigid steel pipe with swivel joints and counterbalances are recommended. When
6-6
PRODUCT TRANSFER
Safety in LPG Design
self-sealing couplings or valves are mounted on the end of loading arms, the loading arm
manufacturer shall provide for this added weight in the counterbalance design in
compliance with the requirements of NFPA 77.
Hard arm connections shall withstand a test pressure of 1.5 times the design pressure of
the system. Hard arms shall be designed and fabricated from materials compatible with
LPG, both liquid and vapor form. They shall also have adequate strength and durability
to withstand the pressures, stress, and exposures to which they may be subjected and
shall be designed to maintain sound mechanical and structural integrity. They shall be
supported in such a manner as to prevent personnel injury and prevent damages or
excessive wear.
When hard arms are attached to the piping, the end of the piping shall be secured to a
concrete anchor or equivalent device capable of withstanding any forces that may be
applied by the movement of a vehicle while the arm is attached. The anchor for the
above application shall be capable of withstanding at least twice the maximum load that
could be applied by the arms singly or in combination. A designated weak joint
(breakaway) shall be built into the piping system to ensure that break-off is at a planned
location if a vehicle pulls away while connected. Self-sealing dry break couplings shall
be provided to prevent uncontrolled discharge of LPG to the surrounding. The piping
upstream of the anchor shall have sufficient flexibility to permit adequate thermal
movement. Thermal relief valves shall be installed to protect against liquid expansion
pressure buildup in the hard arms.
Figure 6.1.5: LPG truck loading through hard arm
Provision shall be provided to depressurize the loading arm to a safe location after
loading or unloading is completed. In some installations the contents of the liquid line
can be shifted into the vapor system or blowdown system. However, if this is not
possible, the amount of liquid to be vented to atmosphere must be kept to an absolute
minimum. This can be achieved by either using dry break couplings (see below) or by
installing block valves at the transfer point as close as possible to each other. The liquid
and vapor contained between the block valves shall be vented into a vertical pipe into
the atmosphere. This pipe may be installed at the truck or at the plant. Vent pipes
(5 mm) at trucks are typically installed at the truck nozzle and end at a point above the
Safety in LPG Design
PRODUCT TRANSFER
6-7
tank. Vent pipes at plants may be larger and usually end at higher elevation in an open
area. The vent exit point shall not be in the vicinity of air intakes or fired equipment.
6.1.6
Hoses in Product Transfer
Hoses shall be used where no other method of product transfer is practical, such as on
hose reels on small bulk delivery trucks. Hoses shall comply with applicable
standards (BS 4089 Hose Standards NFPA77), be designed and certified specifically
for LPG (in liquid and vapor phase) and the materials used in fabrication shall also be
certified resistant to the action of LPG and shall be corrosion resistant. Hoses shall be
marked “LPG” at intervals no more than 3 m. Hoses designed to BS 4089 shall have a
maximum working pressure of 25 bar gauge and a minimum burst pressure of 100 bar
gauge. (Hoses certified to comply with IMO code will have a maximum working
pressure of 20 bar gauge and a minimum burst pressure of 100 bar gauge.) Hoses shall
be hydro-tested to 1.5 times maximum working pressure. Hose connections and
stainless steel hose reinforcement shall be electrically continuous. Wire braid used for
reinforcement shall be made from corrosion resistant material such as stainless steel.
Reinforcing wire within LPG loading hoses shall be in electrical contact with the end
couplings on the hoses to minimize the risk of an electrostatic charge collecting on an
electrically isolated wire within the hoses or on the exterior of the hoses. Intermediate
joints or couplings in a nonconductive hose shall not be permitted because they can
accumulate a charge sufficiently great to spark to an adjacent conductive object.
A shutoff valve at the discharge end of the hose shall be provided to minimize vapor
escape when the hose is disconnected after product transfer. When not in use, hoses
shall be placed on reels or in trays designed to prevent any kinking, torsion etc. to
prevent any physical damage. Particular attention shall be given to potentially damaging
ice formation on the corrugations of metallic hose.
When hoses are used for unloading, bleeder vent valves (connected to a vent stack) are
required to depressurize the hoses when they are not in use. Hoses can be equipped with
Acme screw thread (or equivalent) fittings and capped with a relieving device that will
release any pressure in the hose before threads are disengaged. This relieving device is
typically vented to atmosphere away from the person loading the tank.
Plants using hoses shall keep a Hose Tracking Program in place. Following are the
requirements that should be part of a Hose Tracking Program.
1.
There shall be a list showing all hoses used in the plant. The information on
each hose shall contain: unique hose number, purchase date, quality
certificate, testing conditions, testing schedule, last test date, re-test date,
expected hose retirement date.
2.
A unique hose number, the last pressure test date and re-test date, shall
identify each individual hose in the field. This is to facilitate control.
3.
Operators shall receive training that informs them how to prevent and to
detect hose damage.
More information on hoses used for marine services can be found under “Marine
Cargo Dock Hose.” later in this chapter.
The design criteria for hoses and connections used for transferring LPG liquid or vapor
service at pressures in excess of 35 kPa shall be the following.
Truck
Loading/Unloading Hoses shall be designed as follows: Working pressure of 2,032 kPa
and a Bursting pressure of 10,160 kPa. Hose assemblies shall be designed to withstand
a pressure not less than twice the working pressure, 4,830 kPa.
Unloading facilities should be designed such that the liquid in a hose can be drained into
the vapor system. Liquid contents of hoses shall not be vented to atmosphere. If hoses
cannot be drained, they may be kept under liquid, blocked off at both sides with a
6-8
PRODUCT TRANSFER
Safety in LPG Design
Thermal Relief Valve at one end to protect the hose against excess pressure. The set
pressure of the TRV shall be in accordance with the hose manufacturer’s specifications.
Venting of the liquid at the transfer point (between truck and hose block valve) shall be
performed as described in the previous section “Hard Arms.” The hose length shall be
kept as short as possible.
Hoses shall be supported in such a manner as to prevent personnel injury and prevent
damages or excessive wear. When hoses are attached to the piping, the end of the
piping shall be secured to a concrete anchor or equivalent device capable of
withstanding any forces that may be applied by the movement of a vehicle while the
hose is attached. The anchor for the above application shall be capable of withstanding
at least twice the maximum load that could be applied by the arms or hoses singly or in
combination. A designated weak point (break-away) shall be built into the piping
system to ensure that break-off is at a planned location if a vehicle pulls away while
connected. A self-sealing dry break connector shall be provided to prevent uncontrolled
discharge of LPG. The piping upstream of the anchor shall have sufficient flexibility to
permit adequate thermal movement.
Figure 6.1.6: In case of rupture “Smart Hose” is automatically closed at both ends.
A recent development in hose technology is the “Smart Hose.” by Smart Hose
Technologies. This hose has considerable safety features included in its design. SmartHose will automatically close at both ends if the hose ruptures or if the truck drives
away without uncoupling the hose.
Valve plungers, wedges or flappers installed at both ends of the hose, accomplish
closure. During normal operation they are kept open by a coated cable incorporated
Safety in LPG Design
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6-9
within the hose bore. This cable acts as a compression spring providing thrust in the
direction of both ends of the hose, holding the valves open. Should this thrust be
eliminated due to coupling ejection, hose stretching or hose separation, the valves are
released and instantly seat, stopping flow in both directions.
6.2
Loading and Unloading
6.2.1
Truck Loading and Unloading
This section of minimum standards covers the truck loading and unloading facilities
within ExxonMobil’s fence at a terminal.
The quantity of shipments may be determined by the weight of the shipping vehicle or
by liquid level readings on both shipping and terminal LPG tanks before and after
product transfer. Meters are not normally installed in loading or unloading systems.
Truck
Internal
EBVs
Truck
ESS
Truck
ESS
Break Away
Couplings
EBV's Operated by
Plant Emergency
Shutdown System (ESS)
Electrostatic
Bonding Cable
EBV
LOCAL
EMERGENCY
PUSH BUTTON
EBV
Figure 6.2.1-a: Mini-bulk truck loading
The loading rack shall be designed such that when the truck is parked for unloading or
loading, it shall be able to move away from loading rack to a safe location in an
emergency without backing up. The loading rack should not be located in an area
directly along the longitudinal axis of the horizontal LPG bullets. The loading rack shall
have a concreted paved mat. The concrete mat shall be designed so that the entire truck
will be within the mat during loading and unloading. The concrete mat shall be pitched
so that spills will run to LPG trap. The loading/unloading equipment and support
structures in the loading rack shall be protected with guard rails or stanchions to prevent
damage from vehicles.
Emergency shutdown pushbuttons to activate the Emergency Shutdown System shall
be installed at an easily accessible area at the loading rack and at a safe remote location
at least 15 m from any hazard. This maybe the pushbutton at the filling plant or other.
The emergency shutdown pushbutton shall activate an audible alarm of at least 100 Db
installed at the loading rack and to shut the flow of LPG.
The structure shall be designed to provide structural support for loading equipment and
lighting. The structure shall also provide weather protection for the driver, product and
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Safety in LPG Design
equipment. The structure shall not have any sides or any restrictions that will inhibit
ventilation through the structure. All structural materials installed shall be of noncombustible material. The structure shall have lighting suitable for daytime and
nighttime operation. Lighting shall be an minimum of 100 lux measured at the loading
connection. Lighting enclosure shall be flameproof type.
Figure 6.2.1-b: Principle of the breakaway coupling (Alpha Process Control)
Figure 6.2.1-c: Drybreak coupling (Alpha Process Control)
The unloading systems can use compressors or pumps or a combination of both.
Compressors and pumps shall comply with the requirements in Chapter 4. The fixed
piping shall be constructed of steel. No other material is acceptable. Vapor return lines
shall have check-valves installed to prevent backflow.
When flexible arms or hoses are attached to the piping, the end of the piping shall be
secured to a concrete anchor or equivalent device capable of withstanding any forces
that may be applied by movement of a vehicle while the arm or hose is attached. The
anchor shall be capable of withstanding at least twice the maximum load that could be
applied by the arms or hoses singly or in combination. A designated weak point shall be
built into the piping system to ensure that break-off is at a planned location if a vehicle
Safety in LPG Design
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6-11
pulls away while connected. The piping upstream of the anchor shall have sufficient
flexibility to permit adequate thermal movement.
New or revamped truck loading and unloading points shall be equipped with swivel
hard arms and break-away connections for liquid and vapor return lines. It is highly
important to install these couplings exactly as prescribed by the manufacturer. If, after
installation, the pull-away force cannot be applied as intended, the coupling may not
work. In special cases where a variety of customer trucks are loaded, the additional use
of hoses may be necessary to accommodate the diversity of connection points. Hard
arm material shall be Schedule 80 Seamless Steel - ASTM Specification A-106, Grade
B.
Figure 6.2.1.1-a: Rotary gauge dial
The liquid lines shall be equipped with dry break couplings. Vapor and liquid lines
shall be equipped with automatic, fail safe (thermal/fire actuation) emergency block
valves (EBV), and a local as well as a remote (accessible during emergency) actuation
system to activate the emergency shutdown. The EBV shall be installed in the
transfer pipe within 6 meters of the hose connection or loading arm, per NFPA 58.
Furthermore, there shall be an interlock system, which can prevent the truck from being
moved while hard arms (or hoses in older plants) are still connected.
The piping system shall be designed to accommodate maximum forces originating from
a truck drive-away rupturing the break-away connections. If this cannot be achieved, a
bulkhead with an adequate foundation shall be installed which shall be designed to
withstand 65 kN. If the EBV is located at the rack it shall fail safe under fire
conditions. Actuation of the valves may be automatic by fusible element, which melts
at 120 °C and is 1.5 m at maximum away from the loading connection. In addition to
the EBV a manual quarter turn valve shall be provided at the connection between the
piping and the hard arm. The loading rack area shall be protected against accidental
crash by trucks. An electrostatic grounding (earthing) point shall be provided at
each loading and unloading location. A permissive grounding (earthing) system is
preferred for road vehicles at each loading rack. This shall ensure that the loading pump
shall only work as long as the grounding contact is intact.
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Safety in LPG Design
Locations that use hoses instead of hard arms for product transfer will also need the
items discussed above. If no dry break couplings are provided, a purging system and
good natural ventilation shall be present.
Normally unloading of trucks is achieved by using their own pumps. A pump in the
plant could also unload trucks if the requirements for Net Positive Suction Head (see
NPSH in Chapter 4) for the particular pump can be satisfied. This may be the case if the
truck can be placed close to and elevated above the pump in question.
Figure 6.2.1.1-b: Rotary gauge tube
Hose connections for bulk transfer shall be designed such that they can be emptied of
liquid after loading. They may be left under vapor phase. Hoses on reels of mini-bulk
trucks are an exception because they are constantly connected on the truck side and
therefore provided with a thermal relief valve. Hard arms maybe left under liquid,
however they need thermal expansion protection.
6.2.1.1
Truck Tank Level Measurement
All truck tanks shall be equipped with rotary gauges. A rotary gauge is a variable liquid
level gauge consisting of a small positive shutoff valve located at the outer end of a tube,
the bent inner end of which communicates with the tank interior. The tube is installed in
a fitting designed so that the tube can be rotated with a pointer on the outside to indicate
the relative position of the bent inlet end. The length of the tube and the configuration
to which it is bent is suitable for the range of liquid levels to be gauged. By a suitable
outside scale, the level in the tank at which the inner end begins to receive liquid can be
determined by the pointer position on the scale at which a liquid-vapor mixture is
observed to be discharged from the valve. The more modern version of this device, the
“Magnetel” may also be used.
6.2.2
Rail Car Loading and Unloading
Rail tank car loading racks shall be designed to meet all local regulations and shall be in
accordance with railroad and industry standards. Layout shall provide optimum ease in
positioning tank cars. In some countries, regulatory practices allow loading/unloading a
train where all cars are coupled together. Other countries require uncoupling and
keeping a minimum distance between the cars. A vertical clearance from the track of
not less than 6.7 meters and a horizontal clearance from center of track to loading rack
Safety in LPG Design
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6-13
edge of not less than 2.6 meters shall be maintained. The loading spots shall be level
and on a straight section of track.
New or renovated loading racks shall be equipped with swivel hard arms for liquid and
vapor connections. Hard arm material shall be Schedule 80 Seamless Steel - ASTM
Specification A-106, Grade B. Liquid connections shall have an emergency release as
well as dry break couplings. An alternative would be a purging system for the
coupling. There shall be one fail safe, remote operated emergency block valve (EBV)
in the liquid and vapor lines to the plant tanks. The EBV shall be installed in the
transfer pipe within 6 meters of the hose connection or loading arm, per NFPA 58. The
individual lines to each rail car shall be provided with manual shut-off valves.
Rail Car ESS
Rail Car
EBVs
Melting
in Fire
Adequate Grounding
Through Rails
Manual ESS
on Both Sides
Releasing
when Moving
LOCAL
EMERGENCY
PUSH BUTTON
TERMINAL ESS
More Rail Cars
Liquid
EBV
Vapor
EBV
Figure 6.2.2: Emergency block valves in rail car unloading
National codes normally require that each rail car shall be equipped with an internal
shut-off valve. Ideal loading rack designs incorporate the ability to close all rail car
shutoff valves from the loading rack or the plant emergency shutdown system (ESS).
However, this requires a special rail car design. As a minimum the individual rail carshut-off valve shall be operable at the hard arm and also operate if accident or error
moves the rail car. Push-buttons to operate the ESS shall be located at strategic points,
which provide ready access to operators.
Where hoses are used, valves and connections are necessary between the hose headers
and a vent system to allow draining the loading hose after completing loading but before
disconnecting the hose from the car. Many rail tank cars are equipped with nonconductive bearings and non-conductive wear pads located between the rail car and the
chassis. As a result, the resistance from the tank car compartment to ground through the
rails may not be low enough to prevent the accumulation of an electrostatic charge on
the tank car body. Therefore, bonding of the tank car body to the fill system piping is
necessary for protection against static accumulation. In addition, because of the
possibility of stray currents and to prevent an ignition hazard as a result of such currents,
loading lines shall be bonded to the rails.
The two rails of the siding shall be permanently bonded to the metal loading rack. The
unloading rail spur shall be insulated from the main track to guard against stray currents,
which might cause a spark. Often the insulation point is at the spur entering the site as
shown in Figure “Spacing in marketing LPG bulk plant with cylinder filling” in Chapter
2.
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Safety in LPG Design
Normally a reciprocating compressor shall be provided for unloading rail tank cars.
The compressor shall be sized to provide the desired volumetric liquid unloading rate by
pressurization of the rail car vapor space(s). Manifolding shall be provided to allow
subsequent exhausting of LPG vapor from the car(s), discharging to the bottom of the
LPG tank into the liquid. It is desirable to unload rail tank cars at such a rate that the
operation can be completed by one individual during a normal working period. The
unloading rate will be affected by the size of the tank car and the compressor capacity
available. If unloading has to be finished in shorter time unloading pumps may be
used. If a manifold system is used to unload cars simultaneously, check valves shall be
installed to prevent the return of product to the rail cars.
Provisions shall be made so the loading assembly can be swung to one side and held out
of the way of moving tank cars when not in use. Where a rack is required it shall be of
steel frame construction with nonskid metal flooring. Stairs and the main walkway shall
have handrails.
6.2.3
Marine Loading and Discharge
The design of marine loading and discharge facilities is specific to both the individual
pier and the marine transportation unit(s) selected to serve it. A number of design
issues, both general for marine berths and specific to LPG berths should be considered,
such as site selection, berth layout/spacing, and berth pier and pipeline design to reduce
the risk of vessel collision with loading platforms, transfer lines and berthing structures.
Information on such issues is available in the Marketing Engineering Standard
EE.3M.86 “Marine Facilities, Design, Specification and Evaluation” and DP XV-J
“Safety in Plant Design Docks, Loading Racks and LPG Storage Facilities.” It is
recommended that the designer seek assistance from ExxonMobil Research and
Engineering (EMRE) for guidance in LPG marine facilities matters.
An existing jetty shall be inspected and approved by EMRE for operation prior to any
joint venture commitment. For a new jetty, the following surveys shall be carried out to
establish the design basis:
6.2.4
1.
Bathymetry – to determine water depth for navigation, channels, anchorage
and maneuvering areas.
2.
Tidal Range – to determine tide conditions.
3.
Current – to determine current velocities and directions for surface, mid-depth
and bottom current.
4.
Wind – to determine wind velocities and direction for wind loading
consideration.
5.
Wave/Swell – for terminals at exposed locations to determine wave height,
direction and period.
6.
Geotechnical – to determine soil conditions, extent, thickness, strength and
deformation of soil layers.
7.
Earthquake – to determine seismic conditions.
8.
Environmental – to establish baseline data and provide environmental impact
assessment.
9.
Fleet Data – to establish vessel physical dimension, mooring plans and
manifold details. Data can be obtained from ExxonMobil Supply.
Marine Pier Installations
The jetty shall be constructed of non-combustible materials. A floating pontoon jetty is
not acceptable for LPG operations. The jetty design shall be certified by EMRE
Engineering or approved designated consultant. The jetty design shall include but not
limited to:
Safety in LPG Design
PRODUCT TRANSFER
6-15
1.
the ability to absorb lateral load that is expected when berthing tankers
mooring.
2.
corrosion protection.
3.
spill containment (if jetty is used for loading/unloading multiple products).
4.
vehicle access to facilitate maintenance.
The jetty may be used for loading/unloading multiple products other than LPG provided
it is designed and constructed for LPG operations in addition to meeting relevant
ExxonMobil’s standards and specifications for loading/unloading other products.
Breasting facilities are required to absorb the energy of the berthing vessel, to protect
other facilities and to provide points of contact for the moored vessel. Common
arrangement for tanker pier is shown in Fig 6.2.4-a which shows two free standing
breasting dolphins. Number of breasting dolphins may be increased to accommodate
vessels of larger sizes. Breasting dolphins should be place symmetrically about the
center of the loading platform piping manifold. All berths shall be deep enough and
long enough so that the tanker operated at the deepest draft shall have adequate UKC
(underkeel clearance) during any stage of tide. Berthing dolphins should be well lit and
marked with reflective material to provide clear target for the pilot for night berthing.
The berthing operations should be reviewed to establish the number of tugs and other
berthing aids (bow thrusters, berthing monitoring systems). Issues related to rapid and
fail safe communications, including providing the ship with a control box to shut down
shore pumps and close EBVs in the event of an emergency, should be considered.
Quick release mooring hooks should be considered, so that the vessel can be removed
from the berth area quickly in the event of an emergency.
Mooring facilities should be arranged as symmetrically as possible about the centerline
of the piping manifold. Breast line mooring points shall be located so as to:
1.
2.
Provide mooring line leads as near as possible to 90 degrees to the
longitudinal centerline of the vessel.
As far aft and as far forward as possible.
Spring line mooring points should be located to provide mooring line leads as nearly
parallel as possible to the vessel’s longitudinal axis. Mooring points should be located
to keep all lines at vertical angle less than 25 degrees and not more than 50 m from the
vessel. Mooring structure should be arranged so that all lines in the same service are
approximately the same lengths.
An anemometer shall be provided to monitor wind conditions, so that cargo transfer
can be shut down at pre-determined wind limits. Flammable gas detectors should be
installed in the berth loading/discharge manifold area to detect product leaks and sound
the alarm in the berth area as well as the control center.
Corrosion protection of steel structures above elevation of at least 3 m below riverbed
or seabed shall be provided. Protection may be provided by coatings, increased
thickness of steel member (corrosion allowance) and cathodic protection.
Dock Operations Building
The structure for operator on duty, if provided, shall be located such that the operator
can observe the product transfer system clearly. However, the structure shall be 15 m
from the most hazardous area where ignition is mostly likely to occur. It shall also be
located near a safe emergency evacuation route.
Layout shall provide for structural support, operating area for transfer of product,
gangway access to tanker, spring mooring lines and fire protection. Lighting in all work
6-16
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Safety in LPG Design
areas shall have an average illumination of 100 lux and shall be suitable for both day
and night operations.
Pumps are generally used for loading and unloading vessels. Loading pumps are on
shore and unloading pumps are normally on the ship. Piping shall not be routed below
the jetty.
Loading, discharge and vapor return lines from the Marine Pier to the shore shall have
two emergency block valves (EBV), one at the pier manifold and one at the shore side.
These valves shall be remotely operated. The valves shall either be operable for 15
minutes in the event of a fire by fireproofing power supplies or incorporate fusible
elements, which will allow closure when melted. Valves, which close automatically on
loss of power (fusible elements), shall be designed to limit their closure rate as a result
of power failure in order to prevent hydraulic surge. Emergency shutdown buttons shall
be positioned at the manifold and at the exits to the pier. Consideration shall be given to
Emergency Shutdown Buttons at the Emergency Egress locations as well. Risk
assessment may be used to determine whether the pier shutdown buttons should activate
the pier EBVs only or activate the entire plant emergency shutdown system.
Fig 6.2.4-a: Common arrangement for tanker pier Loading Platform
Electrically insulating flanges are required for stray current protection on marine
loading and unloading lines for both vapor and liquid. Without insulating flanges,
currents could be developed by ship or dock cathodic protection systems or by galvanic
potential differences between ship and shore. Installation shall be such that the pier
piping is insulated from both the on-board piping and the shore piping (the latter to
maintain the separation of the pier cathodic protection system). All plant flow lines
(except for the dock lines as mentioned above), shall be electrically continuous.
Dock lines shall have the insulating flanges as close to the presentation as possible (i.e.
for loading arms the insulation flange shall be installed at the outboard arm, behind the
triple swivel and any support which comes in contact with the ship; for hose strings, one
hose shall be electrically discontinuous). This will ensure electrical isolation between
the vessel and the pier.
An insulating flange, when new, shall have very high electrical resistance, typically over
10 million Ohms. Insulating flanges in service shall have at least 1000 Ohms resistance;
a lower value means that deterioration has occurred and maintenance is needed.
Electrical bonding connections (bonding wire) shall not be used between the vessel
and the pier piping. Because of cathodic protection systems for the dock or ship or
galvanic potential differences between ship and shore, a current may exist through the
Safety in LPG Design
PRODUCT TRANSFER
6-17
bonding wire, which can spark on connecting or disconnecting. In circumstances where
the government authority requires bonding wires, every effort shall be made to educate
the authorities about the dangers of such practice. If a bonding wire is used, the wire
shall include an explosion-proof switch. Before the wire is attached to the vessel, the
switch shall be in the open position. After the wire is attached to the vessel, the bonding
circuit can be closed using the explosion proof switch. The procedure shall be reversed
to disconnect the bonding wire.
All electrical equipment shall be suitable for the electrical area hazard classification
where it is installed.
Normally, LPG Cargo Transfer Equipment is empty when not in use. Appropriate
piping for depressurizing and venting shall be included at the pier manifold. Also,
depending on the operations and vessels involved, vapor return lines may be required.
Critical dock services which shall be protected against fire exposure include the
following: fire mains, dry pipe foam headers, systems associated with the control and
actuation of emergency block valves, quick-release hooks or other emergency facilities,
and any other instrumentation or communications systems which are essential during an
emergency.
Insulating Flanges
if Pier in Salty or
Brackish Water
Emergency
Release
System
EMERGENCY
PUSH BUTTON
ON SHORE
LOCAL
EMERGENCY
PUSH BUTTON
TERMINAL ESS
VAPOR
EBV
EBV
LIQUID
EBV
EBV
Figure 6.2.4 b. : Emergency block valves in marine loading and unloading
As far as possible, the layout of the above systems shall provide at least 7.5 m spacing
from the berth manifolds to avoid fire exposure from three dimensional fires at these
locations. Sections unavoidably extending within the 7.5 m distance shall be
fireproofed. The critical systems shall also be protected against spill fires burning on the
water surface below the dock structure, by locating these systems above the deck or by
fireproofing. An alternative method for the fire main is to install a remote operated
dump valve at each extremity to establish flow in the event of fire (see GP 3-2-3).
6.2.4.1
Marine Cargo Dock Hose
Marine Cargo Dock Hose shall not be used for refrigerated or partially refrigerated LPG
Cargo Transfer. For pressurized LPG, the use of all steel Marine Loading Arms is
recommended, however, under certain circumstances, (i.e. when the throughput rate and
the volumes are small and cargo transfer is infrequent) use of Marine Cargo Dock Hose
may be acceptable for existing facilities. A formal LPG Cargo Transfer Risk
Assessment can be of help in determining whether use of Marine Cargo Transfer Dock
Hose may be used. EMRE's memorandum on “LHG Marine Cargo Transfer
Fire/Explosion Risk Assessment Procedure” (93 CMS2 010) can serve as a starting
point for such an assessment. Marine Loading/Unloading Hoses shall be designed as
follows: Working pressure of 2,415 kPa and a Bursting pressure of 12,075 kPa
GP 3-11-1 shall be used for the purchase specification of Marine Cargo Dock Hose.
The ERE report EE.76E.92 provides details on the purchase specification, inspection
and retirement criteria. The report EE.40E.94 “Marine Dock Hose Technology and
Practice Training Video with its companion Application Guide” is also a source of
useful information regarding hose purchase specification, inspection and testing, hose
handling and retirement criteria.
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Safety in LPG Design
Marine loading (barges) arms/hoses shall be capable of accommodating the combined
effects of change in draft and tidal changes. Marine barges loading arms/hoses shall be
designed and tested periodically in accordance with OCIMF, Design and Construction
Specification for Marine Loading Arms and United States Coast Guard (USCG)
requirements (33 CFR 156) or equivalent. Test pressure shall be 125 percent of
maximum operating system pressure. Hydrostatic test shall be carried out at least once a
year.
Figure 6.2.3.2: Emergency release coupling in marine loading and unloading
6.2.4.2
Marine Loading Arms
New or renovated facilities shall conduct cargo transfer with all steel Marine Loading
Arms. Hard arm material shall be Schedule 80 Seamless Steel - ASTM Specification A106, Grade B. Terminals in pressurized LPG service which use Marine Cargo Dock
Hoses shall consider upgrading to Marine Loading Arms. A formal LPG Cargo
Transfer Risk Assessment can be of help in determining the need for the upgrade.
EMRE's memorandum on “LHG Marine Cargo Transfer Fire/Explosion Risk
Assessment Procedure” (93 CMS2 010) can serve as a starting point for such an
assessment. GP 3-11-2 covers the requirements for the design of Marine Loading Arms
and associated equipment (such as Quick Connect/Disconnect couplers, Accessories,
Range Monitor Systems and Emergency Release Systems (ERS)). Regarding ERS,
reference shall be made to EEEL ERS Guidance Note for GP 3-11-2.
Emergency Release Systems (ERS) are recommended for new and existing LPG arms.
An ERS consists of dual isolation valves at the ship/loading arm connection. The ERS
allows for rapid, automated disconnect in the event of an emergency with little loss of
product. As general guidance, circumstances where LPG transfer without an ERS may
be used are as follows:
1.
Safety in LPG Design
Facility is a loading terminal, and
PRODUCT TRANSFER
6-19
2.
Arm is equipped with a range monitoring system which shuts down loading
pumps and closes remote operated block valve at base of loading arm in event
vessel begins to approach limits of loading arm operating envelope, and
3.
Total arm contents that might be spilled in the event the arm is damaged by
ship motion is less than 500 liters of LPG, and
4.
Surge analysis has been conducted to ensure there is minimal risk of pipe
rupture in event of emergency shutdown described in item 2 above, and
5.
Probability of excessive ship motions is minimal based on historical records
and average wind and current conditions at the site.
Relatively infrequent marine transfers.
6.
The decision whether to equip LPG Loading Arms with ERS can be made after an LPG
Cargo Transfer Risk Assessment. The considerations described above are assessed in
detail in EMRE's memorandum on “LHG Marine Cargo Transfer Fire/Explosion Risk
Assessment Procedure” (93 CMS2 010).
NOTE: Vapor cannot be evacuated from the last tank to be discharged because there is
no vapor line to shore.
Figure 6.2.4.3: LPG ship unloading with compressor without vapor return line to shore
6.2.4.3
Discharging System without Vapor Return
If a ship is equipped with two or more tanks and compressors, it can discharge through a
single line without a vapor return system. This is accomplished by selectively drawing
vapor from one tank in the ship and forcing it into another, causing the discharge. As
liquid is exhausted from the tank, the empty tank may then be used as a vapor source
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Safety in LPG Design
while a loaded tank is being discharged. The piping arrangement and successive steps in
the operation are shown in the schematic flow diagram in Figure 6.2.4.3. Since there is
no vapor line to the shore tanks in this system, the vapor cannot be evacuated from the
last tank to be discharged.
Loading Arm shall conform to OCIMF publication “Design and Construction
Specification for Marine Loading Arm”. Pipelines on pier shall be adequately bonded
and grounded. If excessive stray currents are encountered, insulating flanges or joints
shall be used.
6.2.5
Pipeline Dispatch and Receipt
The designer should consult with the pipeline company and the delivery company
(refinery) to establish detailed mutual agreement on design specifications, ownership,
metering and operating responsibility for the pressure reducing station located in the
plant. Pressure rating of receipt manifolds, type of pressure reducing valve,
overpressure protection and emergency shut-down procedures shall be established.
Point of transfer of product and maintenance/inspection responsibility shall be
clarified.
A remote operated, fail safe emergency block valve (EBV) shall be provided at the
receiving/dispatching station on incoming and outgoing pipelines. This valve shall be
incorporated into the plant emergency shutdown system (ESS). In addition, it may be
equipped with automatic shutdown features (fusible element). Quarter turn ball valves
with hydraulic or pneumatic actuators are the preferred choice. There shall be an
insulating flange to separate the plant piping from the pipeline cathodic protection
system.
Safety in LPG Design
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6-21
7
LPG CYLINDERS
7.1
Cylinder Purchasing Specifications
This section describes the Purchasing Specifications of portable LPG (small) containers
usually referred to as “Cylinders” or “Bottles.”
Cylinders are Loan Delivery Equipment (LDE). This means that they are purchased and
owned by the LPG facility, but stay mostly with the customer for LPG consumption.
They are designed to be refilled at a filling plant. Safety of LPG cylinders is important,
the general public as customers is in direct contact with the cylinders. It is therefore
important that LPG cylinders, together with all associated fittings and controls, e.g.;
valves, gas flow pressure regulators and hoses, be manufactured to the latest
internationally recognized and approved design standards and correctly specified for
their intended duty.
This Purchasing Specification has been produced to assist in determining that the LPG
equipment purchased for market place use, meets acceptable standards of safety,
integrity and reliability. Primarily, this requires special attention to internal cleanliness
of cylinders and quality of elastomers/rubbers, used in valves/regulators and
associated hoses, for safeguarding against LPG leaks and hazardous pressure/flow
conditions.
7.1.1.1
Quality Control Systems
Whatever the type of equipment, all manufacturers should be formally accredited
within an internationally recognized standard for product quality control. As a
minimum, this would mean EN/ISO 9001/2 or BS 5750. If candidate manufacturers are
not currently accredited with these standards they should show evidence of progressing
towards these standards.
Manufacturers should agree to Quality Assurance Auditing of their manufacturing
unit(s) and Quality Control (QC) systems, to be carried out at the discretion of the
purchaser by their designated employee or agent.
Whenever possible, equipment should be certified for LPG service as described in
“Quality Assurance” in Chapter 1.
7.1.1.2
Manufacturing Records
To ensure reliable and efficient quality control, monitoring of equipment integrity and
fault tracing after delivery, manufacturers shall be required to maintain full records of
raw material quality laboratory analyses, or official certification of fit for purpose, for
each batch of material or component used in their cylinders.
Safety in LPG Design
LPG CYLINDERS
7-1
Manufacturers shall also operate a recognized quality control (QC) system, when one is
not specifically required within a design or manufacturing standard. QC records shall be
maintained for each batch purchase.
7.1.1.3
Post Sales Equipment Batch Rejection
A significant change in purchasing policy is the introduction of a means of improving
market place safety by introducing Post Sales Equipment Batch Rejection Criteria.
This requires all equipment found to be faulty either before or after placing into the
market place, to be immediately withdrawn and examined for cause of the fault. The
main concerns being LPG leaks, hazardous gas supply pressures and or any other
malfunction. As part of a purchasing agreement, manufacturers shall agree to
cooperating fully with the investigation and if found to be replicated or unresolved,
consideration being given to replacing the whole batch.
Simply replacing faulty equipment within, or reasonably beyond, a Guarantee period is
inappropriate without investigating and resolving the cause of the fault. This policy
applies to valves, regulators, and hoses as well.
Figure 7.1.2: Typical domestic LPG cylinder
7.1.2
Cylinder Specifications
As a minimum, all cylinders in use or being processed and handled shall comply with
internationally recognized design standards. Where national standards are in force,
these may be used, providing they are at least equivalent to, or derived from, those
standards referred to in sub-section “Manufacturing Standards and Design.” A most
important requirement contained within these guidelines, which is not dealt with by any
of the listed standards, is that of internal cleanliness of the cylinders, not only by
removal of pressure test water, but the prevention of iron oxide and mill scale and/or its
removal. See sub-section “Cylinder Heat Treatment” later in this chapter.
7-2
LPG CYLINDERS
Safety in LPG Design
7.1.2.1
Manufacturing Standards and Design
Cylinders shall be designed, fabricated, tested and marked (or stamped) in accordance
with Department of Transportation (US DOT 4B 4BW-240), equivalent industry
standards or local regulations. Although some of the standards listed below permit the
use of aluminum as a cylinder construction material and brazing for fabrication, neither
of these materials or joining processes are recommended nor should they be used for
refillable LPG cylinders. Only steel and electric arc welding specified within these
standards are recommended.
Examples of appropriate standards are:
1.
US Standards DOT 4B/4BW - 240.
2.
International Standard ISO 4706.
3.
European Standard EN 84/527.
4.
British Standard BS 5045 Part 2 1989.
5.
French Standard NF M88-703.
6.
Malaysian Standard MS 641/642.
7.
Australian Standard SAA AS B239.
8.
Japanese Standard JIS B 8233.
Figure 7.1.2.1: Two piece and three piece cylinders
For reference, the following material specification is quoted, which is used for
DOT 4BA cylinders.
Carbon
Silicon
Manganese
Phosphorus
Sulfur
Phosphorus and sulfur
0.22% maximum
0.45% maximum
1.60% maximum
0.04% maximum
0.04% maximum
0.07% maximum
The material shall be proven to be able to withstand less than 10% of permanent stretch
when undergoing hydrostatic stretch test. All parts of welded cylinder bodies and all
parts welded to the body shall be made of compatible materials. The cylinder can be
fabricated in 2 piece or 3 piece welded design. Figure 7.1.2.1 shows the typical drawing
of a two and three pieces welded cylinder. Opening on the cylinder may be provided
with a boss welded onto the cylinder. Tapered internal threads of ¾” NGT shall be
provided on the boss for attachment of the cylinder accessories. In case DIN 477
Safety in LPG Design
LPG CYLINDERS
7-3
thread is used there shall be clear marking on the cylinder (preferably on the bung)
to distinguish from NGT threaded cylinders. If delivered without valve a lug or cap
shall be provided for each cylinder to cover the opening to prevent foreign material from
entering the cylinder.
7.1.2.2
Manufacturing Measurement Tolerances
The size of portable cylinders shall be up to a maximum of 50 kg. The service pressure
of the cylinder shall be at least 1,660 kPa based on 100% commercial Propane at
temperature of 54 °C. Maximum filling limits for cylinders shall be based on local
standards. In the absence of local standards, or when local standards allow a higher
limit, the maximum safe quantity that can be filled into the cylinder shall be such that
the cylinder will not be more than 95% liquid full at a temperature of 54 °C.
All cylinder dimension tolerances shall be compatible with those of cylinder valves,
filling plant equipment and handling devices. This information shall be obtained from
equipment suppliers and inserted into the purchasing specifications, to suit their
individual circumstances.
Otherwise, serious damage and possible hazardous
situations may develop within the plant.
The following sub-section highlights critical dimensions, which shall be established,
prior to placing orders for cylinders.
Following superficial dimensions shall be fixed:
1.
Overall diameter.
2.
Overall height.
3.
Valve bung thread.
4.
Height of valve filling orifice above cylinder base.
5.
Maximum angle of valve from perpendicular (0.5 degrees from top of valve
bung is recommended maximum).
For most refillable cylinders it is not cost effective to specify both the tare weight and
the internal volume, due to slight variations in diameter, height and metal thickness.
Internal volume shall be specified as it is key to preventing overfilling. The internal
volume specification shall have a tolerance of +1% or less of the rated nominal cylinder
water capacity.
7.1.2.3
Cylinder Attachments
The design of the footring shall be stable and allow the cylinder to stand firmly on a
substantially level surface without any support. The bottom of the cylinder shall have a
minimum clearance of 20 mm from the ground. Design of foot-rings shall comply with
the following:
7-4
1.
Foot rings shall be rolled from steel having equivalent strength as the main
cylinder body and be designed to protect the cylinder against impact damage
if it is dropped. The top of the foot-ring shall be of castellated design to
permit at least six welding lugs and to provide unrestricted air circulation for
the cylinder base, through rectangular apertures for a nominal 60% of the of
its overall rolled length.
2.
The base of the foot ring shall be rolled to at least an internal “J” or “d”
section to provide additional reinforcement and a rounded surface to
minimize damage to cylinder handling and standing surfaces. A flat, sharp
edged finish shall not be accepted. Six equally spaced 5 mm diameter water
drain holes, shall be drilled through the bottom of the “J” section. These
holes shall be angled to allow trapped water to drain away from the foot-ring
and not become self-blocked when the cylinder is standing on a firm flat
surface.
LPG CYLINDERS
Safety in LPG Design
Neck ring valve protection and lifting handles shall be designed as follows:
7.1.2.4
1.
Preferred valve protection is by a neck-ring welded to the cylinder body,
which shall be manufactured from forged or rolled steel, having equivalent
strength as the cylinder body. A steel cap screwed to the bung is allowable.
The disadvantage of the steel cap is that the customer or retailer needs to have
the discipline to always screw it back before returning the cylinder. Valve
protection caps from plastic shall not be permitted.
2.
The neck-ring shall be of sufficient height to allow another cylinder to be
stacked on top without contacting the valve of the cylinder underneath. It
shall also have an external diameter which permits secure, tilt free stacking
but with sufficient clearance for easy manual separation of the stacked
cylinders.
3.
The neck ring shall be open at one side to provide full access to the valve for
fixing top or side connecting LPG gas flow regulators, which ever is
specified. It shall allow for valve apertures, such as gas outlet connection and
pressure relief valve to be positioned to an unrestricted opening in the neck
ring without over stressing the valve tightening torque during its insertion.
4.
For nominal 6 kg to 15 kg capacity cylinders, which may be manually lifted,
the neck ring shall be designed with two handles to permit lifting with two
hands for safe stacking or transporting without strain.
Cylinder Markings
Design Specification:DOT-4B-240
Manufacturer: Keloil
Design Pressure: 1.66 MPa
Year manuf.5-1999 Retst 5-2008
Serial No. E345236
Property of: Esso
Tare weight: 12.5 kg
LPG weight: 12 kg
Water capacity: 26.2 l.
Cylinder shall be marked in accordance with local regulations. Where any of the above
design standards do not specify, the following shall be clearly stamped on the side of the
neck ring, or on the cylinder body top, providing the cylinder body thickness is no
less than 3.48 mm thick.
Figure 7.1.1.4: Cylinder makings
Safety in LPG Design
1.
Flammable.
2.
LP-Gas, LP-GAS, Propane, or Butane.
3.
Design Specification number.
4.
Name or identification logo of manufacturer.
5.
A serial number of the cylinder.
6.
Design pressure (If not included within the design specification).
7.
Date of manufacture (MM/YY).
8.
Tare Weight in kg to nearest 100 g including valve.
LPG CYLINDERS
7-5
9.
Weight capacity of Propane and Butane.
10. Water capacity in liters.
11. Owner’s logo.
12. Re-test date (for future use).
7.1.2.5
Cylinder Heat Treatment
All of the above manufacturing standards require cylinders to be heat treated after
welding. This may be by be stress relieving (annealing) between 625 oC to 650 oC, or
by normalization at about 900 oC. The former is preferred, to reduce the tendency for
internal oxidation of the cylinder surface by heat blistering of mill scale. However, if
national standards require the latter, some means of preventing this phenomenon shall be
employed during heat treatment. For instance, inerting with Nitrogen or providing an
Oxygen reducing atmosphere within the cylinder and/or the heat treatment furnace are
considered appropriate methods.
Heat treatment shall be homogeneous, on either a continuous or batch basis, within a
closely controlled furnace, preferably with Oxygen level control, to minimize external as
well as internal oxidation. Additional safeguard against direct flame impingement on
the cylinders shall also be provided to prevent over heating “hot spots,” on any part of
the cylinder body.
7.1.2.6
Cylinder Finishing
Following the hydraulic testing, all cylinders shall undergo several important finishing
processes before they are ready for receipt for filling processes.
Next , the cylinders shall be shot blasted. Shot blasting standards shall comply with US
Standards SSPC SP6-63; NACE 3, or their equivalents within ISO 8501/1 depending on
the type of paint finish required. It may be carried out in an air blast machine or wheel
propelled using steel or iron grit only as prescribed by ISO 8503/1.
Immediately after shot blasting, the paint finish for the cylinders shall be carried out.
All paint regimes shall include a stove baking process. In addition the following shall
be considered:
1.
When local environmental conditions are highly corrosive, pre-treatment of
cylinders by zinc spraying or zinc phosphating is recommended. The former
is generally regarded the more durable and resistant to impact scratching etc.,
although it may be more expensive to apply.
2.
For environmental reasons water based epoxy paints are preferred. If this is
not feasible, normal epoxy paint finish in accordance with local requirements
and experience.
Internal cleanliness is a highly important quality control requirement for safety of
cylinder LPG in the market place. LPG may suffer odor depletion, “odor fade”, in new
cylinders due to reaction of certain stenching agents with cylinder contaminants.
Therefore, manufacturers shall ensure and guarantee internal cleanliness of cylinders,
especially with regard to free water and particulate matter such as mill scale, “weld
splash” and any other form of iron oxide. Air-blowing and then vacuum cleaning shall
carry out the cleaning process.
Next the tare weight of the cylinders shall be determined:
7-6
1.
Tare weighting of the cylinders, shall be undertaken with a full electronic
load base scale, capable of an accuracy of 0.05% of its full weighing capacity.
2.
All cylinders with an LPG capacity of between 6 and 50 kg, may be tare
weighted to the nearest 100 g.
LPG CYLINDERS
Safety in LPG Design
3.
Tare weights shall be permanently punched onto the cylinder neck-ring or
body.
Valve fitting is the next step in the cylinder manufacturing process:
1.
All valves shall comply with and be approved for their market use
requirements.
2.
An approved, non-hardening valve thread sealant paste or tape shall be
applied. For hygiene reasons, PTFE based sealant pastes are preferred to
lead base. PTFE tape may be used providing it is manufactured to a
recognized national standard, e.g.: BS 4375.
3.
The valves shall be tightened in accordance with the valve manufacturers
specification, or to that value stipulated within an approved valve design
specification referred to in Section "Cylinder Valve Purchasing
Specifications." These torque maximum values shall not be exceeded even to
align the valve outlet and/or pressure relief valve (PRV) to required
orientation.
Leak testing of all cylinders is important step in preventing leaks.
1.
Following fitting of valves, all cylinders shall be leak tested by filling with air
to 7 bar (0.7 MPa) gauge pressure and completely the immersing cylinder in a
water bath and looking for leaks by bubble observation. Any bubbles,
however small, shall result in rejection of the cylinder.
2.
If the leaks are from a valve thread, the valve may be re-tightened to no more
than the torque specification limit. If the leak persists, its cause shall be
traced and remedied. Faulty valve bung threads may be re-tapped or require
the cylinder to be scrapped. No attempt shall be made to re-tap the valve
thread. Faulty valves shall be reported to the cylinder purchaser and/or valve
supplier.
3.
If bubbles are observed coming from the weld of a cylinder, that cylinder
shall be rejected. If two or more weld leaks are observed per batch, the entire
batch shall be rejected unless the source of the leaks is identified and
rectified.
4.
If a leak is observed coming from a cylinder body material itself, all
production with the batch of steel being used, shall be stopped and the matter
reported to the cylinder purchaser for deciding further action.
Prior to delivery, all cylinders, shall be evacuated down to a vacuum (absolute) pressure
of a nominal 10 kPa (75 torr). To prevent damage to paint work, during transport,
cylinders shall be protectively wrapped, as circumstances and experience demand.
Cylinders shall be labeled or otherwise signed in accordance with purchaser instructions
and or local national safety requirements, but as a minimum, this shall include a durable
hazard warning label.
7.1.2.7
Record Retention.
To assist with any incident investigation and or cylinder re-qualification or scrapping,
records shall be kept of all of the following against cylinder serial number:
Safety in LPG Design
1.
Cylinder batch number and order details.
2.
Manufacturing steel batch analysis.
3.
Tare weight.
4.
Water volume capacity.
5.
Test or Working Pressure.
6.
In addition, full records shall be kept of all tests as required by DOT
4B/4BW240 Standard.
LPG CYLINDERS
7-7
7.2
Cylinder Valve Purchasing Specifications
As with cylinders, a wide range of valve designs, types and sizes are in use worldwide.
LPG valves in vapor service may be (a) manually operated, side entry valves and the
most widely used one is called POL (Port-O-Lite) valves with the female (CGA510) or
male outlet connection or (b) automatic, self closing, top entry valves and the popular
systems in the market are Compact, Jumbo and Snap-Tight. LPG valves in liquid
service are always manually operated valves with a dip tube and the most widely used
one is the CGA555 with male connection. Except where it is explicitly forbidden by
national authorities, all cylinders irrespective of valve type shall be fitted with a pressure
relief valve.
7.2.1
Manufacturing Design Standards
All valves and their components, irrespective of design, shall be manufactured in
accordance to a recognized international or equivalent national standard. Examples of
such standards bodies and applicable manufacturing standards are as follows:
1.
International Standards Organization (ISO).
2.
Committee of European Normalization. (CEN/EN) TC 286 WG 2(Draft).
3.
UK LPGA Code of Practice 15 1994.
4.
Japanese Institute of Standards (JIS) B8245.
5.
Malaysian & Singapore Standards.
At the time of writing, an important new European Design Standard designation number
CEN TC 286 WG 2 N67, is in late draft stage, but the forecast date of final versions
preclude its inclusion within these guidelines. In the interim, it is recommended that
existing standards already in use shall be retained, but shall now include the following
additional requirements:
Figure 7.21: Compact, POL, and liquid take-off cylinder valves
7.2.1.1
Dimensions and Materials
No changes to existing valve dimensions shall be made without consulting with
appropriate owner’s personnel, as changes may impact on cylinder filling operations,
market place acceptance and safety.
7-8
LPG CYLINDERS
Safety in LPG Design
All materials shall be compatible with all commercial grades of LPG and chemically
and physically resistant to possible trace contaminants such as re-active volatile sulfides
and water vapor.
7.2.1.2
Valve Body
A Copper/Zinc/Lead (forging brass) alloy designated Cu.Zn40.Pb2 shall be used and
shall provide a tensile strength of 360 - 400 N/mm2 and a Brinell Hardness HB of 80-85.
Suitable standards for the composition and properties of this brass are:
1.
DIN 17 660 -Alloy 2.0402.
2.
BS 2782/2784 Alloys CZ 122 or 128.
3.
JIS H3250, C3771 BE.
The finished valve shall not exhibit internal or external deformation following fitting to
a cylinder after applying tightening torque to the maximum value stipulated within the
valve manufacturing standard, or as advised by the valve manufacture. Cylinder valves
shall have a minimum rated working pressure of at least 1,725 kPa. One-piece valve
body design is preferred to two-piece since there is chance of failure at the threaded
connection causing leakage after prolonged use.
7.2.1.3
Seals and Internal Fittings Materials
All elastomer/rubbers and plastics used for internal valve seals, operation and external
regulator connection seals shall be suitable for LPG at operating temperatures ranging
from –20 oC to 60 oC. They shall also be manufactured in accordance with a
recognized international standard, which pays particular attention to:
1.
2.
Chemical resistance(Swelling/solvency) to lubricants and other normal
occurring LPG components.
Aging.
3.
Low /High Operating Temperatures.
4.
Ozone Attack (Cracking) etc.
Suitable standards, which include the above requirements, are:
7.2.1.4
1.
European Standard EN 549.
2.
BS 6505.
Pressure Relief Valves and Other Internal Fittings
Except where it is expressly forbidden by national authorities, cylinder valves shall
incorporate a pressure relief valve in its body. Pressure relief valves shall be designed to
ensure a minimum LPG release rate and conditions in accordance with NFPA 58 1992,
Appendix E. The pressure relief valve shall relief only vapor pressure. The set pressure
of the relief valve shall be not less than 2,415 kPa.
Although allowed by NFPA 58, fusible plugs are not recommended for LPG
cylinders.
All springs and other internal fittings used in valve designs shall be corrosion proof,
and resistant to chemical attack, sufficient to guarantee protection for at least ten years
irrespective of location.
7.2.1.5
Valve Markings
Valves shall be permanently stamped and or embossed with the following information:
1.
Safety in LPG Design
Manufacturer's name/logo.
LPG CYLINDERS
7-9
7.2.1.6
2.
Date of Manufacture (Month & year).
3.
Pressure Relief Valve Setting.
4.
Marking of valve thread (NGT, DIN).
Liquid and Dual Off-take Valves
Manufacturers of liquid off-take valves, shall ensure that they are specifically designed
for this duty and are manual closing type. They shall be protected against physical
damage. Materials shall be metal and suitable for LPG service. Cylinder valves shall
have a minimum rated working pressure of at least 1,725 kPa. Cylinder valve shall
come complete with a pressure relief valve. The pressure relief valve shall relief only
vapor pressure. The set pressure of the relief valve shall be not less than 2,415 kPa.
Pressure relief valve shall be designed to minimize the possibility of tampering. The
valves shall incorporate an excess flow check device and have outlet connecting threads
which prevent them being connected to a vapor LPG system. Shutoff valve shall not be
located between the excess flow valve and the cylinder. This also applies to dual
purpose (Liquid or Vapor) off-take valves. In addition, such valves shall also
incorporate a self closing device, which prevents liquid LPG release to the atmosphere,
if the valve is accidentally opened before connecting to a sealed system. Valves used
for liquid or dual off-take shall be clearly and permanently marked to indicate liquid
handling. Color coding may be considered.
Where pressure relief valves are fitted, they shall at all times be exposed only to the
cylinder vapor space.
7.2.1.7
Records and Shipping
The valve manufacturer shall keep and maintain records of valve details for each
batch supplied, which include specifications and laboratory analyses of all materials
used for valve manufacture. This shall be sufficient to trace any faults subsequently
found, back to the specific raw material batch supplied by original equipment
manufacturers (OEM).
Pre-Delivery Packing - All valves shall be securely wrapped and packed to prevent
damage and contamination during delivery.
7.3
Regulator Purchasing Specifications
There is a very wide range of regulator designs associated with cylinder LPG. These
guidelines are mainly for non-adjustable regulators, which are directly fitted to
cylinders. Their general principles may be applied to those, which are remotely
mounted, but otherwise directly connected to no more than one or two cylinders. Single
or two stage regulators are acceptable. Cylinder regulators shall be constructed of
materials suitable for LPG service. The connection of the cylinder regulator to the
cylinder valve shall be compatible with the cylinder valve.
Regulators, connected to multi-cylinder manifold installations are not considered within
this chapter. For multi-cylinder installations, a two-stage regulator system, as discussed
in “Container Regulators” in Chapter 10, is typically installed.
Regulator performance characteristics shall be clearly and unambiguously stated and
agreed upon with manufacturer in writing. Once fixed, such performance criteria shall
only be changed by joint agreement and again in writing.
Unless otherwise requested, all regulators shall be guaranteed to be interchangeable
with those in existing use and in so being, provide a safe and otherwise trouble free
fitting to the cylinder valves.
7-10
LPG CYLINDERS
Safety in LPG Design
Regulators may incorporate an excess flow check device, which is designed to cut off
gas flow in event that the gas supply hose to an appliance becomes unattached or is
severed. Such devices shall specifically meet the following acceptance criteria:
7.3.1
1.
Gas flow shut-off is not activated within normal regulator LPG throughput
rating.
2.
Gas “leak-by rate”, following activation, shall not exceed that required by
local/national safety authorities, or a maximum of 60 grams/h which ever is
lower.
3.
It will re-open automatically when the hose is re-connected, but only with gas
appliance valve in the off position.
Manufacturing Design Standards
All regulators, shall be manufactured to an internationally approved current design
standards. Examples of such standards are:
1.
BS 3016.
2.
UL 144.
3.
JIS B8238.
4.
prEN 12864 (Standard in final draft stage since 1997).
Membrane
Spring
Pressure Control Valve
Figure 7.3.1 a: Cylinder regulator principle
7.3.1.1
Materials, Regulator Body, and Internal Fittings
All materials shall be compatible with all commercial grades of LPG and chemically
and physically resistant to possible trace contaminants such as reactive volatile sulfides
and water vapor and salt spray environment attack.
Regulator bodies shall be manufactured from a generally non-corrosive metal only. If
zinc alloy is used, it shall conform to ISO 301:1981.
All springs and other internal fittings used in regulator designs shall be corrosion
proof, and resistant to chemical attack, sufficient to guarantee protection for at least ten
years irrespective of location.
7.3.1.2
Non-metallic Components
All elastomer/rubbers and plastics used for internal and external components shall be
suitable for LPG at operating temperatures ranging from –20 °C to +60 °C. They shall
also be manufactured in accordance with a recognized international standard, which
pays particular attention to:
Safety in LPG Design
LPG CYLINDERS
7-11
1.
Chemical resistance (Swelling/solvency) to lubricants and other normal
occurring LPG components.
2.
Aging.
3.
Low/High Operating Temperatures.
4.
Ozone and Salt Attack (Cracking) etc.
Use of adhesives for fixing regulator components shall not be permitted.
Suitable manufacturing material standards for seals and diaphragms, which include the
above protection requirements are:
1.
European Standard designate pr (preliminary) EN 549.
2.
BS 6505.
In addition to requirements within non-metallic components, materials used for the
manufacture of laminated reinforced diaphragms shall be specifically required to be
tested for resistance to de-lamination in accordance with Standard Pr EN 549. However,
resistance to de-lamination shall also be tested for by immersion in Propylene for 72 h
at 20 (+/-5) oC.
Figure 7.3.1-b: "Snap-On" two stage cylinder regulator (SRG)
7.3.1.3
Regulator Markings
The minimum permanently stamped or embossed markings for regulators shall be:
7-12
1.
Name of manufacturer.
2.
Production date(MM/YY).
3.
Designed outlet pressure.
LPG CYLINDERS
Safety in LPG Design
4.
Grade of LPG.
5.
Maximum rated gas flow.
6.
Direction of gas flow.
Pre-Delivery Packing. All regulators shall be securely wrapped and packed to prevent
damage and contamination during delivery. Regulator packing shall include customer
fitting and operating instructions.
Figure 7.3.1-c: Compact Cylinder Regulator (Kosan)
Figure 7.3.1-d: "Quick-on" cylinder regulator, child proof, single button, excess flow valve (Cavagna)
7.4
Hose Purchasing Specifications
This section of minimum standards covers cylinder hoses used for domestic appliances,
maximum working pressure of 35 kPa and industrial application, maximum working
pressure of 2,415 kPa. Cylinder hoses shall be manufactured to BS, JIS, UL or
Safety in LPG Design
LPG CYLINDERS
7-13
equivalent standard or meets local requirements. The date of manufacture (MM/YY)
shall be permanently marked on each hose. Cylinder hoses shall be fabricated of
materials resistant to the action of LPG both as liquid and vapor. Hoses for industrial
application shall be designed for working pressure of 2,415 kPa with a minimum of 5:1
safety factor. For industrial application, hose shall come with assembly consisting of a
flexible hose, a tee-check valve and a ball valve shall be used. The hose assembly shall
have a design capability of withstanding a pressure not less than 4,830 kPa.
Purchased LPG hoses for customer appliances, shall meet internationally recognized
approved manufacturing standards. Plastic hose shall not be used in LPG service.
Manufacturers shall clearly and durably mark hoses as below and no hoses shall be
purchased unless so marked:
1.
Manufacturers name/logo.
2.
Manufacturing standard number.
3.
Current year of manufacture.
4.
The words "LPG" or equivalent.
Typical cylinder hose manufacturing standards are listed below.
For high pressure hoses, which are pre-fitted with screw connections by the
manufacturer and which are normally used for connecting cylinders to manifolds, high
pressure, or wall mounted regulators.
1.
BS 3121 (Hose Type 2) 1991.
2.
DIN 4815 Part 2.
3.
JIS K6347.
4.
NF Gaz M 88-768.
For low pressure hoses that are normally used to connect regulators to LPG appliances
operating up to 200 mbar gas pressure.
1.
BS 3212 (Hose Type 1) 1991.
2.
DIN 4815 Part 2.
3.
JIS K6347.
4.
NF Gaz D36-161.
The length of the hose shall not exceed 2 m if installed indoors and can be greater than 2
m if installed outside building. However, the length shall be as short as possible. Hoses
shall not be installed with sharp bends or twists and shall be protected against physical
damage. Hoses shall not be concealed from view or used in concealed locations. Hoses
shall not extend from one room to another nor pass through any partitions, walls,
ceilings, or floors.
7.4.1
Hose clips
All hoses must be securely fixed to the connectors. For low pressure connections,
tension clips or screw type clips are sufficient. For high pressure hoses the connection
must be screw type with a compressed clamp which is prefabricated the hose
manufacturer. High pressure hose connections must not be repaired locally but
exchanged against new hose connections. The use of rubber slip ends shall not be
permitted except for domestic appliances where the working pressure is less than 35
kPa.
7-14
LPG CYLINDERS
Safety in LPG Design
7.5
Cylinder Filling Plant
This section of minimum standards covers the LPG cylinder filling plant within
ExxonMobil’s fence.
The cylinder filling plant shall have manual or automatic cylinder filling capabilities,
leak check equipment, weight check equipment, cylinder evacuation units, conveyors
and associated motors. The LPG cylinder filling shed shall be of open shed design. The
floor level shall be 1.1 meter above the road level for easy unloading/loading of
cylinders from/to trucks. The area shall be well ventilated to minimize the accumulation
of LPG vapors.
The LPG supply from the storage to the filling plant shall be by pump. An emergency
shutdown valve shall be provided on the piping system supplying LPG to the filling
facility. Emergency shutdown pushbuttons shall be provided at strategic location/s to
ensure that the filling plant shuts down safely during emergency. Location of the
emergency shutdown pushbuttons shall be located such that they are still operable
during fire situation. The electrical area classification of the filling plant shall be in
accordance with Chapter 2. The LPG filling equipment shall be designed for 1,725 kPa
to accommodate LPG filling with anticipated higher Propane composition.
Public access to areas where LPG is stored and transferred shall be prohibited. To
prevent trespassing or tampering, the LPG filling plant shall be enclosed by an industrial
fence not less than 2 m high unless it is otherwise adequately protected (e.g. within a
greater fenced area). Sufficient clearance shall be provided to allow maintenance to be
performed. Warning signs “NO SMOKING” and “NO OPEN FLAME” shall be posted
in the filling plant. Fire protection and gas detectors shall be provided in accordance
with Chapter 8.
7.6
Cylinder Filling
Cylinder filling may be manual, automated or fully integrated automated. Fully
integrated/automated cylinder filling lines are complex, custom designed equipment and
LPG supply volume and pressure requirements are likely to be specific to each
individual line.
The facilities provided in a plant for handling cylinder maintenance, testing, and filling
will depend on the volume of product to be filled into cylinders, the type and variety of
cylinder to be processed (ranging from 3.9 to 48 kg), and project economics.
Total cylinder filling and storage capacity requirements shall be determined by
establishing the maximum number and size distribution of the cylinders to be filled to
meet average daily/seasonal demand. Then, peak daily demand shall be established in
the same manner. Finally, provision for future business growth shall be added to the
previous two values, since incremental equipment capacity and operating space
(particularly) can be incorporated in a new plant design at a small fraction of the costs
incurred in expansion of the same plant after it is constructed. All of the above
information will be needed to define the number, type and layout of scales.
Average and peak capacity requirement determinations shall include allowances for
interruptions of normal operation by such contingencies as power failures, mechanical
breakdowns, and product supply shortages.
The designer shall next determine the optimum strategy for meeting peak demand, by
balancing capital cost of providing additional instantaneous filling capacity against the
disadvantages of filling and storing cylinders during off-peak periods. Evaluation of the
latter alternative shall include:
Safety in LPG Design
LPG CYLINDERS
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1.
A risk analysis of warehousing a larger inventory of pressurized cylinders.
This risk potential may be partially mitigated by providing remote/satellite
storage depots for filled cylinders. Many national safety authorities regard a
previously used depressurized cylinder awaiting refilling to be as hazardous
as a full one. Local limits on permissible cylinder quantity transported in one
vehicle load may also affect storage arrangement.
2.
A financial analysis of the costs of increased filled cylinder inventory.
Cylinders are usually the largest single asset item in LPG plants that have a
significant filling operation.
Figure 7.5.2: Cylinder filling adapter (SRG)
7.6.1
Cylinder Processing and Filling
The pump supplying the product to the filling shed shall not be oversized since this
would keep the automatic pressure control valve to the supply tank always wide open,
which would impair control. Investment in the “carrousel” system may justify provision
of an installed spare supply pump.
The line feeding the cylinder filling shed shall be provided with a remote operated
emergency block valve (EBV) and several local actuation push-buttons distributed at
strategic locations in the shed to activate the plant Emergency Shutdown System
(ESS). A fail safe, quarter turn ball valve with hydraulic or pneumatic actuators is the
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Safety in LPG Design
preferred choice. The valve shall meet API-607 fire-resistance tests. The valve shall be
located outside the filling shed.
The Emergency Shutdown System shall also activate a valve that closes the air supply to
the filling machine. Should there be a fire, this may prevent escalation by eliminating
the air, which would normally, blow through melted plastic/copper air lines.
7.6.2
Manual Filling System
A Manual filling connection and a beam type scale (preferably double beam) may be
adequate if small quantities of various sized cylinders are to be handled. The scale
weighing range shall be such that the maximum is equivalent to approximately twice the
load, which will be applied by the largest filled cylinder. The scale shall be located in a
platform or within a room specifically designed for cylinder filling. The base and
platform of the scale shall be set flush with the surrounding floor level. Appropriate
means for transporting the cylinders are required. The scale shall be capable of
providing accuracy within the limits prescribed by the local authority.
A well supported manifold shall be provided, at a convenient height, to receive product
from the marketing plant pumping station. The manifold shall include a lateral fitted
with positive shutoff valves. The shutoff valves are fitted with hoses of convenient
lengths to reach the cylinder valve without in any way applying a strain upon the
cylinder. The end of the charging hose is fitted with a quick-acting, positive shutoff
valve and a filling adapter or coupling suitable for attachment to the cylinder valve
being utilized. A steel I-beam provides rigid support to keep all parts in correct
alignment. The manifold piping is 32 mm pipe size for strength and rigidity.
Restrictions are held to a minimum to allow for fast filling.
The fitting at the end of the charging hose may be manually secured to the cylinder
valve and in most instances can be of a type which will permit hand tightening of the
connection. Devices are available where a resilient seating means is used to form a gastight seal without the use of a wrench. The quick-acting shutoff device and the adapter
or coupling may be supported from above by a counter balance so the unit remains in a
convenient location when disengaged and, at the same time, supports the hose end when
attached to a cylinder valve. This minimizes the weight applied to the cylinder and, in
turn, the scales. Units of this type may be installed singularly or with manifolds
accommodating any number of scales required for a particular operation.
As an alternate to the manually secured coupling or adapter at the end of the charging
hose, certain valves are adaptable to the use of a device which may be secured by
hooking, snapping or otherwise engaging the valve with the application of downward
pressure. In other cases a connector may be utilized which engages the valve body,
forcing and sealing the end of the adapter or coupling within the inlet of the valve.
7.6.3
Automated Filling System
An automated filling system may be justified if larger quantities of one cylinder size
have to be filled, possibly including electronic weigh scales to assure greater product
control precision. Simple automatic systems may be justified for modest throughput
levels and are found in many small plants worldwide. Fully-integrated, automated
processing systems are justified where very large quantities of one cylinder size must be
filled. These systems utilize a rotary filling head, or carrousel, similar in principle to a
motor oil filling head. However, special vapor retaining connect/disconnect systems are
incorporated due to high product vapor pressure. Fill weight may be checked
continuously by a load cell-actuated electronic weighing system.
Simple automation can be achieved by using the scales, manifold and charging hose of
the manual system, but installing an automatic valve in the lateral from the manifold. A
Safety in LPG Design
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7-17
sensing device is attached to the scales and is responsive to the upward movement of the
beam. A pneumatic system, using compressed air, permits the control valve at the
manifold to remain in an open position until the movement of the scale beam, which
closes the control valve, actuates the relay. With the poise set at the appropriate point
on the beam (allowance being made for any weight, which may be exerted by the hose)
the flow of incoming liquid shall be shut off when the desired weight is reached.
As a modification of this system, a solenoid valve may be substituted for the control
valve at the manifold with an explosion-proof mercury switch activated by the
movement of the scale causing flow of incoming liquid to cease when the proper weight
has been reached.
7.6.3.1
Unitized Automatic Scales
Scales shall be accurate to within 1/10 of 1 percent throughout their entire range. Scales
shall be of the double-beam type or constructed so that individual adjustment can be
made for tare and net weights. It is recommended that scales be designed for the
following adjustments:
Recommended Range
Tare and Net Contents Fine Adjustments
0.5 – 5.5 Kg
14 g
5 – 60 Kg
113 g
NOTE: A set of check weights shall be available within each filling plant to calibrate
all scales daily.
Figure 7.5.3.1-a: Filling with conveyor
Automated systems are also available which are "unitized" in that the scales and
automatic control device are fabricated as a single unit. These units are constructed
with the control mechanism completely encased within cabinetry. The accuracy of the
scales is reported to be plus or minus 50 grams. In operation, larger tolerances may be
necessary depending upon the fluctuation in the product pressure, speed of filling, the
amount of product to be filled, and friction in hoses, pipes and fittings. The scales are
provided with two roller weights representing the weight of the product to be charged
and the tare weight. Further, a sliding weight for the fine adjustment of the tare is
included. Higher precision is obtainable with “load cell-actuated” electronic equipment.
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Safety in LPG Design
A four scale filling plant is illustrated in Figure “Filling with conveyor” above, using
automatic filling scales with a “keyboard” type of weight adjustment. The filling
head attached to the individual filling hose provides an automatic means of attachment
to cylinder valves with threaded outlets. A conveyor system brings the cylinders to the
four stations and, likewise, removes them. A pneumatic lifting device is located under
each charging position so the scale platform may be elevated through the conveyor,
lifting the cylinder to a position where its weight may be determined.
From Pump
Manifold
Approx 20%
of Scale Beam
Scale Frame
Length
Trip Valve
Filler Valve
Pilot Vapor Pressure
from Air Compressor
Pivot
Point
Trip
Valve
Filling Hose
Scale Beam
Beam in
Raised position
Beam
Horizontal
Beam when Cylinder
is not Filled
Alternate Trip Valve
Installation
Figure 7.5.3.1-b: Typical automatic filling for cylinders
This type of system may be utilized with two or more stationary scales erected after each
other on the chain conveyor. The system includes pneumatic indicators, a counting
device, and stops for fully automated operation of the inlet and outlet of the cylinders.
The capacities are limited by the following operator actions:
1.
Fetching of cylinder from conveyor, and placing on scale.
2.
Connecting of cylinder valve to filling head, adjusting scale, and starting the
filling.
3.
Disconnecting of filling head, removing cylinder from scale, and placing it on
a transport conveyor.
The figure above illustrates an automatic cylinder filling valve attached to a charging
manifold, which may service any number of additional units. The automatic valve can
be isolated from the manifold by a positive shutoff valve. It is necessary to connect a
suitable supply of compressed air to the unit in order to operate the relay pilot and main
shutoff device. The lifting of the scale beam when the desired filling amount has been
reached actuates a trip or “bleed” valve installed on the scales.
The manufacturer indicates that the device is designed with a positive feather-touch
control, which prevents vibration or sudden jars from causing improper cutoff.
Operation of the unit requires no electrical or mechanical power. The automatic
cylinder filling valve is designed specifically for use with a beam scale and is not
adaptable directly for use with a dial scale. When the desired amount of fuel has been
put into the cylinder, the rising scale beam contacts the trip valve and shuts off the filler
valve. A red button indicator appearing on top of the filler valve visually indicates this
condition.
7.6.3.2
Cylinder Handling
Chain conveyor systems may be utilized to eliminate manual transport of cylinders.
With one system the cylinders are loaded directly on to the chain conveyor from a
highway transport or from the empty cylinder storage area and conveyed directly to the
Safety in LPG Design
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7-19
filling scales. The cylinders are taken from the conveyor, placed on the scales for filling
and then returned to a second conveyor, which moves the units either to the highway
transport or to the filled cylinder storage area. Cylinders shall still be inspected for
damage and re-inspection date prior to loading onto the conveyor or at some point in the
system before filling.
A second system employs a continuous conveyor configuration and is particularly
adapted to the charging of 33 kg or 45 kg cylinders. The system can, however, be
adapted for filling smaller cylinders. In this system the empty cylinders are carried to
the filling scales on the chain conveyor and accumulate before the scales. The operator
releases a group of empty cylinders, which pass on to the scales and are stopped by
pneumatic equipment so that they are arranged with each cylinder in the correct position
at each scale.
Cylinder receipt
Control
Visual
external control
for damage, valve
integrity, corrosion
Accept or
Reject
Control
of refurbishment
date
correct filling weight
by reconciling tareweight and actual
weight on independent scale
Control
leak from valve and
bung
Pass or
Refurbish
Read Tareweight
input tareweight into scale
Fix cylinder cap
or seal
Attach filling adaptor
Wash cylinder
Fill cylinder
Load cylinder on truck
or store in plant
Stop filling
Cylinder delivery
move cylinder from scale
Figure 7.6.4-a: Cylinder filling process
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A pneumatic lifting device consists of a bottom frame with bearing brackets into which
the lifting table is placed. The lifting table is raised and lowered by means of a
pneumatic cylinder operated by automatic controls. The charging hose is connected and
the lifting table of the scale is activated in order to lift the cylinder free of the chain
conveyor. After filling is completed and the charging hose is disconnected, the cylinder
is lowered to the chain by releasing the control. As soon as all the filled cylinders are
lowered on to the chain conveyor the “stops” are opened and a new operating cycle
begins. The filled cylinders are then carried to a check scale, which is also built into the
chain conveyor system. This system is ideal for heavy cylinders as it eliminates manual
lifting.
Figure 7.6.4-b: Automated filling plant with carrousel
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7.6.4
Integrated Automated Filling Plant
Integrated, automated filling plants are laid out for high capacity and utilize
specialized equipment. They require extensive preliminary study, and large investment.
The best practice for an integrated cylinder filling process is shown in Figure 7.6.4-a
“Cylinder filling process.” The designer shall rely on packaging experts, both from
within the company and from equipment fabricators and vendors, to design the
equipment and determine the level of automation justifiable. The designer's primary
responsibility will be to provide the packaging specialists with a precise
duty/performance specification for the proposed filling system. This manual confines
discussion of integrated, automated filling systems to a review of the capabilities of
various system components, and a summary of information needed to solicit proposals
for automated systems.
The schematic plant layout (Figure 7.6.4-b: “Automated filling plant with carrousel”)
combines all equipment components discussed above to create an automated plant
capable of continuously filling 1000 to 1200 cylinders (11-13 kg) per hour. One
location operates an 800 cylinder/hour plant with one operator, one forklift driver and
one supervisor.
7.6.4.1
Degree of Automation
Once the filling plant throughput requirements are established, comparative investments
and operating costs for various degrees of automation can be determined. These values
will permit the designer to conclude whether the plant shall have:
1.
Individual weigh scale filling machines.
2.
Chain conveyor systems.
3.
Depalletizers/palletizers.
4.
Fully automated operation incorporating a filling carrousel.
Key factors to be considered in justification of automation are:
1.
Estimated capital cost of equipment components and installation.
2.
Present and projected cost of labor at various skill levels for the life of the
project.
3.
Estimated equipment maintenance costs, preferably supported by actual
experience with identical equipment elsewhere, and applicable local
regulations at the site.
Most recent designs of filling machines allow the processing of cylinders of different
size. Cylinders have to be filled in batches. After one batch is finished the carrousel is
stopped to adjust to the next cylinder height.
7.6.4.2
Carrousel System
For higher processing capacities, the use of a carrousel filler such as shown in Figure
7.6.4.2-a “Typical Cylinder Filling Carrousel” is justified by its efficiency. A carousel
is a rotating table upon which a number of automatic filling scales are equally spaced
around the outer edge. The carrousel becomes an integral part of the chain conveyor
system. Depending on conveyor system complexity, empty cylinders may be routed
from a number of sources to the carrousel. Cylinders accumulate on the carrousel inlet
section of the conveyor, and then are transferred, one at a time, to one of the multiple
carrousel scales by an automatic intake device. As the carousel slowly turns, an
operator places a cylinder on the platform of a scale, attaches the filling head, sets the
net charge weight and tare weight on the scale and opens a valve to start filling. When
the cylinder is filled, the automatic filling valve is tripped to the closed position. The
cylinder may then pass over check scales. The number of scales placed on a carousel
will depend upon the cylinder filling rate required within the plant. Carousels are
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Safety in LPG Design
available with 4 – 36 scales per unit. The size of the carousel shall be selected on the
basis of the expected future filling rate.
Figure 7.6.4.2-a: Typical cylinder filling carrousel (Siraga)
Figure 7.6.4.2-b: Large cylinder filling carrousel (Crisplant)
The following operating sequence is then performed by the carrousel:
1.
Safety in LPG Design
The filling head at the end of a charging hose is either manually or
automatically connected to the cylinder valve. Manual connection may be
LPG CYLINDERS
7-23
necessary with screw type fittings, while automatic connection is feasible
with click-on/clamp-on type fittings.
2.
The scale is adjusted (a) manually for cylinder tare weight, using a single-dial
or digital keyboard weight adjustment, or (b) automatically using selfadjusting scales programmed from a digital keyboard-programmed
computer. Fill weight is predetermined when the size of cylinder to be
processed is selected.
3.
The cylinders are filled in less than one revolution of the carrousel. LPG
supply to each cylinder is shut off automatically when the correct fill weight
is reached.
4.
The filling head is either manually or automatically disconnected as the
carrousel revolution is completed.
5.
The filled cylinders are automatically transferred to a check scale with dial or
digital readout.
6.
After weighing, the cylinders are automatically transferred to the outlet
section of the chain conveyor.
Figure 7.5.4.3-a: Unloading pallets to destacker
Figure 7.5.4.3-b: Pallet feeding into the conveyor system
Carrousels are produced with varying numbers of scales, or filling stations, up to a
maximum of 48. Maximum carrousel filling capacity is approximately 1200 cylinders
per hour. Carrousel design is adaptable to filling all sizes of cylinders.
Where adequate volume demand exits, the filling carrousel permits achieving a
continuous maximum production rate with minimum operating work force. In order to
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Safety in LPG Design
realize the full potential of this equipment, it shall be installed as a part of an integrated
filling plant that incorporates all support facilities needed to operate the carrousel
continuously at capacity.
The filling method briefly described in the prior discussion is predicated upon the
processing of a group or batch of cylinders of a uniform size or capacity, observing the
tare weight of each cylinder, and in turn adjusting the tare weight component of the
scales. Operating in this manner it, is not necessary to adjust the fuel weight component
since uniform cylinders are being processed. It should be noted that by using this
method, a correct filled weight may be obtained regardless of the contents of the
cylinder when presented to the carrousel. In other words, this system ensures that
cylinders will not be overfilled since the scales will trip when the combination of the
fuel weight and tare weight is reached.
Figure 7.5.4.3-c: Check weight scale
A filling procedure using a predetermined quantity is not recommended since it
involves time consuming procedures like cylinder emptying. Utilization of constant
tare weight is also not recommended since different suppliers may manufacture
cylinders over a long period.
7.6.4.3
Cylinder Filling Support Functions
The support equipment items required to create a fully integrated, automated cylinder
filling plant based on the carrousel are as follows:
Equipment to unload cylinders to be filled from the incoming transport and place them
on the conveyor system. Palletization of cylinders is desirable for high volume filling,
distribution, and sales operations.
Safety in LPG Design
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7-25
Equipment to feed a conveyor system shall consist of a fork lift truck, a destacker, and
a depalletizer with a ram to eject the unloaded cylinders onto the conveyor. Ready
access to a second lift truck and driver is desirable in event of destacker, depalletizer, or
palletizer breakdowns. Figure 7.5.4.3-a “Unloading pallets to destacker” shows a
forklift unloading pallets to the destacker.
Cylinder counting equipment: An automatic cumulative counter should be provided
to count all cylinders remaining on the line, which will now be filled, as part of control
procedures. The cylinders next enter the carrousel filling process.
Carrousel Cylinder Filling: Tare weights, connection, filling and disconnection were
described in section 7.5.4.2 “Carrousel System.”
Figure 7.5.4.3-d: Leak detection unit
Check weighing/Overfill detection equipment: The system shall next provide a device
that is capable of accurately weighing moving cylinders and detecting over/underfill
situations. The device shall also eject those over/underfill cylinders from the line onto a
branch “decant/refill” conveyor. The Figure 7.5.4.3-c “Check weight scale” shows an
overfill check weight scale to control whether filling matches correct figures for each
filled cylinder as it moves along the conveyor system, automatically rejecting overfills.
Weight readings are manually checked periodically on a statistical basis to guard against
continuous filling error. Statistical data shall be kept for each carrousel scale as well as
for the overfill check weight scale.
Leak detection equipment: A leak detection device capable of detecting leaks of 0.5
grams/hour, and shunting the leaking cylinders to a reject conveyor is next required.
The automatic leak detection is likely to require review and endorsement by local
regulators. Figure 7.5.4.3-d “Leak detection unit.” shows such a unit, which may
function on the basis of either (a) cylinder pressure drop in 3 seconds, a time interval
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LPG CYLINDERS
Safety in LPG Design
consistent with 1200 cylinder/h. filling rate, or (b) hydrocarbon vapor detection in the
same time interval. In some countries the law requires that dipping the cylinders
completely under water perform leak detection. This is an effective test provided the
water surface is allowed to become calm and is not disturbed by the submersion process.
Figure 7.5.4.3-e “Seal leak detection unit” shows a machine that is capable of detecting
seal leaks.
Figure 7.5.4.3-e: Seal leak detection unit
Cylinder capping equipment: Next, a machine shall be provided to apply either plastic
or metal sealing caps to the cylinder valve outlet connection. Figure 7.5.4.3-f “Typical
valve cap” shows such a sealing cap. If acceptable to local regulators, plastic seals shall
be used because they are probably more effective than metal seals.
Figure 7.5.4.3-f: Typical valve cap
Cylinder washing and drying equipment: The Figure 7.5.4.3-g “Inside of a water jet
washing machine” shows a high pressure jet water washing cabinet without brushes that
cleans the cylinders while they continue to move along the conveyor. Wash water is
Safety in LPG Design
LPG CYLINDERS
7-27
freed of sediment and recycled. High pressure air jets then dry the valve area of the
cylinders. After washing, the cylinders must be manually subjected to a visual
inspection and transferred to a branch “reject” conveyor if they are deficient, for repair
or scrapping.
Figure 7.5.4.3-g: Inside of a water jet washing machine
Figure 7.5.4.3-h: Cylinder stacker
Re-palletizing and shipping equipment: The filled cylinders are now ready for
movement into a palletizer-stacker, and then for removal by lift truck to outbound
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Safety in LPG Design
transport or a filled cylinder storage area. The integrated filling plant's conveyor system
is ideally configured in a loop, so that the filled cylinder outlet is as close as possible to
the empty cylinder inlet, thereby permitting a single lift truck and operator to serve both
ends of the conveyor line.
Figure 7.5.4.3-i: Cylinder painting cabinet
7.6.4.4
Cylinder Maintenance Facilities
Generally it is preferred that cylinder maintenance and re-testing is performed by
specialized third party companies outside the filling plants. Cylinder manufacturers are
best equipped to perform this task. However, if this is not viable, cylinder maintenance
may be performed inside the filling plant. This section of minimum standards covers the
buildings, structures and equipment used for maintaining LPG cylinders. The LPG
cylinder maintenance shed shall be of open shed design. The area shall be well
ventilated to minimize the accumulation of vapor cloud. The shed shall be constructed
of noncombustible materials. The maintenance shop typically includes a shot blasting
unit, spray painting or dip painting equipment, a dry oven, conveyors and associated
electric motors. Utility station such as plant air and plant water shall be provided to
maintenance shop. Plant air and plant water piping shall be clearly identified either by
labeling or color coding. The electrical area classification of the filling plant shall be in
accordance with Chapter 2.
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Cylinder painting equipment: Some plants may include cylinder painting. This
equipment includes a water curtain using recycled water to dispose of over spray mist.
An overhead jack that rotates the cylinder while it is coated by an automatic spray gun
engages cylinders on the conveyor. A protective shroud shall be incorporated into the
rotating jack to protect top cylinder valves normally used in operations. If desired, a
second paint station using a spray gun equipped with a stencil mask can apply a
trademark or text to the cylinder after the base painting coat is applied. Typically, paint
would be applied to a statistical fraction of all cylinders, say one in five. This
intermittent operation would be controlled automatically.
At offsite locations, subcontractors may advantageously perform several of the
operations discussed above.
They may include cylinder painting, cylinder
depressurization, air purging, and subsequent repair and re-qualification of rejected
cylinders and/or their valve assemblies.
Concurrent use of the offsite subcontractor facilities to augment storage space for empty
and/or full cylinders may achieve additional economic benefits. It is recommended that
any design study for a new plant, or for major modification of existing facilities, include
a thorough exploration of incentive for subcontracting support operations of the type
discussed above at locations away from the main plant.
Figure 7.5.4.3-j: Cylinder pallet storage
7.6.5
Purchasing Guidelines for Filling Plants
There are three reliable filling plant manufacturers who are well established in the
market and have produced a large number of safe filling plants. All vendors are deeply
involved in fully automated filling plants, which rely heavily on electronics for
filling/check weigh scales, leak detection and bar code or data matrix control of cylinder
processing and stock control. However, the designers are primarily fabrication and
conveyor systems engineers with relatively limited knowledge of LPG properties and
operations. They are dependent almost entirely on third party filling plant operators for
field experience for developing the practical aspects of their designs. The adverse effect
of intense competitiveness between the manufacturers on quality, performance claims
and guarantees needs to be very carefully safeguarded against. Consequently, it is
important to specify and obtain agreement and or clarification on safety and
performance guarantees etc., in writing, prior to placing orders, on items discussed
in the next paragraphs.
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7.6.5.1
Safety and Integrity of Equipment Design
Vendors should submit designs with due consideration given to the owner’s LPG
Specifications. Consideration should be given to auto- refrigeration, especially of
Propane at –42 °C, irrespective of climatic conditions. The design should also maintain
equipment safety and performance under all anticipated environmental conditions of the
plant location.
The vendor shall provide formal recognized “certification” of all materials of
construction and assembly. This shall include all materials and equipment, “bought in”
by the vendors, with specific attention to elastomer seals, pipe thread and flange
jointing, LPG transfer hoses and electrical parts, including prevention of static
electricity.
All major equipment shall be designed and manufactured to approved standards
acceptable to the appropriate national regulations or LPG Design Guidelines in this
book, which ever are the more stringent. This applies in particular to LPG piping,
flanges and fittings, pumps and compressors and the classification of electrical
equipment.
All main steel work, e.g.; palletizers, carrousel and conveyor chain frames, shall be
fabricated from hot rolled or forged sections. Pressed and folded steel shall not
generally be accepted for major frame members. Possible exceptions may be
permissible for small modular constructions, which can be readily removed or replaced,
if damaged.
Care shall be taken to electrically ground electrically isolated metal parts around the
machinery to prevent accumulation of static charge. In particular, this pertains to metal
parts suspended by rubber for flexibility or metal connections attached to plastic/rubber
tubing.
All machines shall reset automatically. In the case of shutdown or emergency
shutdown, moving parts shall automatically reset back to their starting positions. It is
not recommended to install protective caging between filling machinery. Cylinders
shall be accessible at all times.
All services required of the vendor’s equipment should be thoroughly considered and
agreed upon before signing of contracts, in particular; instrument air supply (air
pressure, volume and quality), electric power (including voltage fluctuation range), and
water (pressure, volume, quality). Detergents for cylinder washing (compatibility with
plant water treatment system) shall be compatible with environmental requirements.
Emergency shutdown system (ESS) considerations shall be included. Vendor
equipment shall be specified as compatible with the existing system and fail safe, i.e.;
sealed against release of LPG during emergency shut down and or failure of operating
media.
7.6.5.2
Performance Criteria and Guarantees.
All vendors should provide overall and specific guarantees of equipment safety as well
as performance. The owner should specify and agree with the vendor’s pre- and post
commissioning “acceptance” criteria, as appropriate, in writing. This is of particular
importance to retrofitting equipment to an existing plant. Previous discussions with all
vendors, revealed serious gaps and shortfalls in their guarantee cover.
Vendors tend to be overly optimistic when making claims about their new
developments. The owner should be careful to request proof of performance and
reliability from case histories, i.e. proven track record. It is obviously important that the
owner does not unknowingly become a proving ground for any relatively new
equipment developments.
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Indemnity clauses differ quite markedly between vendors. The owner should clarify and
agree to the details with the vendor. Such clauses range from possible incidents arising
from, pre- and post-commissioning by vendor personnel, through to market place safety,
unreliability of check scales and/or of leak detection equipment.
Key criteria to be covered include: cylinder filling rates, and tolerances for fill weight,
check weight and leak rates at the required cylinder filling and chain conveyor speeds,
under normal local operating conditions. These criteria would usually require a vendor's
engineer to visit the plant, to be “fully” acquainted with local operating conditions.
Filling scale fill tolerances shall be geared to appropriate National weights and
measurement. It is important to note that the “overall filling accuracy” mentioned in the
ExxonMobil Product Control Manual should not be quoted to the vendor. The owner
shall request “guaranteed” filling accuracy from the vendor, per cylinder size and filling
rate. With the exception of 45 − 50 kg cylinders, most vendors can usually meet
requirements of the Product Control Manual.
7.6.5.3
Pre-Delivery Testing of Equipment.
All equipment shall be pre-tested by the vendor, especially pressure testing of LPG pipe
work and hoses. See also “Performance Criteria and Guarantees” above in relation to
required cylinder filling rates and conveyor speeds. Certification of the safety of all
electric items and circuitry, especially electronic scales and computerized systems shall
be checked. Many of these are now designed to be intrinsically safe, but this should
specifically checked.
7.6.5.4
Installation and Commissioning
The owner should be satisfied that sufficient details are agreed to with the vendor on
how installation and commissioning are undertaken, without prejudicing guarantees
of equipment reliability and performance. This may include direct involvement by
the vendor in both installation and commissioning as well as training of owner
personnel.
7.6.5.5
After Sales Service
All vendors claim to provide a reliable after sales service. Vendor staffing is largely by
personnel involved in equipment manufacture. However, some vendors have or are
establishing local offices, so details of what services are available on a 24 hour notice, as
a minimum, should be obtained.
To augment their normal services, all vendors provide contract Periodic Preventive
Maintenance Services. Details on these services may be obtained and compared with
availability/capability of local skills.
7.6.5.6
Records, Drawings, Maintenance, Spare Parts
The vendor shall provide all records of approval certification and manufacturing
standards for all materials, equipment and tests as appropriate.
Accurate plant drawings and operational service diagrams, especially wiring diagrams,
shall also be supplied, together with maintenance and inspection schedules, with specific
reference to “bought in” items. Note that all vendors are highly dependent upon boughtin materials and equipment which require instructions and recommended maintenance
and inspection schedules from the original manufacturers. Spare parts shall be itemized
with recommended frequency of fitting. This is particularly important for elastomer
seals in liquid phase LPG use.
7-32
LPG CYLINDERS
Safety in LPG Design
7.6.6
Third Party Cylinder Filling Plant
This section of minimum standards covers a third party cylinder filling plant where
ExxonMobil LPG cylinders are filled. Local standards shall be followed. In the
absence of a local standard or if the local standard is less stringent, the following
minimum standard shall apply.
The structure of the LPG cylinder filling shall be of open shed design. The area shall
be well ventilated to minimize the accumulation of vapor accumulation. Pumps shall be
used to transfer the LPG from the storage to the filling plant. At least one emergency
shutdown valve shall be provided on the piping system supplying LPG to the facility.
Emergency shutdown device/s shall be provided at strategic location/s to ensure that
the filling plant shuts down safely during emergency. Location of the shutdown
device/s shall be such that they are still operable during fire situation. The electrical
area classification of the filling plant shall be in accordance with Chapter 2.
Working pressure of LPG to the filling station unit shall be a maximum of 1,725 kPa.
A separate weight check scale shall be provided to check the weight of the filled
cylinders before dispatching them out of the filling facility.
Public access to areas where LPG is stored and transferred shall be prohibited. To
prevent trespassing or tampering, the LPG filling plant shall be enclosed by an industrial
fence not less than 2 m high unless it is otherwise adequately protected (e.g. within a
greater fenced area). At least 2 means of emergency access from the fenced or other
enclosure shall be provided. Sufficient clearance shall be provided to allow
maintenance to be performed. Clearance of at least 1 m shall be provided to allow
emergency access to the required means of egress. The outside storage area for filled
cylinders shall be a minimum distance of 7.5 m from cylinder filling facilities. Relevant
signs such as “NO SMOKING” and “NO OPEN FLAME” shall be posted around the
perimeter and in the filling plant.
7.7
Cylinder Distribution
7.7.1
Distribution Center
Cylinder distribution strategy may, in some cases, require the installation of distribution
centers. This is typically the case where one large filling plant serves several demand
centers. Large trucks transport the cylinders back and forth between the filling plant and
the Distribution Center. From there, small trucks, principally pickup trucks owned by
the resellers or dealers, are used for delivery to end-users.
Regulatory compliance (land status, classification and zoning laws) is a basic
requirement. Sites in industrial zones are preferred, housing and public areas shall be
avoided. The area shall be freely ventilated (not located in a terrain depression). Size
and shape of land shall be large enough to satisfy spacing requirements based on store
capacity stipulated in Table 7.7.1. Safe truck access has to be included in these
considerations. The area shall be preferably fenced with a 3 m high chain linked fence.
LPG cylinders shall be stored upright and well ventilated, preferably in an open-air
loading platform. Where inclement weather can frequently preclude work, or required
by local regulations, the Distribution Center may be provided with a roof, constructed of
noncombustible, lightweight, friable material that would break up quickly in a fire.
Sufficient space shall be maintained between the underside of the canopy and the
highest stacked cylinders to facilitate application of cooling water in the event of a fire
emergency.
Safety in LPG Design
LPG CYLINDERS
7-33
Drains shall be avoided in the floor of the storage place. The platform floor shall be
level and provided with suitable hard standing for LPG cylinder handling. Normally,
the platform would be elevated, such that it is at the same level as the back of the truck.
However, if the platform is at ground level, particular precautions have to be taken that,
when unloading, cylinders are not damaged. Appropriate warning signs are to be
prominently displayed i.e. “No Smoking or naked Flames” “Highly Flammable LPG.”
Electrical fittings used on the platform and under the roof of the storage area shall be
suitable for Zone 2 area classification. Other areas such as yard lighting or office
lighting shall be suitable for normal service.
Table 7.7.1 shows the spacing requirements for Distribution Centers:
Quantity* of LPG
Stored
Size of Largest
Stack
Distance to
Property Line
Kg
kg
m
< 30000
< 7000
8
< 50000
< 9000
9
< 60000
< 10000
10
< 100000
< 10000
11
< 150000
< 20000
12
< 250000
< 30000
15
> 250000
< 30000
20
Table 7.7.1: Spacing to property line in Distribution Centers
*Nominally “empty” cylinders have to be considered as “full” for above calculations
unless the cylinders are stored under the following conditions:
1.
The “full” and nominally “empty” cylinder storage area is clearly marked.
2.
Full cylinders are always stored in the “full” area, nominally “empty”
cylinders are always stored in the “empty” area.
3.
A gangway with a separation distance of at least 3 m is maintained between
the nominally “empty” and “full” cylinders.
4.
Individual unpalletized stacks (either full or empty) shall be separated by 1.5
m distance for accessibility. For palletized stacks the gangway distance shall
be a minimum distance of 2.5 m.
Domestic cylinders shall be stacked in the following manner:
1.
Manual stack: Maximum 3-cylinder high for cylinders up to 15 kg content
and maximum 2-cylinder high for cylinders above 15 kg up to 26 kg.
2.
Wooden Pallets: Maximum 2-pallet high and each pallet at 2-cylinder-high.
3.
Caged Steel Pallets: Filled cylinders - up to 4-pallet high and each pallet at
one-cylinder high. Empty cylinders - up to 6-pallet high).
Cylinders above 20 kg shall be stored upright without stacking.
Fire protection facilities for the Distribution Center are designed for first aid use only.
Following firewater capacities are recommended:
1.
7-34
For LPG storage up to 25 tons 800 l/min at 7 bar for at least 1 hour.
LPG CYLINDERS
Safety in LPG Design
2.
For LPG storage exceeding 25 tons 2300 l/min at 9 bar for 1 hour and at least
2 firewater monitors.
It is expected that public fire brigades would provide backup during LPG fires. Specific
requirements by individual Fire Department Offices may differ and the Project Engineer
will have to present his case to the fire authority for a system, which will suit the
location in terms of associated fire risks.
7.7.2
Dealer and Reseller Cylinder Storage
In some cases dealers or resellers may store considerable numbers of cylinders on their
own premises. The preferred location for cylinders stored for resale is outside buildings.
Fence and door are adequate means of control to the storage premises. Firefighting
capability is normally limited to a 9 kg dry powder extinguishers but depending on
conditions, the local firefighting authorities may require more protection (firewalls etc.).
Storage outside of buildings shall be located in accordance with Table 7.7.2 and at least
1.5 m from any doorway in a building frequented by the public. The table reflects
requirements in NFPA 58, 5.4.1.
Following are the explanations for the column headings in the spacing table below:
1.
Nearest important building or group of buildings.
2.
Line of adjoining property that may be built upon.
3.
Busy thoroughfares or sidewalks.
4.
Line of adjoining property occupied by schools, churches, hospitals, athletic
fields, or other points of public gathering.
5.
Dispensing station.
Horizontal Distance to:
Quantity of
LPG Stored,
1. and 2.
3. and 4.
5.
Kg
m
m
m
< 227
0
0
1.5
227+ to 1134
0
3
3
1134+ to 2721
3
3
3
2721+ to 4540
6
6
6
7.5
7.5
7.5
> 4540
Table 7.7.2: Spacing in reseller cylinder storage (from NFPA 58)
If cylinders are stored inside buildings or structures the buildings and structures shall be
one story in height and shall have walls, floors, ceilings, and roofs constructed of
noncombustible materials. Exterior walls, ceilings, and roofs shall be constructed as
follows:
Safety in LPG Design
1.
Of lightweight material designed for explosion venting, or
2.
If of heavy construction, such as solid brick masonry, concrete block, or
reinforced concrete construction, explosion venting windows or panels in
walls or roofs shall be provided having an explosion venting area of at least
0.1 m2 for each 1.4 m3 of the enclosed volume.
LPG CYLINDERS
7-35
The floor of such structures shall not be below ground level. Any space beneath the
floor shall be of solid fill or the perimeter of the space shall be left entirely unenclosed.
The floor level is preferably 1.1 m above the road level for easy unloading/unloading of
cylinders.
The structure shall be ventilated using air inlets and outlets, the bottom of which shall
be not more than 150 mm above the floor, and shall be arranged to provide air
movement across the floor as uniformly as practical and in accordance with the
following:
Where mechanical ventilation is used, air circulation shall be at least 0.3 m3/min×m2 of
floor area. Outlets shall discharge at least 1.5 m from any opening into the structure or
any other structure.
Where natural ventilation is used, each exterior wall [up to 6.1 m in length] shall be
provided with at least one opening, with an additional opening for each 6.1 m of length
or fraction thereof. Each opening shall have a minimum size of 32,250 mm2, and the
total of all openings shall be at least 720 mm2/m2 of floor area.
Heating shall be by steam or hot water radiation or other heating transfer medium with
the heat source located outside of the building or structure or by electrical appliances
listed for Class I, Group D, Division 2 locations, in accordance with NFPA 70, National
Electrical Code.
Walls of attached structures shall have a fire resistance rating of at least 1 hour. There
shall be no openings. Common walls for attached structures used only for storage of
LP-Gas shall be permitted to have doorways that shall be equipped with 1 1/2-hour (B)
fire doors. Common walls at points at which structures are to be attached be designed to
withstand a static pressure of at least 0.7 MPa per 0.1m shall have the following :
Rooms within structures shall be located in the first story and shall have at least one
exterior wall with unobstructed free vents for freely relieving explosion pressures.
Fire detection and firefighting requirements shall be as per local fire authority
requirements.
7-36
LPG CYLINDERS
Safety in LPG Design
8
FIRE PROTECTION
8.1
Passive and Active Fire Protection
In developing safety and fire protection guidelines for LPG facilities, the greatest
concern is failure of tanks containing LPG whether large tanks or domestic cylinders.
The probability of this type of failure can be made virtually negligible by properly
engineering and operating facilities, in accordance with the guidelines in this manual.
Although avoidance of such risks is of prime importance, it is necessary to protect
against emergency situations that can still occur.
Safety in LPG plant design is incorporated in two ways, by passive protection and by
active means of mitigation. Passive protection is achieved by adequate spacing, by
mounding or fireproofing, and by providing equipment with the appropriate electrical
area classification. Active means of mitigation are achieved by providing emergency
shutdowns, emergency block valves and an adequate firefighting system. Fireproofing
and firefighting equipment is discussed in this chapter. Electrical classification is
discussed in “Electrical Area Classification” in Chapter 2. Also an example of an
“Emergency Shutdown System” is shown in Chapter 2. Emergency block valves on
tanks and loading facilities are discussed in “Emergency Block Valves on Bulk LPG
Tanks” in Chapter 3 and “Emergency Block Valves for Piping” in Chapter 5. The
physical properties of LPG, which are important for understanding the safety and fire
hazard implications, are discussed in Chapter 12 “LPG PROPERTIES.”
Bulk LPG tanks at Refinery, Upstream, and Marketing plants shall be protected with the
application of firewater and/or fireproof insulation. A deluge or fixed spray system shall
protect aboveground spheres, which are discussed in this chapter. Aboveground bullets
shall be protected by a fixed spray system or fixed fire monitors which are capable of
reaching the whole bullet surface area. Insulation may be substituted for deluge or spray
systems, provided fire water is still available by at least a hydrant system. Generally
both fireproofing and a deluge/spray system are not used unless risk analysis shows a
need to further mitigate the effects of fire.
8.1.1
Fireproofing, a Passive Fire Protection
Fireproofing of structural supports, tanks, instrumentation and control tubing, and
valve actuators shall be considered when evaluating the fire protection systems for
LPG storage. The function of fireproofing is to reduce the rate of temperature
increase during fire exposure. The mechanism for reducing the rate of temperature
increase can be absorption of heat through chemical breakdown of the protective coating
or resistance to heating using thermal insulation, depending on the nature of the
fireproofing.
Safety in LPG Design
FIRE PROTECTION
8-1
As a passive fire protection system, fireproofing provides protection without depending
on detection systems or alarms. Mechanical or electrical failures have no effect on its
performance. Fireproofing also provides time to evacuate neighboring areas.
On the other hand, in a fire, fireproofing provides only a finite period of protection.
Without the addition of water, failure of fireproofed equipment will occur if the fire
burns long enough. Further, the type of fireproofing shall be selected carefully to ensure
compatibility with the storage area environment both from the standpoint of fireproofing
application or installation and long-term durability of the fireproofing.
Fireproofing can be used in combination with active water fire protection systems as a
means of protection until the water system is activated, as a back-up in the event the
supply of water is interrupted, or if the water application rate available is less than what
is desirable.
The fireproofing material shall provide equivalent to a fire endurance of 1.5 hours per
UL 1709. The fireproofing system shall be designed to withstand exposure of direct
flame impingement (as per GP-14-3-1) . It shall also be non-corrosive, inert under fire,
resistant to weather and hose streams.
8.1.1.1
Fireproofing Applications
Fireproofing practices for Refineries and Upstream facilities are described GP 14-3-1.
In Marketing LPG storage facilities, fireproofing shall be considered for use in the
following areas:
8.1.2
1.
Structural Supports: Fireproofing used on steel supports for LPG tanks can
prevent collapse in the event of fire. On spherical tanks and vertical
cylindrical tanks, structural supports shall be fireproofed from ground level to
the intersection of the support with the tank shell.
2.
With horizontal cylindrical tanks (bullets), steel saddles and vertical
support steel shall be fireproofed to the intersection of the saddle with the
tank if the saddle is greater than 300 mm height at its lowest point.
Exceptions for horizontal tanks below 7.6 m3 and vertical tanks below 0.5 m3
are discussed in NFPA 58. Thickness of the fireproofing material shall be
provided per “Fire Resistant Coatings” in this Chapter.
3.
Pipe Supports: Fireproofing shall be provided on all pipe supports within 15
m of a tank and on all pipe supports within the spill containment area.
4.
Tank Shell: If, based on substandard spacing in existing installations, a risk
assessment determines a higher risk, fireproofing of the tank shell may be the
only viable solution to alleviate the risk. For new plants it is required to
provide adequate spacing as per Chapter 2 “PLANT SITE.” and fireproofing
shall not be applied considered a solution take credit for substandard spacing.
5.
Emergency Block Valves: Actuators and cabling of Non-fail-safe valves
shall be fireproofed in order to function during the first 15 minutes of an
emergency. The valve body does not need fireproofing since it is fire safe by
design. Fail-safe installations do not need fireproofing since by definition the
fire is supposed to destroy the cabling or ducts and thereby cause short circuit
or pressure loss which in turn closes the valve.
Fire Resistant Coatings
There is a number of fire protective or insulating coatings available that could be
considered for use on an LPG tank. The factors involved in selecting such a coating
include:
8-2
1.
Intended application (i.e., stationary tank, tank truck or railway car).
2.
Amount of fire protection time required.
FIRE PROTECTION
Safety in LPG Design
3.
Use of water sprays and type of water.
4.
Required maintenance.
5.
Material's physical properties.
6.
Material's application properties.
7.
Atmospheric corrosivity.
8.
Weight limitations.
9.
Cost of economics.
With these factors in mind, short descriptions of various fire protective coatings and
thermal insulating systems follow.
8.1.2.1
Dense Concrete
Dense concrete or gunite have been used successfully for many years and are the
materials of choice in refineries, chemical plants and terminals. Concretes made with
Portland cement have a density of 2220 - 2380 kg/m3. Such concrete can be formed in
place or pneumatically sprayed to the required thickness using steel reinforcement such
as galvanized 14 US gauge steel mesh with openings of 50 mm by 50 mm. The mesh
shall be spaced the required distance from the substrate, usually half the thickness of the
concrete or gunite. See ExxonMobil Engineering Report EE.50E.88 for a recommended
formulation and mixture.
Concrete and gunite are durable and can be satisfactorily applied by most contractors.
The disadvantages of these concretes include relatively high weight, high thermal
conductivity, need for steel reinforcement and the installation cost and time involved in
forming them in place. The underlying steel shall be coated prior to applying the
concrete or gunite, and, also, in corrosive atmospheres, it shall be top-coated.
8.1.2.2
Lightweight Concretes
Non-proprietary lightweight concretes are made of lightweight aggregates such as
vermiculite and perlite and cements that are resistant to high temperatures.
Additionally, a number of lightweight cementitious concretes (such as Carboline's
Pyrocrete 241, et. al.) are available and have been used. Chloride-containing
lightweight concrete shall not be used because of their innate corrosion potential. Dry
densities range from 400 to 1270 kg/m3. Pneumatically applied material is about 20
percent heavier than lightweight concrete poured in place.
Lightweights are often sprayed, troweled or formed in place using reinforcing mesh.
The substrate shall be coated and reinforcement shall be required. A top coat for the
lightweight concretes is recommended to prevent moisture from penetrating; otherwise,
corrosion may occur, along with cracking and spalling in freezing climates.
Lightweight concrete materials are fairly durable and have limited maintenance
requirements. They are capable of withstanding direct flame impingement up to
1090 °C; they can withstand thermal shock and high-pressure hose steams; and they can
be satisfactorily applied by most contractors.
The disadvantages of lightweight concrete materials include the need to maintain a good
surface coating so that moisture cannot penetrate. Additionally, lightweight concretes
are also more susceptible to mechanical damage than are dense concrete materials.
8.1.2.3
Pre-formed Inorganic Panels
Pre-formed inorganic panels are pre-cast or compressed fire-resistant panels made of a
lightweight aggregate and a cement binder or a compressed inorganic insulating material
such as calcium silicate and perlite. The panels are attached to the substrate by
mechanical fasteners that are designed to withstand exposure to fire without appreciable
Safety in LPG Design
FIRE PROTECTION
8-3
loss of strength. An example of this type of material is COROC II and Hubilite
insulating panel boards.
When panels are used outdoors, an external weatherproofing system is usually required
to prevent moisture from penetrating. All joints shall be caulked or sealed with mastic.
Pre-formed materials have several advantages including: they can be applied cleanly;
there is no curing time; and they have low conductivity. One disadvantage of preformed materials is the necessity for labor-intensive application when these panels must
be fitted to tanks because of their diameter; therefore, they may not be practical.
8.1.2.4
Masonry Blocks and Bricks
Masonry blocks of lightweight blast-furnace slag (used as coarse aggregate) are
sometimes used. These units are laid up with staggered joints not more than 6 mm
thick. The joints shall be made using fire-resistant mortar, such as a mixture of 1 part
lime, 4 parts Portland cement, and 12 parts perlite.
Brick and block are no longer commonly used because of their high installation cost and
fairly extensive maintenance requirements. Brick-and-block assemblies tend to crack
and admit moisture, which can lead to serious corrosion and spalling. They should
provide adequate protection where currently installed, but should not be used in new
installations.
8.1.2.5
Subliming, Intumescent, Ablative Organic Coatings
Organic coatings (such as “Chartek” or “Pittchar”) can provide fire resistance through
one or more of the following mechanisms:
1.
Intumescent epoxies expand to several times their volume when exposed to
heat and form a protective insulating ash or char at the barrier that faces the
fire.
2.
Subliming mastics absorb large amounts of heat as they change directly from
a solid to a gaseous state.
3.
Ablative mastics absorb heat as they lose mass.
Organic coatings are sprayed on the substrate in one, two or more coats, depending on
the required thickness. Reinforcing fabric or wire is needed for application to LPG
tanks, tank trucks and railroad cars.
The main advantage of organic fire resistant coatings is that they are lightweight. They
are suitable for use on existing equipment supports that may not be able to handle
additional weight or are located in less accessible areas.
Because of complex application characteristics, the need for adequate film thickness and
proper bonding to the substrate, only vendor-approved, experienced applicators should
be employed. A disadvantage is that they may tend to shrink while curing;
specifications should therefore indicate the wet thickness that will yield the required dry
thickness. To ensure proper application and thickness, a qualified inspector should
frequently check the applicator's work. A manufacturer's representative is frequently
used to supervise the application and to serve as the inspector.
Substrate preparation and priming of the steel is important to adequate bonding.
Intumescent coatings require a top coat to prevent moisture from penetrating and
causing failure. The surface coating should be inspected and renewed according to the
vendor's recommendations.
The use of fire hoses on an intumescent coating during a fire may be detrimental; part of
the protective char may wash away. Also, coatings may be less durable than more
8-4
FIRE PROTECTION
Safety in LPG Design
traditional concrete materials when subjected to mechanical impact and abrasion.
Several coatings should not be used where equipment or piping must be steam cleaned.
See also GP 19-1-1 “Paints & Protective Coatings.”
8.1.2.6
Thermal Insulating Systems
Two thermal insulating materials are recommended not only as insulating materials but
also as fire resistant coatings. Suitable materials are listed in GP 14-3-1. A material like
foamed glass block is applied in two layers, each 50 mm thick, with staggered joints. A
steel jacket is also required. This steel jacket serves two purposes:
1.
In the event of a fire it holds the insulation in place and prevents it from
shattering and failing structurally.
2.
It provides protection from fire hose impingement during fire fighting
functions.
These systems are frequently used where thermal energy conservation is required; foam
glass can be designed and used for both cold and hot service. Poor fitting of the
cladding over any porous insulation or gaps in insulation blocks can lead to corrosion
under insulation discussed below.
Sphere
Watertight cover
to protect column
insulation
Figure 8.1.2.7: Corrosion protection of sphere columns
8.1.2.7
Corrosion Prevention behind Coatings
Experience has shown that improper protection of the steel under fireproofing or
improper application of the fire proofing can lead to corrosion behind fire protective
coatings. Concrete has been used extensively, and corrosion has been found frequently
behind concrete and gunite coatings. The causes for such corrosion have been
determined in several applications and include:
Safety in LPG Design
FIRE PROTECTION
8-5
1.
Improper surface preparation and application.
2.
Protective steel coating not applied when required.
3.
Inadequate concrete mixes (compressive strength as low as 103 - 138 bar).
4.
Inadequate rain shedding designs employed.
5.
Little or no maintenance after application.
6.
No external protective coating applied to fire resistant coatings.
Each of these factors in its own way can be applied to each of the insulating or fire
protective coating discussed above. It is important that each fire protective system be
properly applied and maintained. Special care shall be taken when designing
fireproofing for sphere columns. The upper end of the sphere column shall have an
inclined watertight steel girder which positively prevents the ingress of water between
concrete and column.
8.1.2.8
Selection of a Fire Resistant or Insulating Coating
For stationary LPG bullets or spheres the system of choice as noted previously is dense
concrete or gunite. Others have been used but, if an insulating system is required, then it
is recommended that foam glass block be applied. To protect foam glass from flame
and fire hose impingement, it shall be jacketed with a steel jacket.
Alternatives to concrete are lightweight cementitious coatings or proprietary lightweight
concretes such as Pyrocrete 241, which has been used successfully within the company
for a number of years. These require no protective jacket.
Coatings on tank trucks or rail cars are normally provided only when required by local
regulations. Intumescent epoxy coatings have been applied successfully on over-theroad tank trucks in some Far East locations for several years. When required for tank
trucks or railway cars, an intumescent epoxy “Chartek III” or PPG's “Pittchar” shall be
used, mainly because of their ability to absorb road shock damage.
8.1.2.9
What Thickness Is Required?
There is no easy answer to this question and it pertains to each of the fire protective
coatings discussed previously. There are several guidelines including the following:
1.
The most conservative guideline is API 2510 which states that the fire
resistant coating must give 1-1/2 hours protection in a UL 1709 fire when
exposed on a 10W49 steel beam. There is no indication if it is to be exposed
in a contour or box design, but for organic coatings and lightweights the
contour design is recommended.
2.
Global Practice 14-3-1 recommends that for a critical application to a 19 m3
capacity or greater tank, fireproofing with 38 mm of high density concrete or
gunite shall be used.
3.
Independent testing facilities, such as Germany's BAM F90 where test
requirements are less demanding than API 2510, require 90 minutes
protection for an LPG tank using venting LPG as coolant in a non-controlled
fire.
4.
Other National laws (in US and overseas) regarding both rail and truck
transportation require various criteria affecting thickness.
It is recommended that for LPG tanks or other stationery storage API 2510 be followed.
For truck tanks, there are no clearly defined procedures; therefore, it is recommended
that national rules be followed. Do not follow the advice of proprietary coating
manufacturers unless there is documented laboratory testing that such coating thickness
meets local or national law. Additionally, consider long term durability (10-20 years) in
8-6
FIRE PROTECTION
Safety in LPG Design
the atmosphere, resistance to mechanical damage, need for thermal insulation, corrosion
of the underlying steel, ease of repair, and others.
8.2
Fire Protection System Design Philosophy
Most LPG fires originate as smaller fires that have the potential to become larger and
more hazardous. Such fires may not occur as a result of tank failure, but because of
pump or piping leaks, or tank overfills. Human failure, such as overfill or improper
water draining, can also lead to an LPG release. Unless controlled, the leak can ignite
and the fire can escalate rapidly. A primary objective of the features described in this
section is to break that chain of events and control incidents at an early stage. See also
API Standard 2510A “Fire-Protection Considerations for LPG Facilities.”
Design for optimum safety of LPG facilities varies with differences in the size and type
of operations and in plant operating environments. The greatest concern is fire, and the
fundamental philosophy is as follows:
1.
Prevent releases and fires by proper design, sound operating practices, and
regular training of personnel.
2.
Provide a system of valves (Emergency Block Valve) that can be operated
locally or remotely, to isolate storage tanks and transfer equipment in case of
a leak, fire or other emergency.
3.
Provide a means of protecting LPG tanks and tanks from overheating in case
of fire.
The properties of LPG are such that a small release, if not controlled, can escalate very
quickly into a major event. Therefore, fire protection equipment shall be designed for
rapid actuation and for the largest credible scenario.
The first step in determining fire protection requirements is to divide the plant into risk
areas. DP XV-G, “Equipment Spacing” defines areas separated by 15 m of relatively
open space as separate risk zones. Fire can normally be contained to one risk area.
Spacing, firewalls, drainage, and emergency block valves are design elements that will
limit the spread of a fire and allow firefighting access.
For example, in a large plant the marine piers, truck loading, and cylinder filling areas
may be separated by spacing and segregated drainage. The figures for spacing given
in Chapter 2 “PLANT SITE” are considered adequate for this separation. A small
redistribution center would be considered one risk area.
Cooling by firewater is the basic fire protection for LPG distribution facilities. The
extent and capacity of the firewater system is based on the assumption that only one
major fire will occur at one time. Thus, the sizing and layout (Design Practices XV-J)
of major components of the system is based on the fire contingency at the risk area
having the largest requirements. The system shall be sized to provide firewater for the
fire area and for cooling all equipment in the vicinity of the fire area. Sizing guidelines
for Refining and Upstream facilities are discussed in Design Practices XV-I and GP 3-23. For Marketing terminals and bulk plants, the system configuration and spacing is
outlined in Chapter 2 “PLANT SITE” and sizing of the firefighting facilities is discussed
below.
While the main purpose of the firewater system is to provide cooling in case of fire it
shall be mentioned that monitor and hose streams, and water spray systems, can also be
used to disperse vapor releases.
The maximum firewater flow requirement is generally for the LPG tank area. Required
water rates for these and other risk areas are in “Firewater Pumps in Plant.”
Safety in LPG Design
FIRE PROTECTION
8-7
Fire protection for redistribution centers, for bulk customer and dealer installations,
shall be designed as required by local law. However, a risk assessment may require
higher than normal protection. This has to be decided case by case. Guidelines for
emergency planning can be found in the ExxonMobil Marketing Operations Guide
(Grey Book) “PLANT EMERGENCY PLANNING.”
Requirements in this chapter cover the basic needs of most facilities, but modifications
may be necessary for specific situations. Local regulations shall always be followed.
Equally important, fire protection equipment shall be suitable for the response capability
of the plant itself and of public emergency agencies and mutual aid partners.
Emergency planning shall reflect all of these factors. Development of those plans and
testing them with practice drills will often show where improvements can be beneficial.
8.2.1
Firewater System, an Active Fire Protection
Reliability is a primary consideration in the design and layout of the firewater system.
The system shall be designed for easy testing to assure dependability, adequate flow
rate, and adequate coverage of the protected equipment.
In a plant handling other products in addition to LPG, an integrated system is normally
provided. Firefighting foam may be available for gasoline or other fuels, but it shall not
be used on an LPG spill or fire. Application of foam will not control vapors or
extinguish an LPG fire; in fact, by adding heat, it is likely to increase the vaporization
rate with unpredictable and undesirable consequences.
Provisions for fire protection shall comply with the requirements of API STD 2510 and
API Publication 2510 A.
8.2.1.1
Firewater Source
A large body of unlimited water such as the sea, a lake, or a river is the preferred
firewater source. The suction pipe shall designed such that it is always fully submerged
irrespective of tidal conditions. Where this is not available, wells or a municipal
supply can be used if sufficient capacity and reliability are available. Municipal sources
used for drinking water shall be protected against contamination by positive means, such
as a break tank with the water inlet at the top above the maximum level. The fire water
system shall be suitably protected from freezing where necessary.
Where the source is limited, water storage shall be provided, sufficient to provide full
design flow for a minimum of 4 hours for Marketing Terminals and 6 hours for
Refining and Upstream Facilities without interrupting other essential users at the
plant. The difference between both is based on experience and the complexity of units.
Also, the water source must be capable of supplying one half the maximum firewater
demand on a continuous basis after the storage capacity has been used. Storage can be
in tanks, ponds, or reservoirs. Refining or Upstream facilities shall have a connection
between the cooling water system and the firewater system as an emergency back-up.
8.2.1.2
Firewater Pumps in Plant
The main firewater pumps shall have a total capacity no less than the demand of the
largest risk area. At least two pumps shall be provided, with independent power
sources for drivers. This may consist of one diesel plus one electric or, two diesel
powered pumps. Alternatively, the drivers of all firewater pumps may be electric
motors provided a backup diesel generator system capable of supporting all firewater
pumps’ drivers is available. The decision should be based on reliability of electrical
supply.
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FIRE PROTECTION
Safety in LPG Design
Rated capacity of any single pump normally does not need to exceed 50% of the total
requirement. For small terminals where the pump size is the same or smaller than local
fire trucks, consider providing connections so the fire truck can operate in parallel in the
event of failure of one of the pumps.
RAILWAY
RAIL CARS
Spray Water
Spray Water
Spray Water
Remote
Impoundment
Hydrant
Hydrant
Bullets
Deluge
System
MOUNDED
TANK
Sphere
Spray Water
MOUNDED
TANK
Earth Mound
Pumps
M o n i t o rH y d r a n t
Monitor
Hydrant
Propane Loading
Spray Water
Spray Water
Spray Water
Butane Loading
FIRE
Firewater Main Grid
WATER
TANK
FILLED
CYLINDERS
FIRE
PUMPS
Hose Reel
Spray Water
EMPTY
CYLINDERS
OFFICE
BUILDING
CYLINDER
FILLING
GATE
Figure 8.2.1: Typical Firewater system in a Marketing Terminal
Where support by local fire brigade is weak or questionable, three pumps, each
providing 50 percent firewater design capacity, would normally be considered to
provide sufficient reliability. The third pump would be available when one of the first
two is down for maintenance or fails in service. Likewise, two 100 percent pumps may
be specified. One would be "primary" while the other would serve as a spare. Pumps
are normally sized no larger than 568 m3/h but can be larger if a single firewater demand
exceeds this. When two 50% pumps are provided, the site shall have a plan for backup
protection during maintenance downtime of one pump. Upstream, due to the remote
nature of many facilities, requires 100% coverage at all times, i.e. two 100% pumps or
three 50% pumps. Diesel drivers shall match the power requirements of the pump.
Engines shall be equipped with a closed-circuit cooling system employing a water-
Safety in LPG Design
FIRE PROTECTION
8-9
cooled heat exchanger or a radiator with fan. It shall be noted that the horsepower
requirement is greater for a radiator with fan cooling system than for a water-cooled heat
exchanger system. This shall be carefully considered when determining the size of
engine needed. Water quality (hardness, corrosivity, sludge content) shall be evaluated
when determining the type of cooling system provided. Heat exchangers are prone to
plugging and failure under conditions of poor water quality and may prove to be
unreliable under such conditions. Enough fuel shall be on hand at the Diesel pumps to
operate at maximum fuel use capacity for four hours in Marketing and six hours in
Refining and Upstream Facilities.
The pumps shall have remote and local start capability. Primary fire pumps shall be
sequentially and automatically started by a drop in fire main pressure. Further pressure
drop shall automatically start the secondary fire pumps. Manual starting of the primary
and secondary fire pumps is acceptable provided that the facility is manned 24 hours and
proper procedure for fire water startup has been established. The staged pre-set timers
are typically set at 5 – 10 second delays.
Care shall be taken to ensure that the chosen system allows pumps to be tested with
minimal impairment of the total fire protection system. Additionally, the flowmeasuring device shall be located for accurate testing throughout the pump capacity
curve.
Figure 8.2.1.2-a: Firewater Diesel
System design depends on the combination of pumps and pipe sizing to deliver proper
pressure to spray systems, hydrants, and other users. At least 5.5 bar gauge pressure
shall be available at design rate at the farthest point in the mains from the pumps. Handheld hoses are difficult to control above 8 bar gauge. To stay within this operating
range, pump discharge pressure shall not be less than 8.6 bar gauge at rated
capacity; 10 bar gauge is a reasonable maximum.
A flat pump curve is necessary because a wide range of flow rates will be needed for
different contingencies. The pressure rise at shutoff shall not exceed 20%, and at 150%
of rated capacity the head shall not be less than 65% of design. The firewater rate will
depend on the exposure. Following rates from NFPA 2510A are used to calculate the
firewater requirements:
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FIRE PROTECTION
Safety in LPG Design
Tank Cooling (radiation):
Tank Fire Engulfment:
Jet Fire Impingement:
4.2 liters per minute per m2;
10.5 liters per minute per m2;
1000 - 2000 liters per minute; at point of contact
For a three sphere (20 m diameter) installation, the basis for the firewater rate would be
as follows: assume the central tank on fire. The two adjacent tanks would need cooling.
The calculation would be:
Tank area: π x 202 = 1256 m2
Tank on fire would need: 1256 x 10.5 = 13188 liters per minute
Adjacent tanks would need: 2 x 1256 x 4.2 = 10550 liters per minute
For potential jet fires add: 2000 liters per minute
Total firewater demand: 25783 liters per minute
For larger tanks credit may be taken for tank areas not under direct
flame radiation.
System pressure shall be controlled at the pump discharge by a pressure controller,
bypassing excess flow back to the water source. Higher pressures, and additional
controls, may be needed if the plant is very large or there are significant differences in
elevation.
Figure 8.2.1.2-b: Firewater pump
Continuous positive pump suction shall be provided. Priming devices are not
recommended because of concerns about reliability. This means that submerged vertical
centrifugal pumps are needed to lift suction from the sea or other waterway. Horizontal
centrifugal pumps are suitable only if the source is above the pump; for example, from a
tank. Suction screens or strainers shall be provided if foreign material is present which
could plug the suction lines or pumps. Either traveling or double removable screens,
cleanable with the pump in service, shall be used.
A flow meter is recommended, such as an averaging type pitot tube, to permit testing the
performance of each pump. The instrument can be located in the main firewater grid
piping, where it can also be used to measure flow to deluge and spray systems. It can
also be located in the bypass line back to the source.
A jockey pump shall be provided on a pressurized wet pipe firewater system to
maintain water pressure. The jockey pump shall be sized such that it will pressurize the
Safety in LPG Design
FIRE PROTECTION
8-11
firewater system to 690 kPa gauge in 2 minutes. Firewater main pressure shall be
monitored at a continuously manned location and alarm upon low pressure in the
system. The jockey pump is not considered as part of the overall plant firewater
capacity, although it may be used for other utility purposes. Shutoff valves shall be
provided on each pump discharge to permit maintenance work without disrupting the
firewater system. Shutoff valves shall be positive shut-off valves.
8.2.1.3
Firewater Distribution Piping in Plant
A grid or looped pressurized firewater system shall be provided, capable of supplying
water at the required rate to any part of the plant. The fire mains enclose each risk area
in a loop, and the loops are interconnected to form a grid. Isolation valves shall be
provided so that in the event of any single piping failure:
1.
No more than 300 m of pipe containing users (hydrants, hose reels, sprays,
monitors, etc.) can be lost; and
2.
The piping to only two adjacent sides of any risk area can be lost.
Piping within a risk area supplying more than two users shall be connected to two
separate sections of the fire main separated by a valve in the main. Lines to two or more
users shall also be valved at each end where they connect to the main.
Fire mains shall be sized by hydraulic calculations to supply required rates. A
minimum 150 mm nominal pipe size is recommended. Pipe flow velocities shall not
exceed 6 m/s in any area of the distribution system piping. In freezing climates, all
sections of the piping system that are normally filled with water shall be buried 300 mm
below the frost line. Firewater connections to monitors, hose reels, sprays etc. in
freezing climates shall be winterized. Above-grade sections shall normally be dry per
GP 3-2-3 para. 3.17. Piping material shall preferably be welded steel; flanged cast iron
may also be used, except in Upstream applications. Buried steel pipe shall be suitably
coated and wrapped for corrosion protection. Cement lined pipe shall be required for
salt water service, and is recommended for fresh water, to minimize corrosion. The
residual pressure at the hydrant outlet shall be the basis for determining the hydraulics
for the firewater piping system. The minimum residual pressure shall not be less than
690 kPa gauge.
Pipe which is one size larger than calculated required pipe NPS shall be used in
designing firewater piping system to deliver the specific flow rates since internal
corrosion and scale formation in unlined steel pipe may reduce the flow capacity over
time.
8.2.1.4
Firewater Deluge for Tanks
A water deluge is a system where water is applied at the top of a tank and allowed to run
down. Lines shall be at least 75 mm in size. In addition, a fixed spray system shall be
used to wet the lower hemisphere of the sphere. Where monitors are available which
can fully wet the lower hemisphere, they may be substituted for the lower spray system.
This type of system is most effective for spheres because the tank geometry assists in
evenly distributing the water. It can be built from large diameter piping that is not prone
to plugging and is more likely to survive an explosion. A disadvantage is that soot and
carbon from a fire may inhibit wetting the surface, particularly the underside of the
sphere. The lower spray systems, or monitors, are used to protect the underside. A
recent change in the GP-3-2-3 requires now firewater coverage of the lower hemisphere
If necessary, weirs shall be used to improve water distribution and prevent “dry spots.”
Especially the area behind columns needs careful ducting of deluge water. The
adequacy of the water coverage shall be determined by means of performance tests. If
water could accumulate behind weirs they shall be provided with drain holes to prevent
corrosion.
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FIRE PROTECTION
Safety in LPG Design
Water is usually applied over the tank by a single large nozzle at least 38 mm diameter,
with inverted bowl deflectors to direct the flow downward. Manholes, piping
connections, and ladders often interfere with uniform coverage, so weirs and baffles can
be used to improve distribution.
The system shall be manually operated from a safe location outside the spill containment
area and at least 15 m from the tank being protected. The location of the actuating valve
shall be prominently marked. In locations with limited manpower, the system shall be
remotely operable from a manned location. Deluge valves shall be designed to be easily
reset without removal of faceplate or other disassembly of the valve. Deluge valves
shall be designed to fail in the open position on loss of control power.
8.2.1.5
Firewater Sprays
Water spray systems consist of a network of small spray nozzles, arranged in rows or
grids over the tank or equipment being protected. This type of system is more
susceptible to plugging, which results in reduced and unequal water application. Water
spray systems are also more susceptible to damage from an explosion than deluge
systems or monitors.
Figure 8.2.1.5-a: Large bore firewater vortex spray nozzle
A spray system is normally dry, with open nozzles, and is operated by a single valve.
The system shall be manually operated from a safe location outside the spill containment
area and at least 15 m from the equipment being protected. The location of the actuating
valve shall be prominently marked. In locations with limited manpower, the system
shall be remotely operable from a manned location or may be activated automatically.
A skilled and experienced specialist, using hydraulic calculations should design systems;
otherwise it is likely that flows will not be uniformly distributed. Several features are
needed to minimize plugging. A strainer with a valved 50 mm blow-off connection
shall be installed in the main feeder pipe to the sprays. Mesh size shall be 50% or the
spray nozzle diameter. Nozzles shall have a minimum orifice diameter of 13 mm;
diffusers or deflectors to form the spray pattern shall be external. The maximum
openings of the strainer shall be 6 mm and the ratio of free screen area to pipe crosssectional area shall be no less than 3:1. Water distribution within the nozzle discharge
Safety in LPG Design
FIRE PROTECTION
8-13
pattern shall be uniform; hollow-cone patterns shall not be used. The pattern shall be
reasonably unaffected by changes in water flowing pressure within the anticipated
pressure range. Nozzles shall spray well-dispersed droplets throughout the discharge
pattern, with approximately 85 percent of the droplets ranging from 200 – 400 microns
in diameter. A waterspray nozzle pressure of not less than 414 kPa is acceptable.
Carbon steel piping downstream of the strainer shall be internally galvanized, and flushout connections shall be provided. Corrosion-resistant stainless steel and copper-nickel
alloy is preferred to galvanized pipe, although they are more expensive.
Spray systems shall be designed as shown in Figure 8.2.1.5-b. If at all possible flush
valves should be installed at the end of the header which can be opened before each test.
In cold climates the system must be self draining.
Spraywater
Settling rust and debris
Figure 8.2.1.5-b: Spray nozzle design minimizing plugging
Spraywater
Settling rust
and debris
Figure 8.2.1.5-c: Unfavorable spraywater design leading to plugging.
8.2.1.6
Hydrants in Plant
A sufficient number of hydrants shall be installed to supply the required water rate to
each risk area. They provide backup protection in case a primary system such as a
deluge or water spray is disabled. Hydrants can provide water to pumper trucks, mobile
monitors or hand-held hoses, directly from the plant firewater system. Depending on the
number and size of hoses, a hydrant can supply 130 to 170 m3/h of water. When a
pumper truck is used, it can boost the pressure, increasing flow to 250 m3/h.
Hydrants shall be located within 50 m of any point where water will be required.
However, they shall be accessible in emergencies, and shall be along roadways and not
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FIRE PROTECTION
Safety in LPG Design
within risk areas. Maximum spacing between hydrants is 90 m, measured along roads
or accesses ways. Hydrants shall be located on at least two sides of each LPG tank so
the tank can be reached by at least 3 streams of water from hose lines not longer than
90 m.
Hydrants shall be 102 mm diameter with two 65 mm hose connections. Each hydrant
shall have two valved and capped hose connections, and a larger valved and capped
connection to supply a pumper truck. The length of suction hose required shall not
exceed 7.5 m. Connections shall be compatible with municipal and mutual aid
firefighting equipment. Hydrants shall be self-draining in freezing climates. The steel
hydrant barrels shall be hot-dip galvanized after welding. Unless a local fire department
hose connection is specified male hose thread, 65 mm to NST (refer to NFPA 194) may
be used.
8.2.1.7
Firewater Monitors may be Fixed or Mobile
Fixed monitors provide practical and flexible firewater coverage, and shall be
considered if emergency response forces are limited. A monitor can be quickly aimed,
activated and locked in position by a single person, who is then free for other tasks. A
monitor has an effective range of about 30 m, and if strategically placed can protect two
or more risk areas. Monitors can be bolted directly to hydrants with no additional
piping. They shall be accessible in an emergency, between 15 and 30 m from the
equipment protected. Depending on ship size, monitors on piers may have to be
remotely operated.
Monitors shall be of brass or bronze construction, with double ball bearing swivels and a
locking device. Nozzles shall be about 110 m3/h capacity, adjustable for fog or straight
stream type.
In terminals, tank truck and tank car loading and unloading positions shall be protected
by fixed water monitors with adjustable fog-to-straight-stream nozzles located on each
side of the loading and unloading installations. Installation of fixed water monitors shall
be considered at large or high-risk consumer installations.
Mobile monitors shall have the same capabilities as described above but are more
flexible because they can be moved around to cover larger areas. However, they require
more time and manpower to deploy. The monitor shall be furnished with two hose
connections, and two 15 m lengths of hose, which can be stored on the trailer. Each
connection requires a check valve to protect against a burst hose. For rapid deployment,
hoses shall remain connected to a hydrant or valved outlets from a firewater main.
8.2.1.8
Internal Firewater Flooding of Tanks
Pressurized LPG storage at refineries and upstream gas plants shall be provided with
water flooding facilities to inject water and displace LPG in the lower portion of LPG
tanks, in the event of a tank leak, per GP 3-2-3 para 9.4. Marketing facilities may
provide connections if required by risk considerations. The key to such an installation is
to have water pressure available to overcome the sphere pressure. In the case of
Propane or Propane rich storage, or sometimes Butane storage, higher pressure water is
provided via a pumper truck.
8.2.1.9
Hoses and Hose Reels
Fire hoses are for use by plant or municipal brigades, usually as backup for deluge and
spray systems and monitors. Two to four men may be needed to handle a typical 65 mm
size hose. The time needed for deployment can be reduced if hoses and nozzles are
available at the plant. Hoses shall be stored in cabinets because they can be deteriorated
by solar radiation. Nozzles shall be constant-volume, adjustable combination straight
Safety in LPG Design
FIRE PROTECTION
8-15
stream/fog types, made of brass. Live hose reels shall be permanently connected to the
fire main system.
One person can put hose reels into action more quickly than the larger hoses where a
team is required. Fixed hose reels typically hold 30 m of 32 to 38 mm hose of the firm
type that can be stored on the reel without collapsing.
8.2.1.10
Drainage Against Area Flooding During Firefighting
Spills of LPG liquid (and heavier-than-air vapor releases) shall drain away from product
storage, transfer areas, and buildings. It is also important to avoid accumulation of
water where it could interfere with emergency actions. The drainage system for each
individual risk area shall be sized for the maximum rate of rainfall, or design firewater
rate, whichever is greater. Plant-wide drainage capacity is usually set by the maximum
rainfall rate, assuming this exceeds the firewater requirement for the largest risk area.
8.2.1.11
Dry Powder Fire Extinguishers
The preferred strategy for fighting an LPG fire is to isolate the fuel at its source and to
cool exposed equipment. LPG fires shall not be extinguished unless the fuel supply can
be isolated; otherwise a vapor cloud may form and create a greater hazard.
However, portable fire extinguishers shall still be provided in LPG plants, primarily to
protect against fires involving other materials. In plant areas outdoors, type BC or ABC
dry chemical extinguishers are most suitable. Portable extinguishers with about 9 kg of
agent can readily be carried and used by one person. Typically two extinguishers shall
be provided at strategic locations throughout plants. Examples for such locations are:
marine piers, truck and rail racks, cylinder filling sheds, pumps, compressors; and
redistribution centers.
Larger wheeled units with about 70 kg of agent are more powerful, but normally two
people are needed for handling and maneuvering. They shall be placed near truck
loading racks, and can be considered on piers unless congestion would limit their
mobility.
Dry chemical extinguishers are also effective indoors, but the agent is messy and can
damage electrical and electronic equipment. Carbon dioxide extinguishers are
recommended for substations, computer rooms, and similar locations with sensitive
instruments and switch gear.
Halon extinguishers (type BC) are no longer
recommended because of environmental concerns. Pressurized water extinguishers can
be considered for offices where combustible materials such as wood, paper, and plastic
are the main sources of fuel. Extinguishers for use indoors shall be mounted near exit,
and shall be of a size that can readily be carried around in the building.
8.2.2
Protection Requirements
8.2.2.1
Aboveground Tanks – Bullets and Spheres
Spheres shall be protected with either a water deluge system, per GP 9-2-1 (preferred),
or a fixed spray system discussed later in this chapter. Water deluge systems are not
recommended for horizontal bullets, or for protection of transfer operations which shall
use firewater sprays. Although fireproof insulation may be substituted for deluge or
sprays provided fire water is also available, generally deluge and sprays are preferred.
Spray systems shall be used on horizontal LPG bullets, cylinder filling and storage
areas, and truck and rail loading/unloading areas. For horizontal bullets, a fire water
monitor system capable of wetting the whole bullet surface may be substituted for
sprays. If three or more tanks are closely spaced, less than 15 m shell-to-shell, they shall
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FIRE PROTECTION
Safety in LPG Design
be fireproofed if a fire water monitor system is used, per GP 14-3-1. This is because the
close spacing will make fire water coverage more difficult.
At least two portable fire extinguishers each having a minimum capacity of 9 kg of dry
chemical with a B: C rating shall be provided. Emergency controls, if provided, shall be
conspicuously marked, and the controls shall be located so as to be readily accessible in
emergencies.
8.2.2.2
Buried/Mounded Drums
Mounding or burial of storage drums shall be considered as adequate protection without
further need of firewater for these tanks. For mounded drums, the sand shall be
stabilized with a layer of grout or other material to prevent erosion from rain or
firewater hose streams. At least two portable fire extinguishers each having a
minimum capacity of 9 kg of dry chemical with a B: C rating shall be provided.
Emergency controls, if provided, shall be conspicuously marked, and the controls shall
be located so as to be readily accessible in emergencies.
8.2.2.3
Pump and Compressor Stations
A live hose reel shall be provided covering the entire area of the pump and compressor
area with 30 m long hose. Live hose reel shall be permanently connected to the fire
main system and the nozzles shall be complete with shutoff ball valves. Adjustable fogto-straight- stream nozzles shall be provided on live hose reels equipped with 30 m of 32
or 38 mm fire hoses. At least one portable fire extinguishers having a minimum
capacity of 9 kg of dry chemical with a B: C rating shall be provided. Emergency
controls, if provided, shall be conspicuously marked, and the controls shall be located so
as to be readily accessible in emergencies.
8.2.2.4
Truck Loading and Unloading Facilities
Tank truck and tank car loading and unloading positions shall be equipped with dry
chemical fire extinguishers as follows: one 15 kg dry chemical extinguisher for each
four positions. Since the only safe way to extinguish an LPG fire is to shut off the
product supply, dry chemical fire extinguishers are required for other purposes, such as
small spills or burning materials after the product has been shut off. At consumer
locations, tank truck unloading positions shall be equipped with dry chemical fire
extinguishers.
Either a water spray system or fixed water monitors shall be designed for truck
loading and unloading area. Sufficient number of 30 m long live hose reel shall be
provided to cover the entire area of the truck loading/unloading area. Live hose reel
shall be permanently connected to the fire main system and the nozzles shall be
complete with shutoff ball valves. Adjustable fog-to-straight-stream nozzles shall be
provided on live hose reels equipped with 30 m of 38 mm fire hoses.
At least two portable fire extinguishers each having a minimum capacity of 9 kg of dry
chemical with a B: C rating shall be provided for the truck loading/unloading area.
Emergency controls, if provided, shall be conspicuously marked, and the controls shall
be located so as to be readily accessible in emergencies.
8.2.2.5
Vaporizer
At least one portable fire extinguisher having a minimum capacity of 9 kg of dry
chemical with a B:C rating shall be provided. This requirement is optional if the
vaporizer is installed near LPG tanks where fire extinguishers are already provided.
Safety in LPG Design
FIRE PROTECTION
8-17
8.2.2.6
Cylinder Filling Plant
Water spray system shall be designed for filled cylinder storage area. Sufficient
number of 30 m long live hose reels shall be provided to cover the entire area of the
cylinder filling plant. The hose reels shall be permanently connected to the fire main
system and the nozzles shall be complete with shutoff ball valves. Adjustable fog-tostraight- stream nozzles shall be provided on live hose reels equipped with 30 m of 32 to
38 mm fire hoses.
At least two portable fire extinguishers each having a minimum capacity of 9 kg of dry
chemical with a B: C rating shall be provided for the LPG filling plant. Emergency
controls, if provided, shall be conspicuously marked, and the controls shall be located so
as to be readily accessible in emergencies.
8.2.2.7
Cylinder Storage
All storage areas shall clearly display a warning notice with the words “LPG STORE.”
Warning signs “STOP MOTOR”, “NO SMOKING”, “FLAMMABLE GAS” shall be
posted at all LPG cylinder storage area. The locations of the signs shall be determined
by local conditions, but the lettering shall be large enough to be visible and legible.
Sufficient number of 30 m long live hose reel shall be provided to cover the entire area
of the cylinder storage area if the water capacity of the total LPG cylinders stored
exceeds 15 m3. Live hose reel shall be permanently connected to the fire main system
and the nozzles shall be complete with shutoff ball valves.
Adjustable fog-to-straight-stream nozzles shall be provided on live hose reels equipped
with 30 m of 38 mm fire hoses.
Fire-fighting foam shall not be used for LPG fire.
At least two portable fire extinguishers each having a minimum capacity of 9 kg of dry
chemical with a B: C rating shall be provided for the cylinder warehouse.
8.2.2.8
Cylinder Maintenance Shed
Sufficient number of 30 m long live hose reels shall be provided to cover the entire area
of the cylinder maintenance.
At least two portable fire extinguishers each having a minimum capacity of 9 kg of dry
chemical with a B: C rating shall be provided for the cylinder maintenance shed.
8.2.2.9
Firewater at Marine Piers
Firefighting monitors shall be installed at the berth, primarily to provide
coverage/cooling for the berth manifold area and the cargo transfer equipment. They
may also provide a method for dispersing LPG vapors in the event of an accidental
release. Berth monitors shall provide assistance to the vessel's firefighting system and
shall be able to cover the vessel's manifold area.
In addition, berths receiving vessels in the international trade shall have ISGOTT fire
water connections installed at the berth to provide the vessel with fire water in the event
of an emergency. Fire water connections for a fire boat to connect and increase the
berth's firewater capacity shall be considered. If firewater connections are installed,
they shall be located at least 60 m away from high risk areas. Detailed description of the
firewater requirements and firefighting equipment requirements for marine terminals are
provided in EMRE's report EE.5TT.81 “Fire Protection and Safety Guidelines for
Marine Terminals.”
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FIRE PROTECTION
Safety in LPG Design
The following fire protection shall be provided:
8.2.3
Installation
Requirements (Minimum)
Barge pier or wharf for
transfer of Class I products,
or any product heated above
its flash point, and Class I
products in drums
Fire main and hydrants with firewater supply
of at least 340 m3/h
Portable fire protection including monitors
and portable fire extinguishers
Flammable Gas Detectors
Flammable gas detectors may be provided as early warning. Installations shall be based
on risk considerations. Plants close to housing that may be unmanned during the night
would need higher protection than plants in uninhabited areas.
A minimum level of leak detection is already called for on all new pump seals. The
integrity of the pump seal can be further upgraded to one of the higher sealing categories
described in Chapter 4, “Shaft Sealing,” to both reduce the chances of pump seal failure
and provide better containment should the pump seal fail. In addition to pump seals,
new compressor seals, depending on risk considerations may also be provided with a
leak detection device.
Leak detection may also be provided around loading/unloading locations, which do not
have continuous surveillance, and around LPG tanks, through the use of flammable gas
detectors. Decisions to provide this additional monitoring would typically be made as a
result of a risk analysis. The analysis would typically consider local conditions, the
proximity of the storage to the fence line and populated areas or to process equipment
and ignition sources, and the volume of storage or frequency of loading/unloading
operations. When flammable gas detectors are applied to loading/unloading points, the
design shall be tolerant of hydrocarbon that will be present around the transport vessel
due to breaking connections. If not, false alarms will render the system ineffective by
frequent nuisance alarms. Additional detail on flammable gas detectors is provided in
Design Practices Section XV-K, “Flammable Gas, Toxic Gas, and Fire Detection
Systems.”
LPG detection system shall be designed as “PRE-FIRE” early warning system. As such,
automatic water application system (i.e. deluge, water spray) shall not be activated by
LPG detection system alone.
Open path detectors are now reliable and a few instruments can cover large areas. To
detect leaks from groups of equipment or facilities within the same line of sight, openpath gas detects shall be installed. For example, open path gas detectors can be used to
detect release of LPG from a row of pumps or compressors. Their signal may be used to
activate the Emergency Shutdown System. An audible alarm is needed at the control
room and, if the control room is not permanently manned, also a local audible alarm.
Some local regulations always require local alarm. During times when the plant is not
manned, the alarm may be transmitted to a security company.
Also point detectors are acceptable. They may be installed to detect leaks from
individual release sources. Individual release sources includes pump seals, compressor
seals, flanges, safety relief valves venting to atmosphere, sewer vents and small
pipes/connections which are prone to failure due to vibration or corrosion. One point
detector may be used to detect gas leaks from several individual release sources which
are located nearby. However, the coverage or sensing area of the point detector must
not exceed those specified by the manufacturer. If coverage area is too large for one
detector, additional point detectors shall be installed.
Safety in LPG Design
FIRE PROTECTION
8-19
Detectors shall initiate an alarm at the following:
1.
A measurement of 20 –2 5 percent LEL - at least 100 decibels audible and
visual alarms. The alarm condition shall not be considered cleared until the
specific detector reading has dropped below 20–25 percent LEL. Operator
response shall be required to clear the audible and visual alarms.
2.
A measurement of 50 – 60 percent LEL – activate an emergency ALARM
and at least 100 decibels audible / visual alarms. The alarm condition shall not
be considered cleared until the specific detector reading has dropped below
20 – 25 percent LEL. Operator response shall be required to investigate cause
of emergency, take corrective action before clearing the audible, visual alarms
and resetting the emergency ALARM system.
These detector shall sound alarm at the site and at a constantly attended location if the
site is not continuously manned.
The detectors shall be installed not more than 100 mm above grade level. At least one
gas detector shall be installed at each of the following locations:
1.
LPG tanks.
2.
Truck loading/unloading rack.
3.
Cylinder filling shed.
4.
Jetty - near manifold flanges.
Gas detectors shall be positioned based on site conditions and requirement taking into
account of manufacturer’s recommendation.
8.2.4
Fire Detectors
Minimum requirements can be supplemented with additional fire protection, depending
on each plant's situation. Factors which may make this desirable include limited
emergency response capability; proximity to populated areas or public roads; risk from
adjacent facilities; future development of the area; availability of utilities; and
topography of the site. Fire detectors are an appropriate design feature to provide early
warning thus reducing risks. Additional fire detector details are provided in Design
Practices Section XV-K “Flammable Gas, Toxic Gas, and Fire Detection Systems.”
Numerous mechanical and electronic devices are available to detect the environmental
changes created by a fire. The most sophisticated type is the optical flame detector,
which “sees” a fire directly, without depending on air currents to transport smoke or hot
gases to a sensor. This is a significant advantage outdoors. Flame detectors can be
useful in emergency response by reducing the time interval between the outbreak of a
fire and its control, which is important in an LPG installation.
When an optical device is desired, the infrared (IR) detector designed specifically for
hydrocarbon fires is recommended. IR detectors are more reliable and less vulnerable to
false alarms than ultraviolet (UV) detectors. The ability to discriminate between fires
and false alarms can be improved by adding a second sensor, either IR or UV.
Considerable care and expense are needed in design, installation, and maintenance if
flame detectors are to be beneficial. A detector can only save time in identifying an
emergency, and there may be better ways to do it. Flame detectors shall be considered if
all the following conditions exist:
8-20
1.
Continuous surveillance not feasible with available personnel.
2.
Early warning by other means not feasible: gas and seal leak detectors.
3.
Capability exists for prompt response: fire suppression, shutdown, and
isolation, whether automatic or manual.
FIRE PROTECTION
Safety in LPG Design
4.
Available technical expertise for equipment selection, design, installation.
5.
Ongoing maintenance support available.
All flame detectors have a wide field of view, typically around a 90° angle. This
suggests a small number of detectors could cover a large area, but there are other factors
to consider. Response is not instantaneous, because the detector to satisfy its design
criteria for a fire must accumulate enough information. The sensitivity and response
time of a detector depend on fire size, distance, and the position of the fire in its field of
view. And as distance increases, obstructions become more of a problem, and false
alarms become more likely. Requirements for audible alarms are as discussed under
“Flammable Gas Detectors.”
Heat sensing detectors, also discussed in DP XV-K, are an alternative to optical flame
detectors. Response from heat sensing detectors is slower, but false alarms are reduced
compared with optical detectors. Two types of point heat sensing devices, pilot heads
with fusible plugs and nylon tubing, have been used with success to automatically
actuate deluge or spray systems. When fire detectors are deployed at locations with
spray or deluge systems, the site shall evaluate automating the deluge/spray system with
the fire detection. In the case of LPG tanks, prompt actuation of deluge/spray systems
will reduce any BLEVE potential.
Safety in LPG Design
FIRE PROTECTION
8-21
9
TRANSPORTATION
9.1
Means of Product Movement
This chapter discusses the various types of equipment used to convey product between
refineries and marketing terminals, from terminals to distributors, or from terminals to
end users. Ship or rail car normally transports large quantities while road transportation
dominates transportation in smaller parcels. The LPG is transported in pressure tanks,
which are mounted to ship, rail car or truck. Ships are also often equipped with
refrigerated tanks.
The discussion in this section cites basic, generally accepted standards for equipment
design that will provide satisfactory LPG transportation if more specific national
standards do not exist. Need for “custom design” can be avoided since vendors have
standardized equipment designs for tank trucks, portable containers and rail tank cars.
General guidelines are defined to assist in selecting the most efficient truck power trains
and suspensions for various types of service.
9.2
Road Bulk Transportation Equipment
Two classifications of road transport are considered:
9.2.1
1.
Large capacity transports for point-to-point product delivery from a supply
source to a single destination.
2.
Smaller route delivery trucks (Mini-bulk) designed to load at a terminal
and deliver product to a number of end users efficiently grouped on a
periodically served loop route.
Truck Design and Procurement
As a guide for the design, fabrication, testing, and inspection of a road transport unit,
reference DOT (ICC) Specification MC331, “Cargo Tanks Constructed of Steel
Primarily For Transportation of Compressed Gases” and NFPA 58, Section 6 “Vehicular
Transportation of LP-Gas” shall be referenced. All welded piping on truck transports
shall be fabricated and tested in accordance with ASME Code for pressure piping,
Section 3, Petroleum Refinery Piping B31.3.
LPG road transport units may be in the form of a single tank mounted on a semi-trailer
with the motive power and a portion of the load being carried by a tractor. In certain
areas, motor vehicle restrictions and/or road conditions may necessitate the use of a unit
consisting of a tank mounted directly on to a truck frame. The primary design goal is
transportation of the maximum legal payload safely and efficiently by minimizing the
weight of the vehicle and tank. Local regulations will determine the configuration that
Safety in LPG Design
TRANSPORTATION
9-1
best achieves this objective. To select the most efficient and safest vehicle for a given
location, procurement has to consider terrain, weight regulations, road characteristics,
frequency of stops, round trip mileage and local tax structures, and speed limits as
follows:
1.
Hilly terrain and steep grades will require comparing the advantages of
larger engines versus transmissions with more forward speeds, considering
total drive train cost and vehicle weight effects. Engine weight will affect
payload. Two-speed rear axles are another (somewhat costly) possible design
solution. More braking capability will also be required.
2.
Stringent weight limits may justify high cost light weight components such
as fiberglass cabs and aluminum wheels; however, rough road surfaces may
make affect light weight components such that they fail prematurely.
3.
Rough road surfaces may warrant investing in more sophisticated tractor
suspensions, and shock absorber-mounted orthopedic seats for drivers, to
reduce driver fatigue and maximize driver safety and efficiency.
4.
In congested traffic conditions with a high percentage of stop and go driving,
engine speed versus torque characteristics will affect the amount of gear
shifting required, and correspondingly, driver efficiency. This may warrant
truck automatic transmissions, but their advantages should be weighed against
increased unit cost, lower fuel efficiency, and loss of some payload capacity.
5.
Cost and spare parts availability, and the ability to provide adequate
maintenance should also be considered.
6.
In several countries Diesel engine powered trucks are mandatory. However,
with all the electronics now built-in modern Diesel trucks they may serve as
ignition sources as do gasoline driven vehicles.
The vendor should offer at least two power train options for each truck proposal, and
these should be supported by geared speed versus engine speed charts.
9.2.2
Basic Design Considerations
Before selecting a specification for an LPG road transport unit, local regulations shall be
explored thoroughly since regulations from the following authorities may have to be
taken into account:
1.
Authority concerned primarily with pressure tanks.
2.
Motor vehicle authority.
3.
Weights and measures authority.
4.
Agency primarily concerned with the transportation of a hazardous
commodity.
5.
Authorities responsible for the operation of bridges and tunnels.
It is recommended that Specification MC 331 and Division III of NFPA 58 be utilized as
a guide for the design, construction, inspection, and testing of road transports in the
event that a mandatory specification does not exist. Specification MC 331 embodies a
tank designed and constructed fulfilling the requirements of the ASME Boiler and
Pressure Vessel Code, Section VIII for unfired pressure vessels. Tanks intended for
static storage must not be used for deliveries.
Tanks shall have metal tank data plates welded to the tank in a conspicuous and
accessible place with vessel code and class to which it is made, manufacturer name and
tank serial no, water capacity, minimum and maximum working and design pressures,
date of original and subsequent tests, operating temperature ranges.
9-2
TRANSPORTATION
Safety in LPG Design
9.2.2.1
Working Pressure on Truck Tanks
According to Specification MC 33,1 the design pressure shall not be less than 6.9 bar
gauge nor more than 34.5 bar gauge. NFPA 58 limits are more restrictive. It is
recommended that a working pressure of 17.25 bar gauge be adopted for all LPG road
transport units. This working pressure would allow use of the vehicle for Propane,
Butane and their mixtures. If there is a long range use for a transport vehicle exclusively
for Butane it may be designed for 10.75 bar gauge. In addition, the design is to take into
account the allowance for vehicle acceleration and deceleration both horizontally and
vertically. Acceleration and deceleration shall be assumed as 1 g (9.81 m/sec2)in
direction of travel, and 2 g in the transverse horizontal direction, vertical acceleration
both upwards and downwards at 5 g.
9.2.2.2
Tank Openings and Valves
Tank nozzles and valves are fitted internally, recessed into the tank shell or positioned
so to minimize the risk of impact damage and to prevent unauthorized access. NFPA 58
requires that tank fittings and appurtenances be protected against damage by either: their
location, e.g. behind the vehicle frame or bumper, a protective housing or recessing.
The following indicates the opportunities available.
Pressure relief valves may be of a recessed type in which working parts of the valve do
not extend beyond the shell of the tank. As an alternate, an internal spring-type safety
relief valve may be installed within a well or recess lowering all working parts below the
surface of the shell. PRVs on transportation tanks shall be tested or replaced every 5
years. Where local codes explicitly permit longer testing times, they may be followed,
up to 10 years. In some countries (Central Europe) the installation of PRVs on trucks is
prohibited and the tanks are designed for the maximum pressure specified by local
codes.
Gauging devices, such as visible float gauges, rotary gauges, fixed liquid level gauges,
and pressure indicators, may be installed within recesses. The pressure gauge must be
connected to the vapor phase.
Liquid or vapor connections necessitate the use of pipe, valves, etc. to extend beyond
the primary valve installed within the tank flange or coupling. Internal Excess Flow
Valves on all liquid outlet lines and vapor return lines The accidental removal of the
exterior portion of the valve will, in most cases, permit the valve element within the tank
to remain closed, or if open at the time of an accident, will effect automatic closure. The
liquid inlet line shall be protected by a backflow check valve. All filling and discharge
connections to tank to be provided with quick closing internal valves, and a manual or
automatic shut-off valve. All inlets and outlets must be labeled to designate function.
All liquid and vapor lines shall be capable to be closed by a Truck Emergency Shutdown
System. Shutoff shall be manual at two points at the front and rear end of the truck tank
and by a remote handheld device carried by the driver. Activation maybe electrical or
by radio.
NFPA 58 also requires certain valve arrangements on vapor and liquid openings to
prevent excessive discharge of gas in the event a connection is accidentally broken. For
vapor and liquid withdrawal openings either a shutoff valve located as close as possible
to the tank in combination with an excess flow valve in the tank, or an internal valve
with excess flow protection shall be required. For vapor and liquid inlet openings either
a shutoff valve located as close as possible to the tank in combination with a back-flow
check valve or an excess flow valve in the tank, or an internal valve with excess flow
protection shall be required. From all the NFPA options, liquid inlets are recommended
to have only back-flow check valves and not excess flow valves. Vapor inlets are
recommended to have excess flow valves, as check valves would not allow for liquid
withdrawal in the event of an overturn.
Safety in LPG Design
TRANSPORTATION
9-3
The control mechanism for self-closing internal valves shall be arranged with an
interlock so the forward motion of the vehicle will release the valve holding mechanism
and cause the valve to close shall the valves be accidentally left in the open position.
This may be accomplished through an interlock with the brake system or the ignition
system. An interlock with the brake system is recommended. There are a number of
packaged control devices available for this application.
9.2.2.3
Piping, Tubing, and Fittings on Truck Tanks
It is recommended that only Schedule 80 pipe be utilized if the joints are welded or
welded and flanged. This shall conform to ASTM Specification A-106, Grade B, or
equal. This is more stringent than section 3.3 of NPFA 58, which permits Schedule 40
piping.
All valves, fittings, pumps, pipework and accessories to be located behind under-run
protection bars or be located as such, that they are protected to minimize risk of damage,
or leakage in a vehicle accident or roll-over. Installations where the pump is not
mounted directly on the liquid discharge connection shall be arranged with a minimum
length of suction pipe sloping downward from the outlet to the pump suction without
irregularities or pockets.
Figure 9.2.2.3: Lifting lugs on tank trucks will help should recovery be needed
Threaded joints shall be minimized. On any vehicle the use of threaded joints shall be
reduced to an absolute minimum. The maximum size thread tolerated on a truck
installation is 32 mm. All welding fittings (welded fittings to be socket weld) shall be
compatible with Schedule 80 pipe. All threaded fittings shall be extra heavy forged
steel (3000#). All threaded connections shall be secured with pipe joint compound
specifically approved for liquid LPG. The use of Teflon tape is discouraged since
during a fire it will melt and create additional leakage and add fuel to the fire. Note that
this recommendation is different from that given for cylinder valves since those are of
low melting material anyway.
All welded piping shall be fabricated and tested in accordance with ANSI Code for
pressure piping, Section 3, Petroleum Refinery Piping B31.3.
9-4
TRANSPORTATION
Safety in LPG Design
Ideal designs minimize the need for flexible connections by attaching as much
piping/equipment to the tank as possible. If piping or equipment is attached to the truck
chassis provisions shall be made to compensate for stresses and vibrations in the
piping system. The piping system configuration and its restraints shall provide
flexibility.
The filling connection shall lead directly into the vapor space of the tank. Spray filling
will lower the pressure in the tank and does not have the adverse effects associated with
fuels spray loading. If two filling connections are utilized, at least one connection shall
be fitted with a tube to the top of the tank in order to facilitate the final filling of the
tank.
The manhole in the tank shall have least 610 mm in diameter. Tank must have the
lowest possible center of gravity which when fully loaded, does not exceed 1.75 meters.
Overall stability is important and a static tilt angle of 25 degrees must be possible when
fully loaded.
9.2.2.4
Anti-Surge Baffles in Truck Tanks
An LPG tank, when in transit, is only partially filled with liquid; therefore, relatively
severe “loading” may develop as a result of the movement of the cargo. The possibility
of including anti-surge baffles within the tank shall be reviewed with the transport
supplier, considering local operating conditions before a specific design is finalized.
9.2.2.5
Safety Controls on Trucks
Interlocks: In the operation of a tank truck delivery unit, there are a number of
procedures which are repeated several times in the course of a working day; therefore,
the possibility of an error and in turn a potential hazard is always present. These can be
eliminated as there are safety controls available, which either program the operator's
activities or act as an override to correct any inappropriate action on the part of the
operator. The following is an indication of the operations which may be monitored or
which may be automatically performed:
1.
An interlock may be applied to the truck braking system to prevent
movement while product is being transferred.
2.
A lock may be placed on the vehicle so it is incapable of being moved until
the dispensing hose has been returned to the vehicle and attached in an
appropriate location.
3.
A lock may be placed on the vehicle until the filling hose has been
disconnected from the liquid filling connection.
4.
A lock may be placed on the vehicle until the liquid discharge valve or any
other quick closing internal valve has been released and closed.
5.
A lock may be placed on the movement of the vehicle until the power take
off has been disengaged and the pump has ceased to function.
6.
A safety control is available which can be programmed for a tank truck
delivery unit depending upon the type of equipment used. As an example, the
device can be programmed so it is necessary to set the brakes first, open the
main liquid discharge valve, engage the pump, then, complete the discharge
operation. The hose shall be returned and secured, the pump disengaged and
the main liquid discharge valve closed before the vehicle can be moved.
The tank, when on truck, shall not exceed maximum allowed overall height (in the
country) and not to be closer than 150 mm to prime mover/tractor cab. Chassis and
running gear to have local statutory approval and meet local axle load limits. Turntable
coupling (where fitted) in good condition, meeting manufacturers specification and
design.
Safety in LPG Design
TRANSPORTATION
9-5
Suspension to be appropriate for local terrain, bushings in
compliant with relevant statutory regulations and meeting
Braking system to be fully compliant with relevant statutory
working order. All tires (including spare) to have minimum of
of sizes and type on same axle.
good order, and fully
manufacturers design.
regulations and in full
3 mm tread, no mixing
Landing legs are optional (i.e., weight consideration) but if not fitted, trailer to have
pads on underside of trailer for portable landing legs, designed and capable of
supporting a fully loaded trailer.
Drive shaft Protection: Consideration shall be given to protecting the tank from being
struck by the vehicle drive shaft in event of a universal joint failure. Housings or baffles
can be used.
Figure 9.2.3.3-a: Typical single tank semi-trailer instrumentation
Truck lights: Trucks, trailers, and semi-trailers transporting LPG shall not be equipped
with any artificial light other than electrical. Lighting circuits shall have suitable overcurrent protection (fuses or automatic circuit breakers). All wiring shall have sufficient
current carrying capacity and mechanical strength, and shall be secured, insulated and
protected against physical damage. Tank/Trailer side clearance lights and rear traffic
indicator, brake lights to be fitted and in working order, and comply with local statutory
regulations. Electrical wiring to be installed and protected (i.e., in conduit) to avoid fire
or short circuit in normal conditions of use. Semi trailer to be in electrical continuity
with prime mover/tractor. Connectors for air and electrics to be in good condition and
mounted on the trailer such that the cables will not be fouled by prime mover mounted
equipment.
Under-run bar shall be fitted to rear of rigid or trailer, across width of the tanker. The
bar to be secured to the subframe. Near and off side under-run protection shall be fitted
and secured to the subframe. All wheels shall be fitted with mudguards.
Minimum of 4 brass earthing pins to be fitted (2 each side), connections the body to be
free of paint, dirt and grease. An earthing cable (15 m) to be fitted adjacent to pump
outlet.
Delivery hoses to be fitted with “pull away couplings”(either on truck or at all plant
side) and to be in good condition and fit for purpose, and securely stored on trailer/tank
during driving.
Trailers to have 2 x 9 kg dry chemical fire extinguishers (one on each side), in addition
to prime mover 2 x 9 kg dry chemical fire extinguishers.
Capacity of tank/trailer to be clearly labeled (50 mm letters) near fill point of trailer.
All wheel hubs to be equipped with full complement of bolts, correctly tightened.
9-6
TRANSPORTATION
Safety in LPG Design
Toolbox (lockable) to be fitted to appropriately store tools/adapters/fittings.
9.2.3
LPG Truck Discharge System
9.2.3.1
Pumps on Trucks
While LPG pumps at the refinery or terminal fill the tank trucks relatively quickly the
discharge of product from the truck will take longer since it has to be done via the pump
installed on the truck. A truck tank may be fitted with a pump flanged directly to the
liquid outlet. At the completion of the operation, the vapor contents will remain within
the tank as the pump is incapable of transferring vapor.
Truck pumps are operated through a power take-off by the truck engine. Mostly
positive displacement sliding vane pumps are used on trucks and therefore shall be
equipped with a suitable pressure actuated bypass valve permitting flow from the
discharge to the suction or to the tank. Centrifugal pumps are not suitable for outboard,
power take-off, transport unloading. After selecting a pump, the manufacturer's
recommendation shall be observed in selecting a strainer. A pump shall be specifically
designed for liquid LPG and preferably be flanged. It shall also be designed and
recommended for use on a delivery truck.
Figure 9.2.3.2-a: Bypass valve
The capacity of the pump should be selected considering the average volume of each
individual delivery or discharge and the rate at which the product can be received. The
following is offered as examples for selecting pumps on trucks typically used for
customer delivery.
Pumps for tank truck delivery service are available with a discharge capacity of 380
liters per minute. This type of pump is suited to filling relatively large fixed storage
tanks with high capacity filler valves.
When the average tank being filled is approximately 760 liters capacity and the filler
valve has a relatively low capacity, a pump capable of 110 to 190 liters per minute may
be adequate.
A pump should also be analyzed from the standpoint of maintenance as it is important
that the wearing parts can be replaced with minimum effort. A pump, which requires
removal from the installation in order to be repaired, is not desirable.
Safety in LPG Design
TRANSPORTATION
9-7
The unit shall also be capable of evacuating the same tanks, which it normally would be
charging. In normal operation, it may be necessary to evacuate the contents of a fixed
field LPG tank because of a defect in the unit or because of the necessity of moving the
unit. With the inclusion of the evacuation feature in the tank truck delivery system, the
liquid can be withdrawn from a supply tank and discharged into the delivery tank;
therefore, the unit has the capability of charging or fueling without depending upon a
pump or compressor at the plant.
Selection of a pump establishes the maximum discharge rate and an appropriate meter
can be selected. The meter shall have a sufficiently high range so it can accurately
measure the maximum anticipated discharge rate through the system. Various
accessories may be added to the meter depending upon the type of dispensing procedure
desired.
9.2.3.2
Bypass Valve for Truck Pumps
The selection of a bypass valve is dependent upon the capacity of the pump and shall be
sufficient to prevent the overpressuring of the system and, in turn, excessive wear on the
pump. A bypass valve, which is capable of sensing complete closure of the discharge
line and opening for full pump capacity, is desirable. The Figure “Installation of bypass
valve in truck piping system” below illustrates where to install it in the system. A
typical bypass valve is designed to bypass full pump capacity when the valve at the hose
end is closed.
Figure 9.2.3.2-b: Installation of bypass valve in truck piping system
9.2.3.3
Hoses and Hose Reels on Trucks
LPG tank trucks may be able to utilize plant hoses when discharging at the plant. With
this arrangement the plant hose can be equipped with a positive shutoff valve at its
outer end, thereby eliminating the necessity of bleeding or venting the hose upon
completion of the operation.
If the discharge operation is performed using a hose or hoses provided with the road
transport, a suitable venting arrangement shall be included in the transport piping system
so the hose can be safely vented after the unloading has been completed (see sections
6.1.5 and 6.1.6). A tubing vent may be run from a point just downstream of the
outboard transport shutoff valve to the top of the tank. The hose, complete with plugs or
caps, shall be suitable for liquid LPG and shall be rated for a minimum working pressure
of 17.25 bar gauge or, preferably, 24.2 bar gauge. When carried on a transport, hoses
shall be contained within a tube designed for the purpose and attached to the vehicle or
tank with closures at both ends.
9-8
TRANSPORTATION
Safety in LPG Design
The marketing plant loading facility provides hoses or conduits for the tank truck
delivery loading. Swivel hard arms are recommended for this purpose.
An evacuation hose approximately 10 to 20 meters in length shall be provided to
connect the suction connection on the tank truck delivery unit with a fixed storage tank
from which the liquid contents are to be withdrawn. The hose shall be approximately 25
mm nominal size and be rated for a working pressure of 17.25 bar gauge.
A dispensing hose, 19 mm or 25 mm nominal size shall be employed. The length of the
hose shall be determined considering the maximum distance from a safe convenient
discharge location for the tank truck delivery unit to the most remote fixed storage tanks.
The hose, as in all cases, shall be specifically designed for LPG service and be rated for
17.25 bar gauge working pressure.
Figure 9.2.3.3: Typical single tank semi-trailers
Where necessary, a vapor return hose shall also be provided with the length being
equal to that of the dispensing hose; the size shall be 13 mm or 19 mm nominal. A hose
reel is recommended as an accessory on a tank truck delivery unit since it reduces wear
and tear on the hose, ensures that the hose is properly stored when the vehicle is in
motion and can aid in developing a more efficient delivery operation.
9.2.3.4
Shutoff Valves on Truck Tanks and Hoses
A shutoff valve utilized as the primary valve in any mobile tank connection shall be of a
quick closing internal type with a means of remote control whenever possible.
Limitations regarding the use of these valves are usually dependent upon size. Some
relatively small connections cannot be fitted with this type valve as they are not
commercially available. The mini bulk distribution system requires the driver operator
to watch the tank while being filled but also requires him to be able to shut down the
LPG flow on short notice. This is generally done by a handheld remote wireless
connection to the truck valves. In case the button on the instrument is released, the
valve on the truck closes and the pump stops.
Safety in LPG Design
TRANSPORTATION
9-9
Figure 9.2.3.4: Typical retail cargo vehicle LPG transfer system
9.2.3.5
Liquid Meters
When liquid meters are used, they shall be selected to operate within the manufacturer's
recommended minimum/maximum rated capacity.
Materials used in the construction of meters shall be suitable for use with LPG and
maintain suitable performance over the range of operating conditions the meter will be
subjected to. Cast iron shall not be used unless the material is of an approved grade
having adequate ductility and impact resistance over the full pressure and temperature
range of the system.
Liquid meters shall have the following accessories installed, either as part of the meter
assembly or externally, to enable accurate meter readings.
A fine mesh strainer at the meter inlet. The typical coarse mesh strainer used in the
pump suction is not adequate for the meter.
A differential valve to provide a back pressure against the meter and the pump and to
maintain the system pressure above the product vapor. This prevents vaporization of the
liquid as it passes through the meter.
9-10
TRANSPORTATION
Safety in LPG Design
A vapor eliminator to remove vapor from the liquid before it passes into the meter. The
eliminator consists of a small tank with either a float-operated mechanism or a constant
bleed orifice to circulate LPG vapors back to the LPG tank.
Consideration shall be given to the requirements for a temperature compensator. A
temperature compensator converts the measured meter volume reading at the
temperature of the product as it passes through the meter so the meter will register the
equivalent volume at a standard temperature of 15.6 °C.
When liquid meters are used to measure the volume of LPG being transferred from one
tank to another or from a pipeline, meters and accessory equipment shall be installed in
accordance with the procedures stipulated in the API MPMS 5.1. Meters shall be
protected from accidental damage either by their location or by other structural means.
Meters shall be accessible to personnel for operational purposes and taking meter
readings. Inlet and outlet piping shall be arranged and supported so an excessive load is
not imposed on the meter.
Figure 9.2.3.5: Liquid meter installation at rear side of mini-bulk truck (Sasso)
9.3
Road Cylinder Transportation
Similar requirements discussed above under “Truck Design and Procurement” also
apply for cylinder transportation trucks. The road conditions, grades, etc., which the
vehicle must traverse shall be determined first, then the loading which will be applied to
the vehicle shall be analyzed. The vehicle shall possess adequate capacity for the “pay
load” and be sufficiently powered to negotiate the roads over which it must travel, at a
reasonable speed. To properly handle industrial cylinders during unloading, trucks
should be equipped with mechanical or hydraulic tail-gates.
Dual axle trucks and tractor-trailers (articulated trucks), purchased for the transport of
cylinders, shall be carefully analyzed to determine that they are capable of handling
current or future pallet designs. The bed, together with sideboards or stakes, shall be of
sufficient strength to support and retain the maximum number of filled cylinders
expected. The height of the bed shall be considered in relation to the loading or
Safety in LPG Design
TRANSPORTATION
9-11
unloading dock. A tractor-trailer shall be equipped with a “landing gear”, of
sufficient strength to support a full load, so that the tractor may be utilized for purposes
other than supporting the tractor-trailer during loading or unloading operations.
Following are international codes, which may be referenced for information on road
cylinder transport:
1.
The Australian Standard 1678 “Emergency Procedure Guide - Transport of
Compressed and Liquefied Gases.”
2.
The Australian Code for “Transport of Dangerous Goods by Road and Rail.”
Figure 9.4: Typical rail LPG unloading site
9.4
Rail Tank Cars
There is limited value in a detailed discussion of LPG rail tank car design in a manual
intended for international use. Pressure tank cars have long been used extensively for
LPG transport. However, their capacity and external dimensions, as well as external
fittings and running gear, are determined by the widely varying characteristics of the
various national railways (e.g. rail gauge, quality of roadbed, minimum allowed radius
of main line rail curvature, banking of curves, and maximum design train speed).
Therefore the designer of rail receiving or loading facilities or the purchaser of rail tank
cars is best advised to seek the guidance of rail transport regulatory authorities and
equipment suppliers in the jurisdiction where the facility is to be constructed or
modified.
9-12
TRANSPORTATION
Safety in LPG Design
LPG rail car appurtenances are arranged such that they are protected in case of
accident or derailment. One way of doing this is to concentrate all tank penetrations in
a vertical cylindrical protective dome, with a hinged top cover, located at the top center
of the horizontal tank. Another way is to provide a recess on the side of the tank and
locate appurtenances therein. In countries (Central Europe) where risk from derailments
is considered low, the appurtenances have been installed below the tank.
9.5
1.
Pressure Relief Valves are installed in the vapor space and are of the internal
spring type. The fabricators will determine the start-to-discharge pressure
setting and flow capacity of the devices. Certain countries do not permit the
installation of pressure relief (Central Europe). Tanks on such rail cars are
designed to contain the maximum pressure.
2.
Liquid Outlet: Usually there will be two liquid outlets provided within the
recess/dome. Each shall be fitted with a manually controlled positive shutoff
valve under which is fitted an excess flow check valve. The intake of the
liquid draw-off is located at the bottom of the tank.
3.
Vapor Connection: A single vapor connection is provided, fitted in a similar
manner to the liquid outlet except that the excess flow check valve terminates
in the vapor space.
4.
Gauging Device: A gauging device in the form of a slip tube gauge is utilized
to determine the liquid level within the rail car.
5.
Sampling System: Some rail cars have a tube extending to the bottom of the
tank and fitted with an excess flow check valve connecting to a positive
shutoff valve within the dome. This installation may be used to take a sample
of the cargo to verify the contents or to confirm that the tank has been
completely unloaded.
Marine
Barge or ship either as a containerized shipment or as bulk cargo may transport LPG.
As a containerized shipment, cylinders or portable containers may be utilized provided
the cylinders or containers meet basic requirements. Care should be exercised in
preparing the shipment so it will meet the regulatory and safety requirements, which
may be applicable to the vessel, the waterways traveled and the ports at which the cargo
is loaded and unloaded.
As bulk cargo, non-refrigerated LPG may be transported by barge or by tank ship.
Barges may be constructed with containers mounted directly to the deck or may be
integrated within the structure. The decision should be dependent upon the anticipated
condition of the waterways traveled.
As a tank ship cargo, the product may be refrigerated in order to use low pressure
tanks, which reduce containment weight and increase cargo weight. The design of a
tank ship should be developed considering the condition of the product at the loading
point, the facilities available at the delivery point, the volume of product available when
loading and the storage or receiving capacity at the delivery point. In particular, if a
refrigerated LPG cargo ship is designed to supply pressurized storage terminals the ship
shall have sufficient heat exchange equipment to increase the temperature of the
product downstream of the unloading pump to ambient levels. While heating could be
accomplished on shore or the receiving tank may be designed to accept low temperature
LPG, designing the ship without heat exchange equipment would limit its delivery
capabilities.
When selecting marine transportation for LPG, ships with vertical multistage onboard
pumps units should be preferred. The ships pumps will have to provide sufficient head
to overcome the pressure in the ambient temperature LPG tanks.
Safety in LPG Design
TRANSPORTATION
9-13
Design of marine LPG transportation equipment is a specialized endeavor, which
benefits from technical experts in the field. ExxonMobil Research and Engineering
Company and ExxonMobil marine transportation specialists should be consulted when
in selecting designers and constructors. In addition EMRE has published extensive
design guides, and can offer assistance in the design and construction of marine pier
facilities for LPG.
The basic design code for the cargo containment and transfer systems on bulk LPG ships
is the International Code for the Construction and Equipment of Ships Carrying
Liquefied Gases in Bulk (referred to as the “IGC Code”) published by the
International Maritime Organization (IMO). This code conforms to the requirements of
most countries of registry, exporting countries and importing countries. A notable
exception is the United States, which has regulations for both U. S. flag ships, and
foreign flag ships that call at U. S. Ports. These regulations, administered by the U. S.
Coast Guard, are defined in the Code of Federal Regulations, Title 46, Chapter I, Part
38, Liquefied Flammable Gasses, Part 153, Ships Carrying Bulk Liquid, Liquefied Gas,
or Compressed Hazardous Materials and Part 154, Safety Standards for Self-Propelled
Vessels Carrying Bulk Liquefied Gases.
In addition to requirements for the liquefied gas containment and transfer systems, ships
should meet numerous other national and international standards specified by the ship's
country of registry, classification society, and intended port states. These standards will
usually be established at the outset of a project to suit the ship's intended service.
9-14
TRANSPORTATION
Safety in LPG Design
10 CUSTOMER INSTALLATIONS
10.1
Cylinder Bank Installations
This section of minimum standards covers the cylinder bank installation requirements
for industrial and commercial areas where ExxonMobil LPG is supplied.
Public access to areas where LPG is stored and transferred shall be prohibited. To
prevent trespassing or tampering, every LPG storage place shall be enclosed by a fence
or cage or ventilated cabinet. Sufficient clearances shall be provided to allow
maintenance and safe exchange of cylinders.
Figure 10.1.1: Multiple cylinder installation
10.1.1
Design of Cylinder Banks
LPG cylinders are preferably located outside of buildings and in well ventilated
surrounding. The cylinders shall be installed under shade whenever possible. They
shall not be installed directly under windows or adjacent to doors. They may be
positioned against walls and secured by chains against tumbling. Where unavoidable,
cylinders may be located inside buildings. Locations shall be carefully chosen such that
Safety in LPG Design
CUSTOMER INSTALLATIONS
10-1
leaking LPG could not accumulate in basements and that the location would not impact
on escape routes. If necessary, PRV outlets must be piped to a safe location outside the
building. At a safe location the gas will be allowed to dissipate to a concentration below
lower flammable limits. Mechanical ventilation, air-conditioning intakes, openings of
buildings or sewer system openings shall not be close (less than 1.5 m) to such locations.
Local regulations may govern cylinder group installations. In the absence of local
regulations, typically, 20 industrial 50 kg LPG cylinders may be installed in one group.
Depending on customer needs it could be less but the total number must not be more
than 30 industrial cylinders. If more LPG is used (requiring more than 30 cylinders to
be installed) a container may be a better (safer and more economic) solution. If this is
not feasible a second group of cylinders may be installed but a minimum distance of
7.5 m from the first group must be kept. The cylinders are all connected to a manifold
which is divided at the center by an automatic switchover regulator. 50% of the
cylinders are in use while the other 50% are in reserve.
10.1.2
Installation of Cylinder Banks
LPG cylinders shall be installed at location least frequented by personnel and necessary
precautions shall be taken to prevent tampering. Cylinders shall be installed
aboveground and set upon a firm foundation which shall be substantially level.
Flexibility shall be provided in the connecting piping. Cylinders shall be positioned so
that the pressure relief valve is in direct communication with the vapor space of the
cylinder. Cylinders shall not be stacked one above another when in use. Loose or piled
combustible material and weeds and long grass shall not be permitted within 3 m of any
cylinder.
Fire protection shall be provided in accordance with local regulations and local
authority shall be consulted for final approval. As a minimum, there shall be one 9 kg
dry powder fire extinguisher located at a safe distance from the cylinder bank.
10.1.3
Vaporization Rate in Cylinders
In order to maintain a sufficient flow of vapor to a user, it is important to properly
estimate or calculate the vaporization rate. Depending on local code requirements for
customer controlled piping; insufficient vapor rates could result in either a trip of the
vapor supply or a loss of flame and continued feeding of vapor. In the latter case,
continued flow of LPG may result in gas accumulation and a potential for explosion.
The vaporization rate of an LPG container or cylinder is influenced by many factors.
Precisely calculating the vaporization rate of a given cylinder is difficult. In the
majority of cases when 11 kg or 12 kg domestic cylinders are used, the required
vaporization rate is sufficiently low so that problems do not arise. Typically, regulators
for domestic cylinders allow a take-off rate of 1 to 1.5 kg/h. In the event that larger
cylinders are utilized, the vaporization rate from a single cylinder may not be sufficient
to meet customer demand. In this case, multiple cylinder installation can be used.
If there is any doubt regarding the vaporization capacity within a cylinder or a bank of
cylinders, the pressure within the cylinder(s) may be monitored and, in this way, a
substantial decrease in cylinder pressure may be interpreted as an indication that
additional vaporization capacity is required. By so doing, the lack of available vapor or
a shutoff of the vapor supply may be prevented. For customer comfort two cylinder
banks may be installed. Such installations may permit automatic switch-over from one
bank to the other if the first bank is empty. However, in that case the customer should
get an alarm so that a new set of filled cylinders can be ordered.
If a mixture of Propane and Butane is used, the former will vaporize preferentially. At
the end of the vaporization process the liquid heel in the cylinder will consist only of
10-2
CUSTOMER INSTALLATIONS
Safety in LPG Design
Butane. As a result the vapor pressure in the cylinder will keep dropping throughout the
vaporization.
The Figure 10.1.3-a: shows how approximately a 70% Butane, 30% Propane mixture
evaporates. At the end of the evaporation process all Propane is vaporized. In the
calculation it was assumed that the temperature in the cylinder was constant at 25 °C.
Of course, this assumption is for very little consumption and may therefore not be
realistic, but it serves to demonstrate the effect.
Weight Fraction Butane
1
Composition of Liquid
0,8
0,6
Composition of Vapor
0,4
0,2
0
11
10
9
8
7
6
5
3
2
1
0
1
0
Mass Re m aining in Cylinder, kg
Pressure in Cylinder, bar g
Figure 10.1.3-a: LPG composition as cylinder is emptied
4
3.5
3
2.5
2
1.5
1
0.5
0
10
9
8
7
6
5
4
3
2
Mass Re m aining in Cylinder, kg
Figure 10.1.3-b: LPG pressure change as 10 kg cylinder is emptied
The Figure 10.1.3-b: shows the pressure drop during the above vaporization process. If
the temperature would be allowed to drop, the pressure may drop even lower.
Safety in LPG Design
CUSTOMER INSTALLATIONS
10-3
Estimates of vaporization capacity for one 45 kg cylinder filled with Propane, based on
experience, are given in the Figure 10.1.3-c. When necessary, variations from the
conditions in the table should be considered when sizing a specific installation.
7,0
full
Vaporization, kg/h
6,0
75%
5,0
4,0
50%
3,0
25%
2,0
1,0
0,0
-20
-10
0
10
20
Ambient Temperature, C
Figure 10.1.3-c: Vaporization from 45 kg Propane cylinder depending on ambient
temperature and % of fill. Note that 1 kg/h of Propane equals 13.8 kW
The German “Flüssiggas Handbuch” (TRF1996) suggests the following sizing criteria
for cylinder vaporization:
Offtake
Characteristic
Cylinder Size
5 kg
11 kg
33 kg
Short Term
up to 1.0 kg/h
up to 1.5 kg/h
up to 3.0kg/h
Periodical
up to 0.5 kg/h
up to 0.8 kg/h
up to 1.8 kg/h
Continuous
up to 0.2 kg/h
up to 0.3 kg/h
up to 0.6 kg/h
Table 10.1.3: Vaporization from different size cylinders depending on gas use.
Short term = 15 min, Periodical = 30 min on, 30 min off.
10-4
CUSTOMER INSTALLATIONS
Safety in LPG Design
10.1.4
Icing or Sweating on Cylinders.
Any indication of “icing” or “sweating” on the cylinder is an indication that the
vaporization capacity is being exceeded and additional cylinders should be installed. As
the vaporization capacity is exceeded, ice will form on the exterior of a cylinder in
relationship to the humidity within the surrounding atmosphere. During periods in
which the air is relatively dry, icing or sweating may not occur; however, the surface of
the cylinder will be noticeably chilled and, with an increase in humidity, ice or water
dew accumulation may be anticipated. As ice forms on the exterior of a cylinder it
provides an additional barrier to the transfer of heat; therefore, the overall efficiency of
the cylinder as a vaporizing unit is diminished. Dew on the exterior of the cylinder has
the same effect. It will tend to vaporize and take heat of vaporization from the cylinder
cooling it further down. As noted above, insufficient vaporization rates may lead in
certain situations to a loss of flame with subsequent accumulation of LPG vapor.
10.2
Containers at Customer Sites
This chapter covers the spacing and site requirements and the design of aboveground
and underground or mounded LPG containers for domestic and commercial storage
which are refilled on site. It does not cover cylinders connected in use in banks, or
cylinders in storage awaiting use or distribution. Typically these containers are between
1 and 300 m3 in size.
Containers for domestic storage and commercial storage containers should be fabricated
to conform to ASME codes, or an equivalent standard when recognized by local
regulations. The code followed shall be acceptable to the regulatory authority where the
container is to be used. Procedures to establish Design Pressure, Design Temperature,
and Critical Exposure (Minimum Design) Temperature are the same as explained in the
section “Bulk Storage” in Chapter 3.
When the LPG supplier (ExxonMobil) owns containers, the responsibility for the
containers remains with the supplier. Therefore, the use of an inspection service is
recommended to assure continuing quality assurance oversight throughout the contract
with a container manufacturer.
Each container shall be completely identified by the nameplate, which is permanently
attached to the container. Nameplate requirements may vary based on local codes.
Typical commercial or domestic containers with fittings are shown in Figure: 10.2.7.
They are designed for horizontal installation, with dual lifting lugs for deployment.
Four steel feet permit setting the container on a pre-poured concrete foundation slab.
Where there is potential for flooding, the container shall be fixed to the foundation slab.
The foundation must be heavier than the total buoyancy of an empty tank.
10.2.1
Spacing and Location of Containers
Spacing of containers to the nearest important building, property line, or other
containers at customer sites shall be in accordance with requirements in the country. In
absence of any local requirements spacing as per Table 10.2.1 is suggested.
The number of containers in a group is limited to 6 containers. If more than one such
installation (group of 6 containers) is made, each installation shall be separated from any
other installation by at least 7.5 m. Do not apply the minimum distances between
containers to such installations. The designer shall not install multiple containers
simply for the purpose of reducing spacing to the property line. The need for multiple
containers shall be justified based on operating requirements.
Safety in LPG Design
CUSTOMER INSTALLATIONS
10-5
Maximum water
capacity (m3 )
Minimum separation distances (m)
Aboveground
container
Of any
single
container in a
group
Of all
containers in a
group
Underground or mounded
containers
From
buildings,
prop. line
or fixed
source of
ignition
From buildings, property
line, etc. to
Between
containers
Valve
assembly
Container
shell
Between
containers
Less
than 0.5
1.5
2.5
1
2.5
0.3
0.3
0.5 to
2.5
7.5
3
1
3
1
1.5
2.5 to
9.0
27
7.5
1
7.5
3
1.5
9.0 to
135
450
15
1.5
7.5
3
1.5
135 to
337.5
1,015
22.5
11
3
3
Above
337.5
2,250
30
¼ of sum
of
diameters
of adjacent
containers
15
3
3
Table 10.2.1: Spacing of containers at customer sites
The following shall apply to above ground containers installed along side of buildings:
1.
ASME containers shall be located and installed so that the discharge from the
container pressure relief device is at least 1 m horizontally away from any
building opening that is below the level of such discharge. And not less than
1.5 m in any direction away from any exterior source of ignition, openings
into direct-vent (sealed combustion system) appliances, or mechanical
ventilation air intakes.
2.
The filling connection and the vent from liquid level gauges on ASME
containers filled at the point of installation shall be not less than 3 m in any
direction away from any exterior source of ignition, openings into direct-vent
(sealed combustion system) appliances, or mechanical ventilation air intakes.
LPG containers, whether aboveground or underground or mounded, shall be installed in
the open air outside buildings. Containers to be located downgrade and downwind from
possible ignition sources. LPG containers shall not be stacked one above the other. The
area including LPG containers and related equipment shall be enclosed by an industrial
type fence at least 2 m high unless it is otherwise protected, e.g. being within a larger
fenced area or otherwise isolated from public access. Unless the area is smaller than
50 m2, there shall be at least two means of exit at adjacent sides of the fence. The gate
shall open outwards, shall not be self-locking and shall open into an unobstructed open
space. An exception to the above is when the container is provided with a positive
means of denying access to valves and fittings other than pressure relief valves, e.g. by a
ventilated hinged cover that can be locked, or by a blank flange or plug on drain
connections. Where damage from vehicular traffic is a possibility, means of protection
shall be provided, e.g. by the use of crash barriers, bollards or non-continuous toe
walls.
10-6
CUSTOMER INSTALLATIONS
Safety in LPG Design
No permanent source of heat shall be located within 1.5 m of an LPG container. LPG
containers shall not be located directly beneath electrical power cables. For cables
carrying less than 1.0 kV, the LPG container shall be sited at least 1.5 m from a line
drawn vertically downwards from the power cables. For cables carrying 1.0 kV or
greater voltage, the distance shall be increased to 7.5 m.
No horizontal separation shall be required between an aboveground LPG container and
underground containers containing flammable or combustible liquids installed in
accordance with NFPA 30.
Fire protection of LPG containers at customer sites shall follow requirements of the
local fire services.
10.2.2
Designing Customer Storage Systems
10.2.2.1
Piping Arrangement for Customer Installation
Storage containers at consumer locations shall be designed for 100 percent Propane to
provide for future flexibility of product mix.
Figure 10.2.2: Consumer LPG Facility
Figure 10.2.2 illustrates a consumer LPG facility. This figure shows container
connections and service line for a permanently mounted container at a consumer
location. If transport hoses can reach the storage container, piping from connections (1)
and (2) can be eliminated. Valve outlets shall be fitted with hose adapters. The liquid
volume between the block valve at the unloading vehicle and the block valve of the plant
must be minimized.
Vent line valves at unloading connections shall be equipped with spring-loaded
actuators that must be manually held open. Thermal relief valves shall be located
between all shutoff valves. LPG vapor piping systems downstream of the first-stage
pressure regulator shall be sized so that all appliances operate within their
manufacturer’s specification.
The container valving may be different in case a vaporizer is required. In such cases, a
liquid off take to a vaporizer may be installed, with a pressure regulator on the
Safety in LPG Design
CUSTOMER INSTALLATIONS
10-7
container vapor outlet itself. This pressure regulator system would serve as a back-up
during vaporizer shut-down.
10.2.3
Sizing Of Containers and Vaporization Rates
Sizing of containers involves two aspects. One is the maximum consumption per hour
required by the customer. The other is the supply and shipping logistics. Maximum
consumption can be satisfied by natural or enforced vaporization. Containers for the
former need to be larger than for the latter. According to supply logistics containers
shall be sized to receive incoming shipments while maintaining a minimum reserve of
the number of days demand equal to one-half of the shipping time from the supply
source. LPG is commonly shipped to consumer locations in bulk in trucks or rail tank
cars
10.2.3.1
Natural Vaporization
An important characteristic of LPG is that, when in a storage container, it can use the
heat from the surrounding air to change from a liquid to a vapor. When withdrawal
starts, heat is taken initially from the liquid itself, resulting in a drop of liquid
temperature. Because the product is at a lower temperature, heat starts to flow from the
surrounding air through the container wall into the product. The rate at which this heat
flows depends primarily on the temperature difference between the air and the product.
The greater this difference, the larger the rate of heat transfer. Normal Butane will
remain a liquid at atmospheric pressure when ambient temperatures are below its boiling
point of 0 °C. Propane will remain a liquid at atmospheric pressure when ambient
temperatures are below its boiling point of – 42 °C. At temperatures above these boiling
points, rapid vaporization will take place as long as the pressure is at or slightly above
atmospheric.
Figure 10.2.3.1: Vapor pressures of Butane and Propane mixtures
10-8
CUSTOMER INSTALLATIONS
Safety in LPG Design
LPG is typically stored as a liquid under pressure. Under inactive conditions, each
product in a closed container has a defined pressure versus temperature relationship i.e.
the vapor pressure (see Figure 10.2.3.1). Regardless of the amount of liquid in a
container and provided some vapor space exists, the internal pressure will correspond to
the vapor pressure of the product at the temperature of the liquid.
When the liquid stored is 100 percent Butane or 100 percent Propane, the vapor
conditions will be uniform. However, when the product is a mixture of Butane and
Propane, the vapor above the liquid mixture will always have a higher percentage
volume of the lighter product (Propane), regardless of the proportion in the liquid state.
Since Propane will vaporize at a faster rate than Butane, the first quantity of vapor taken
from such a container will have a higher content of Propane than the succeeding vapor.
As vapors are withdrawn, both the vapor and the liquid will have increasingly higher
Butane content.
When vapor is being withdrawn from a container, the internal pressure is lowered until
the rate of conversion from liquid to vapor is equal to the rate of vapor withdrawal. If
too much vapor is withdrawn the pressure drops to dangerously low levels which may
lead to flame-out or other undesirable situations. For such installations a vaporizer may
be needed, which is described later in this chapter. The size and the shape of a container
are important when vaporization must take place within the container. Given the same
volume a long container brings more vaporization as compared to a short one.
The container surface area that is in direct contact with the liquid is an important factor
known as the wetted surface area. The liquid surface area in contact with the vapor
has very little effect on vaporization. More heat will be transferred for vaporization
when a container is full than when it is nearly empty. In addition, the following factors
will have an influence on the container vaporization rate:
1.
Ambient temperature of the atmosphere.
2.
Size and color of the container.
3.
Exposure to solar radiation.
4.
Amount of frost, ice or insulation on the container wall.
5.
Circulation of air around the unit and wind conditions.
Precise calculations can be developed in order to determine vaporization rates, however,
due to the variation in various factors mentioned in the above list, reliance upon the
estimated formula will usually suffice in the selection of a container for Propane only.
10.2.4
Sizing Containers for Vaporizing Liquid
Certain factors must be known in order to determine the proper size for a container when
it is to be used to vaporize the liquid stored in it to replace vapor being withdrawn from
the space above the liquid. These include:
1.
Maximum quantity of LPG to be vaporized per hour.
2.
Number of hours per day that vapor is required.
3.
Minimum allowable container pressure or minimum allowable inlet pressure
to first-stage regulator.
Type of product or percentage of mixture.
4.
Safety in LPG Design
5.
Minimum anticipated ambient temperature during period of maximum
vaporization.
6.
Relative humidity of atmosphere at time of minimum ambient temperature
and maximum vaporization.
7.
Minimum liquid level in container at time of maximum vaporization.
CUSTOMER INSTALLATIONS
10-9
There is quite some experience with company engineers to calculate or estimate
container sizes. The following is not meant to replace those methods or experiences.
The methods described below shall help the inexperienced engineer to understand
container sizing. These methods in this guideline are conservative in sizing LPG
containers for natural vaporization, i.e. they will be rather on the large side than too
small.
10.2.4.1
Sizing Small Containers for Natural Vaporization.
Calculation Example 1: The customer needs 9 kg/h at a pressure of 1 bar. The coldest
outside temperature is 20 °C. The LPG mixture is 30% Propane and 70% Butane.
First determine the maximum possible temperature differential. According to Figure
10.2.3.1 at 5 °C the pressure of a 30/70 mixture would be just above 100 kPa (1 bar).
With a minimum outside temperature of 20 °C the usable temperature differential would
be 20 – 5 = 15 °C. Entering the x-axis of Figure 10.2.4.1 at 15 °C temperature
differential and going to the next container size above 9 kg/h it turns out that a 1.75 ton
container would be sufficient for the purpose.
Calculation Example 2: The customer has a 5 ton container and needs LPG at a
pressure of 0.5 bar. Can he run on 100% Butane if the coldest outside temperature is 19
°C? How much LPG can be taken off?
According to Figure 10.2.3.1 the pressure of 100% Butane at 19 °C is about 90 kPa (0.9
bar). The temperature at 50 kPa (0.5 bar) is 10 °C. So, the usable temperature
differential would be 9 °C. Entering the x-axis of Figure 10.2.4.1 at 9 °C shows that
Butane in a 5 ton container could still vaporize Butane at a rate of about 12 kg/h.
25
5 ton
Vaporization, kg/hr
20
3,5 ton
15
1,75 ton
10
1 ton
5
0,5 ton
0
0
5
10
15
Usable Temperature
Differential, C
Figure 10.2.4.1: Vaporization of LPG in 0.5 to 5 ton container (remaining container volume 25%)
10-10
CUSTOMER INSTALLATIONS
Safety in LPG Design
10.2.4.2
Sizing Larger Containers to Satisfy Natural Vaporization Requirements
Often, containers larger than 25 tons are equipped with vaporizers. However, in tropical
climates with almost constant temperatures customers may prefer to save the operational
cost of the vaporizer. When using LPG mixtures the designer must anticipate that the
pressure in the container decreases as the liquid level drops. This is due to the fact that
Propane boils off first (see Figure 10.1.3-b). In Figure 10.2.4.2 the vaporizing capacity
at a container level of 25% volume is shown for a 10 ton, a 25 ton and a 50 ton
container, containing 100% Propane. The minimum pressure anticipated is 0.5 bar. If
the container contains more volume obviously the vaporization will be higher but this is
not taken into account since only the lower vaporization is of interest. Depending on
vaporization requirements, such containers may need a refill once they reach 25%. If
only LPG at certain mixtures is available (and not pure Propane) the customer must be
aware that the pressure in the containers drops as the container level decreases (see
Figure 10.1.3-b) since first the Propane components vaporize.
250
50 ton
Vaporization, kg/hr
200
25 ton
150
100
10 ton
50
0
-20
0
20
40
Ambient Temperature, C
Figure 10.2.4.2: Vaporization of Propane in 10 to 50 ton containers (at a container volume of 25%
and a delivery pressure of 0.5 bar)
10.2.4.3
Sweating, Ice or Frost Formation
The formation of ice or frost will reduce the flow of heat into the liquid. Water present
in the air will condense onto the container’s outer surface when the surface temperature
is below the dew point of the air. The dew point temperature of the air varies with the
degree of humidity and the dry bulb temperature of the air.
When the surface temperature of the container is at or below 0 °C, any water on the
surface will freeze to frost or ice. Frost or ice on the container surface will act as
insulation and reduce the rate of heat transfer from the air to the liquid. Therefore, the
withdrawal of vapors from a container shall be limited to that volume which can
vaporize without reducing the product and container shell temperatures to below the
dew point whenever that temperature is below 0 °C.
Safety in LPG Design
11
CUSTOMER INSTALLATIONS
10-
Typically, in tropical climates ice or frost formation is not a problem. However,
sweating can often be seen on containers. This is a sign of too high vapor load for this
container. Often there are multiple container installations which were designed properly
but not operated properly. The users or fillers often leave only one container on line
(which is then sweating). All containers must be opened such that the pressure in the
system is as high as intended by the designer. Continuously withdrawing LPG vapor
from an underground or mounded container is not recommended since the gas supply
would not last long and serious sweating on the container surface would accelerate
corrosion Such containers installations typically have vaporizers.
10.2.5
Enforced Vaporization by Means of Vaporizers
The alternative method to obtain vaporization of the LPG liquid uses separate
vaporizers. The LPG liquid is piped directly from the bottom of the storage container to
the vaporizer. Heat required for vaporization is provided by electricity, steam, hot water
or water-glycol solution. A direct-fired vaporizer is not allowed.
Vaporizers offer advantages even when the expense of providing the heat for the
vaporization is considered. If the size of the container required for natural vaporization
is larger than the volume required for receipt and reserve volumes, it may be possible to
install smaller storage capacity and save in total investment by using a vaporizer. Also,
the gas from a vaporizer will match the characteristics of the entering liquid and will be
nearly uniform throughout the withdrawal of liquid from the storage container. While
the pressure during natural vaporization drops as the liquid level is lowered during
consumption it will remain constant when using a vaporizer installation. One drawback
of the vaporizer installation is the fact that all components, also the Oily Residue will
pass the vaporizer. Therefore provisions must be made (KO drum) to collect oily
condensate after the vaporizer.
Vaporizers shall be designed, constructed, installed and tested in accordance with a
recognized and appropriate pressure container code, NFPA 58, 59 and manufacturer
recommendations. LPG vaporizers supplied shall be from reputable manufacturers
approved by ExxonMobil.
Supply
Supply
High Pressure
Regulator
Low Pressure
Regulator
Vaporizer
Commercial/Domestic Supply where
Vaporizer is not Required
Feedback System
Supply
Supply
Vaporizer
Vaporizer
Pump
Semi Feedback System
Pump
Direct Feed-out System
Figure 10.2.5: Typical container and vaporizer installations
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CUSTOMER INSTALLATIONS
Safety in LPG Design
The Figure 10.2.5 “Typical container and vaporizer installation” illustrates container
supply installations schematically. The installation on the upper left side of the figure
shows a typical Propane container. Because of its high vapor pressure, a vaporizer is
normally not required. A first stage high pressure regulator is usually installed in the
container dome. A second stage regulator is installed in the supply line immediately
before distribution to the consuming appliances. Two stage regulators are recommended
for maximum operating reliability by virtue of more uniform gas pressure, and enhanced
safety in the event of a regulator failure.
The remaining three installations on Figure 10.1.3 show frequently used configurations
for vaporizers and containers. These are typical of commercial and domestic Butane
supply systems. The liquid supply line shall preferably originate in a supply valve with
dip tube located in the dome, rather than as shown (for simplicity). If there is a remote
possibility that the stored liquid temperature could fall as low as the –7 °C, the boiling
point of liquid Butane, the arrangement shall be a “feedback” or “semi-feedback”
system. Then, the container would not be subjected to a vacuum, which might
compromise it structurally or affect gas supply pressure at appliances. The “direct feedout” system would be suitable for Propane in situations where a vaporizer is needed to
provide comparatively large instantaneous gas capacity from limited storage. Need for
the indicated optional pump will depend on the relative elevations of the container and
the vaporizer, line pressure drop, and the operating pressure range in the container.
When designing the system the minimum possible ambient temperatures have to be
taken into account so that vapors are not subjected to dew point conditions (see Table
10.2.5).
Press
Propane
Mix
Mix
Mix
Mix
Mix
Mix
Mix
Mix
Mix
Butane
100
90/10
80/20
70/30
60/40
50/50
40/60
30/70
20/80
10/90
100
Bar (g)
Dew Point in ºC
1.0
-43.0
-36.0
-30.0
-25.0
-20.0
-15.5
-12.0
-9.0
-6.0
-3.0
0.0
1.5
-33.5
-26.0
-18.5
-14.0
-9.5
-5.0
-1.5
2.0
5.0
8.0
11.0
2.0
-26.5
-19.0
-11.0
-6.0
-1.5
3.0
6.0
10.0
13.0
16.0
19.0
2.5
-20.0
-13.5
-5.0
0.0
4.5
9.0
12.0
16.0
19.0
22.6
25.5
3.0
-14.0
-6.0
0.0
5.5
10.0
14.0
17.5
21.5
24.5
28.0
31.0
3.5
-9.0
-2.0
4.6
9.5
14.0
19.0
22.0
26.0
29.5
33.0
36.0
4.0
-5.5
2.0
8.5
13.5
18.0
23.0
26.5
31.0
34.0
37.5
40.5
4.5
-2.0
5.5
12.0
17.5
22.0
27.0
30.0
34.5
38.0
41.5
45.0
5.0
1.0
9.0
15.5
21.0
25.5
30.0
34.0
38.0
42.0
45.5
49.0
6.0
7.0
16.10
21.5
27.0
32.0
36.5
40.5
45.0
49.0
52.5
56.5
7.0
12.0
20.0
27.0
32.5
37.5
42.0
46.5
51.0
55.0
59.0
62.5
8.0
17.0
25.0
31.5
37.0
42.5
47.0
52.0
56.0
61.0
65.0
68.0
9.0
22.0
29.5
36.0
42.0
47.0
52.0
57.0
61.0
65.0
69.5
73.0
10.0
26.0
33.5
40.0
46.0
51.5
56.0
61.5
65.5
69.0
73.5
78.0
Table 10.2.5: Dew points in different LPG mixtures in relation to pressure
Sizing of a vaporizer is not complicated. The basis figure to be known is the peak
consumption. For a single appliance this is the hourly consumption (kg/h). If the unit is
started up daily add 50% for accommodating the additional consumption during start-up.
If there are multiple consumers (multiple restaurant kitchens) consumption for all
appliances shall be added up since the may all consume at the same time (lunch and
evening peak hours). Choice of the vaporizer is optimum if two or three units would
cover multiple consumers. Depending on required reliability one single consumer may
be adequately served by one single vaporizer. It shall be borne in mind that vaporizer
installations typically have an emergency back-up with an natural vaporization line
Safety in LPG Design
13
CUSTOMER INSTALLATIONS
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directly from the container.
The use of a vaporizer is indispensable for
underground/mounded container installations
It is important that there be a clear understanding as to where the designer's
responsibilities end in the system leading to the point of end use. Generally, the
point of demarcation is at the outlet of the high pressure regulator for non-vaporizer
systems, or at the container's liquid and vapor connections in the case of vaporizer
systems; however, this demarcation shall be confirmed formally in a written document
executed by supplier and customer.
10.2.5.1
Installations of Vaporizers
This section of minimum standards covers indirect fired LPG vaporizers to be installed
in end user facilities using ExxonMobil supplied LPG. Local standards shall be
followed. In the absence of a local standard or if the local standard is less stringent, the
following minimum standard shall apply.
Vaporizer houses shall not have drains to sewers or sump pits. A strainer shall be
installed in the liquid inlet to the vaporizer. A knockout pot shall be installed at the
outlet of the vaporizer to remove heavy hydrocarbons. Vaporizers shall be located in
accordance with the minimum distances from other equipment in accordance with Table
10.2.5.1.
Exposure
Minimum Distance
Required (m)
Aboveground LPG Container
3
Relief Valve of Underground LPG Container
3
Point of Transfer (Truck)
3
Nearest Important Buildings or Adj. Property Line
1.5
Table 10.2.5.1: Minimum Distance Required for LPG Vaporizers
10.2.6
Installation of Containers
10.2.6.1
Aboveground Installation
Aboveground containers shall be placed on concrete foundations. If the location could
be flooded the container shall be fixed and the foundation shall provide adequate
anchors or weighting such that the container would not float, even if empty. Product
identification and safety signs shall be installed at the container. The “NO SMOKING”
sign shall be clearly legible at the safety distance applicable and from points of access to
the storage site. Grounding requirements are defined in “Grounding Connections for
Tanks” in Chapter 3.
10.2.6.2
Underground Installation
Underground containers shall be protected from superimposed loads, e.g. vehicular
traffic loads, either by fencing or protecting them with a reinforced concrete slab or
other load-bearing means. If the area for the container is not fenced off, the container
manhole cover and container fittings shall be protected against damage and tampering.
Mounded containers shall be protected either by fencing off the area around the mound
or by other adequate means.
Underground containers shall be surrounded by sand that is not aggressive in terms of
corrosion. Minimum coverage on top is 0.3 m of sand. A concrete slab may be added.
If traffic moves over the slab, no load shall be conveyed to the tank. Mounded tanks
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CUSTOMER INSTALLATIONS
Safety in LPG Design
shall be covered similar to the requirements (0.9 m) mentioned in Chapter 3. For
smaller tanks the cover may be less provided the cover cannot be eroded by rain or
firewater.
Underground installation greatly reduces fire risk and security problems. Where spacing
is tight or not available, underground installation may be chosen.
Figure 10.2.6.1: Typical container
Underground containers and their fittings, valves, pressure relief valves, gauging devices
and regulators shall be adequately protected against corrosion. Proper drainage shall be
provided for the housing dome to eliminate accumulation of water. The following
guidelines shall be observed when installing underground containers:
Safety in LPG Design
15
1.
Only ASME containers constructed for underground service and marked
accordingly shall be installed underground.
2.
All fittings, including any plugged openings, are plugged tight and free from
leaks. Containers may be pressurized with air or LP-gas vapor to make
certain there are no leaks.
3.
The container is purged in accordance with generally accepted industry
practices. (See NPGA Safety Bulletin 133 and the LPG Safe Operations
Guide).
4.
Rust, dirt and other foreign matter have been cleaned from the surface of the
container, and the container has been visually inspected for gouges, dents, pits
or other defects.
5.
The external surface of underground tanks shall be Grit blasted to SA 2½
standard or chemically treated and coated with an adequate paint system with
specialist advice. Shop applied coatings are preferred, but field applications
are also acceptable. Irrespective of type and application method of coatings, a
holiday test on the coating shall be carried out immediately prior to
installation on site.
6.
All points of contact shall be protected while the container is being loaded
and transported. Damage to the protective coating shall be prevented.
7.
All underground containers and piping shall be cathodically protected unless
written tests of soil samples indicate that it is not required. Failure to provide
cathodic protection can cause hidden corrosion and leaks as well as
weakening of the tank wall over time.
8.
The bottom of the hole shall be level and free of rock. If rocks are present, a
150 mm bed of sand shall be used. For completely buried tanks, the hole
CUSTOMER INSTALLATIONS
10-
shall be dug to a proper depth to provide for the housing dome to extend far
enough above ground level to prevent entrance of water (50 to 150 mm is
common practice), allowing for grading away from the dome.
Figure 10.2.6.2: Underground installation
9.
The top of the container shall be at least 150 mm below grade, unless the
container might be subject to abrasive action of physical damage from
vehicular traffic or from other causes such as in LP-gas service stations. In
this case, it shall be placed not less than 600 mm below grade or equivalent
protection shall be otherwise provided (such as by the use of a concrete slab)
to prevent imposing the weight of a loaded vehicle directly on the container
shell (NFPA 58- Par.3.2.4.8 a).
10. For mounded systems, the same general procedure shall be followed, except
that the housing dome would be above ground, and the above ground surface
area of the tank shall be covered with at least 300 mm of earth or sand.
11. In the flood plain and high water level areas, provisions shall be made to
adequately secure the container to the ground, or to a concrete slab, to prevent
flotation. Local soil conditions may require other provisions to allow proper
drainage from within the housing dome.
12. Precautions shall be taken to prevent damage to the tank coating while
lowering the tank into the hole and while back filling. Any damage to the
coating shall be carefully repaired. Any small unprotected areas of a coated,
wrapped and cathodically protected tank and piping system will be subject to
concentrated corrosive action resulting in the possibility of severe metal loss
and ultimately a leak.
13. Back fill shall be free from rocks or similar abrasives. Clean, dry sand is
preferred. See Figure 10.2.6.2.
14. Because the installed container should be adequately anchored if flotation is a
possibility and shall be leak tight with a slight positive nitrogen pressure, it
could be maintained empty. However, it is preferred that the container be
filled immediately after installation is complete.
15. Where underground containers are installed in locations subject to infrequent
vehicular movement, sufficient provision shall be made to prevent the weight
of such vehicular traffic from damaging the container or appurtenances. The
top of the tank shall be at least 600 mm below grade or be protected by a
concrete slab or equivalent.
16. Barriers shall be provided to protect the housing dome, relief valve discharge
stacks, filling risers and any appurtenances that extend above grade level.
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CUSTOMER INSTALLATIONS
Safety in LPG Design
17. Product identification and safety signs shall be installed adjacent to the
housing dome and filling risers. The “NO SMOKING” sign shall be clearly
legible at the safety distance applicable and from points of access to the
storage site.
18. For underground tanks that are embedded in watertight concrete casings and
sand-bed, observation wells with PVC casing of at least diameter 50 mm with
0.5 mm slots shall be installed. Observation wells shall be installed at two
diagonal corners of the underground tank(s) and shall extend to a depth of
600 mm below the bottom of the tank(s).
10.2.7
Container Fittings and Piping
Figure10.2.7: Typical fittings needed on containers
Containers, regardless of size, shall be equipped with the following:
1.
pressure relief valve (PRV).
2.
fixed level dip tube.
3.
filling connection.
4.
level indicator.
5.
pressure gauge.
6.
drain valve.
7.
inspection nozzle.
8.
multivalve.
9.
bottom liquid off take (optional).
10. vapor offtake connection.
11. Regulator.
All directly connected appurtenances shall be closely grouped on the top center of the
horizontal container shell, and protected against mechanical damage by a steel dome
with a hinged cover. On larger containers all the nozzle connections may be installed on
the manhole cover if possible. The pressure relief valve shall not be installed under the
dome, as this would interfere with discharge.
Piping requirements for customer installations vary widely from country to country.
Therefore, it is recommended that local codes and requirements be followed. If there
are no local codes, NFPA 58 Section 3.2 shall be followed.
Safety in LPG Design
17
CUSTOMER INSTALLATIONS
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10.2.8
Container Valves and Accessories
10.2.8.1
Filler Valve
For new fill line installations, an isolation valve (preferably a ball valve) shall be located
at the tank with a single back check filler valve adjacent to it. When it is necessary to
locate the filling connection at a point remote from the filler valve, the filling
connection shall be fitted with a single back check filler valve. A positive shutoff valve
can be installed immediately behind the single back check filler valve in order to
provide maximum safety. Double back check filler valves shall not be used in place of
an isolation valve and single check valve in new installations.
Figure 10.2.8.1: Double back check filler valve
10.2.8.2
Vapor Service Valve
A vapor service valve may be provided as a separate unit or may be incorporated as
part of the combination valve. The valve shall preferably incorporate a back seating
feature. An excess flow check valve shall not be incorporated with the inlet of the
device, unless local regulations ask for it. With an excess flow check valve installed
there is a possibility of an interruption in service to gas-consuming devices due to high
demand. This could cause a flameout followed by resumption of flow when local codes
do not require safety shutdowns on loss of gas supply.
10.2.8.3
Liquid Service Valve
An internal positive shutoff valve shall be installed for liquid off take service from the
bottom of the container. For liquid off take service from the top of the container, an
excess flow check valve shall be installed within the container, and a positive shutoff
valve installed immediately adjacent.
All containers 475 liters or more in capacity shall be fitted with a connection for the
purpose of emptying the container of liquid. This requirement may be satisfied by the
use of a bottom mounted lock type excess flow check valve installed within the
container, which shall normally be plugged. Removal of the plug and the installation of
a nipple or adapter to which a positive shutoff device is attached can activate the
connection for the withdrawal of liquid. In order to ensure the proper procedure in
activating the valve, an instruction tag shall be attached to the plugged excess flow
check valve.
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CUSTOMER INSTALLATIONS
Safety in LPG Design
10.2.8.4
Excess Flow Check Valve
In domestic storage containers, excess flow check valves shall be included as integral
parts of vapor return valves. They shall also be installed either as an integral part of the
liquid service shutoff valve or within the liquid outlet of the container with a separate
shutoff valve installed immediately adjacent. Excess flow valves permit the flow of
liquid or vapor in either direction. Excess flow is controlled in only one direction (the
direction of the arrow stamped in the valve). If flow in that direction exceeds a
predetermined rate the valve automatically closes.
Figure 10.2.8.4: Excess flow check valve
10.2.8.5
Liquid Level Measurement
Visible Float Gauge: A visible float gauge is the first choice for level measurement in
customer containers. It shall be installed under the valve guard or hood for protection.
The liquid level can be observed without discharging LPG vapor or liquid, however,
accuracy may not be always adequate.
Rotary Gauge: A rotary gauge may be used in customer containers if there is no hazard
in connection with the LPG discharge necessary for the measurement.
Fixed Level Gauge: Each container shall be fitted with a fixed liquid level gauge. The
gauge shall be fixed to indicate an 85% filling level.
Slip Tube Gauge: A slip tube gauge is not recommended on containers of 10,000 liters
or lower capacity. A slip tube gauge shall only be installed when there is a strong need
for greater measurement accuracy than float gauge or rotary gauge can provide, and
when electric power is unavailable to operate a servo-gauge or radar gauge which was
discussed in Chapter 3.
10.2.8.6
Pressure Indicator
Each container in excess of 10,000 liters capacity shall be fitted with a pressure indicator
(PI), which may be part of a combination valve. In containers 10,000 or less in capacity,
a pressure indicator is an optional feature; if used, there shall be a shutoff valve between
the container and the PI.
10.2.8.7
Regulators
All regulators shall be designed and installed in accordance with NFPA 58 or
equivalent. Regulators for outdoor installations shall be designed, installed, or protected
so their operation will not be affected by freezing, sleet, snow, ice, mud, or debris. This
protection is permitted to be integral part of the regulator. All materials used to
construct the regulators shall be resistant to the action of LPG under service condition.
Safety in LPG Design
19
CUSTOMER INSTALLATIONS
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Regulator shall be designed for outdoor installation. Regulators shall be incorporated
with an integral relief valve. or shall have a separate relief valve to limit the regulator
outlet pressure. An integral or separate overpressure shutoff device shall be provided to
shutoff the flow of LPG vapor when the outlet pressure of the regulator reaches the
overpressure limits. Regulators with an overpressure protection device and a rated
capacity of more than 147 kW (10.6 kg Propane per hour) shall be permitted to be used
in two-stage systems where the second-stage regulator incorporates an integral or
separate overpressure protection device.
Figure 10.2.8.5: Visible float gauge
Integral two-stage regulators shall be provided with a means to determine the outlet
pressure of the high pressure regulator portion of the integral two-stage regulator.
Exception: Automatic changeover regulators shall be exempt from this requirement.
Integral two-stage regulators shall not incorporate an integral pressure relief valve in the
high pressure regulator portion of the unit. Regulators shall be designed so as to drain
all condensate from the regulator spring case when the vent is directed down vertically.
At low flow rates, a single stage system may be suitable. Check with manufacturers. At
higher flow rates it is typical to use two stage pressure regulation. The first regulator
reduces the pressure to about 1.5 bar gauge (150 kPa) and the second regulation step
lowers it to about 30 millibar gauge (3 kPa). For multiple tank installations only one set
of regulators shall be installed as opposed to individual sets on each tank. This is to
prevent “pressure cycling” of the system, which would be caused by slight differences in
regulator adjustment or different heat input or vaporization.
Aluminum or zinc is permitted for approved regulators. Zinc used for regulators shall
comply with ASTM B86, specification for zinc-alloy die casting, or equivalent
standards. Nonmetallic materials shall not be used for upper or lower casings of
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CUSTOMER INSTALLATIONS
Safety in LPG Design
regulators. Regulators shall have the manufacture date (MM/YY) permanently marked
on the body.
First-stage and second stage regulators shall be installed outside of buildings. The first
stage regulator shall be as close to the storage as practical. First-stage or high-pressure
regulators shall be directly attached or attached by flexible connectors to the vapor
service valve of a container or to a vaporizer outlet. The regulators is permitted to be
installed with flexibility in the interconnecting piping of manifolded containers or
vaporizers.
Figure 10.2.8.7: Container regulator
The point of discharge from the required pressure relief device on regulating equipment
installed outside of buildings in fixed piping systems shall be located as follows:
1.
More than 1 meter horizontally away from any building opening below the
level of discharge.
2.
More than 1.5 meter in all directions from any source of ignition, opening
into direct-vent appliances or mechanical ventilation air intakes.
3.
Not beneath any building unless the space is well ventilated.
Installation of the regulator shall minimize accumulation of LPG condensate. Regulator
inlet piping shall be cleaned at the time when the regulator is installed as foreign
particles that entered the regulator may cause it to malfunction. The pressure regulators
shall be installed in location where tampering of the regulator by unauthorized personnel
is prevented.
10.2.8.8
Multivalve
The multivalve combines the double back check filler valve, vapor equalizing valve with
excess flow, pressure relief valve with protective cap and chain, service line shutoff
valve, fixed liquid level gauge, float gauge opening and plugged pressure gauge opening
in one unit. Using various available combination valves that combine all of the fittings
required can reduce the number of shell penetrations. This Single Outlet System creates
a higher and more congested equipment profile within the dome, and is likely to increase
maintenance time. The disadvantages should be weighed against the reduction in shell
penetration achieved.
Safety in LPG Design
21
CUSTOMER INSTALLATIONS
10-
10.2.8.9
Pressure Relief Valves
All LPG tanks, vaporizers, positive displacement pumps’ discharge shall be provided
with one or more spring loaded or pilot operated pressure relief valves. Sizing of the
valves will depend on the container surface and is described under: “Pressure Relief in
Marketing Terminals” in Chapter 3.
Suitable thermal relief valves shall be provided on liquid lines that can be blocked
between two shutoff valves. Other equipment that can be blocked between shutoff
valves shall be provided with protection from overpressure due to thermal expansion of
the liquid.
Figure 10.2.8.8: Multivalve for vapor and liquid withdrawal
The pressure relief system shall be protected from the closure of any block valves
installed between the tank and the pressure relief valve or between the pressure relief
valve and its discharge vent outlet. This protection may be achieved by one of the
following procedures:
1.
Installing the pressure relief valve without block valves.
2.
Providing excess pressure relief valve capacity with multiway valves,
interlocked valves, or sealed block valves arranged so that isolating one
pressure relief valve will not reduce the capacity of the system to below the
required relieving capacity.
3.
Locking or sealing the block valves open with a lock. The key must be in
possession of an authorized person.
Multiple pressure relief valves can cover total required relief valve capacity. These shall
be installed with a manifold that includes provision for selectively closing off any
particular relief valve to permit removal for inspection while the remaining valves
provide for the discharge capacity required for the container. Alternatively, Multiport
relief valve may be used.
Weep holes on the bottom of pressure relief valve stacks shall be equipped with a 90
degree elbow to deflect a discharging vapor stream away from any container shell or
piping.
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CUSTOMER INSTALLATIONS
Safety in LPG Design
Discharge vents shall lead to the open air or to a flare system. Positive design and
operational steps shall be taken to prevent the discharge of liquid LPG from atmospheric
vents. Such steps include automatic shutdown of filling operations prior to overfilling.
Discharge vents shall be protected against mechanical damage. If discharge vents
relieve to the atmosphere, they shall be designed to prevent entry of moisture and
condensate. This design may be accomplished by the use of loose-fitting rain caps and
drains. Drains shall be installed so that the discharge will not impinge on the tank or
adjoining tanks, piping, equipment, and other structures. Discharge vents shall
terminate a minimum of 3.0 m above grade with a final discharge vertically upward.
Discharge shall be to an area that has the following characteristics:
1.
2.
3.
The area prevents flame impingement on tanks, piping, equipment, and other
structures.
The area prevents vapor entry into enclosed spaces.
The area is above the heads of any personnel on the tank, adjacent tanks,
stairs, platforms, or the ground.
Pressure relief valves on equipment within buildings shall be piped to a point outside the
buildings and shall discharge vertically upwards.
Pilot operated pressure relief valves shall be designed so that the main valve will open
automatically and protect the equipment if the pilot valve fails. Pilot operated valves
shall be provided with a backflow preventer.
10.2.8.10 PRV Testing Requirements
Pressure relief valves in LPG service normally operate in a clean, non-corrosive
environment. Furthermore, PRVs are constructed of corrosion resistant materials, and
are installed so as to be protected against the weather. Because of added odorization, a
leak around a PRV is likely to be discovered during inspection. Pressure relief valves in
LPG service have shown a good reliability over the years. However, since no
mechanical device can be expected to remain in operative condition indefinitely, it is
recommended that the PRV be replaced when the container is tested/reconditioned, or
more frequently if required by local regulations.
Safety in LPG Design
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CUSTOMER INSTALLATIONS
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11 AUTOMOTIVE LPG
11.1
Automotive LPG Stations
11.1.1
Design of Automotive LPG Equipment
This section sets out the minimum design standards for the LPG facilities in a service
station for the refueling of motor vehicles running on LPG. The general minimum
standards for bulk installations and for different components as laid down in the other
sections shall apply except where specifically modified by this section. Local standards
shall be followed. In the absence of a local standard or if the local standard is less
stringent, the following minimum standard shall apply.
LPG tanks at Retail Outlet Stations may be mounded or buried to eliminate the risk of a
BLEVE. Furthermore, there are limited requirements with regard to minimum distances
to buildings in the neighborhood. A mounded or buried system will require an internal,
submerged pump. External inspection for corrosion damage is not easy, cathodic
protection is recommended.
Since there is no formal formula to calculate the LPG tank capacity the following should
be considered. The frequency of unloading operations shall be minimized since each
additional unloading operation constitutes an increased risk. The tank shall be sized to
allow for a weekly supply pattern, e.g. 2% of annual sales volume.
Aboveground tanks shall be installed with sufficient liquid head as required by the
design of the pump. Underground tanks situated below driveway shall be adequately
protected by reinforced concrete slabs or chamber designed by a qualified structural
engineer. The manhole cover and the tank fittings open to access from the top shall be
protected against damage and tampering.
For underground tanks, submersible pumps of a reputable make specifically approved
by ExxonMobil shall be installed. Installation shall be in strict accordance with the
manufacturer’s instructions. Submersible pumps must be installed in barrels such that
the pump unit can be taken out for servicing without having to gas free the whole tank.
Following are international Codes on Automotive LPG:
11.1.1.1
1.
Autogas, CPR 8-1, CPR 8-1s, Netherlands (in Dutch).
2.
The Australian Standard 3509 “LP Gas Fuel Vessels for Automotive Use.”
Remote Operated Emergency Block Valves
The Emergency Shutdown System (ESS) shall be designed and executed as a fail safe
system. All valves with a diameter larger than 1.6 mm shall be part of the Emergency
Safety in LPG Design
AUTOMOTIVE LPG
11-1
Shutdown System. There are 3 types of motive energy to operate the Emergency Block
Valves: Hydraulic, Air and LPG vapor. The valve shall be a fail safe, spring loaded,
quarter turn valve meeting API 607 fire-resistant testing.
1.
Systems driven by LPG vapor: This type of motive energy is
recommended. It has the advantages of sufficient vapor always being
available and a relatively simple system layout.
2.
Air driven system: This type of motive energy is recommended if reliable air
supply is readily available. Air supply should be separate from air used for
pressuring tires since the latter may be emptied by users. Also air for
Emergency Block Valves may need drying since condensed water can cause
internal corrosion in the valve actuators.
Hydraulic system: This type of motive energy is not recommended since even small
leaks influence its reliability.
Emergency shutoff valves shall be installed as close as practicable to the liquid and
vapor inlet/outlet connections on the tank, except where a back flow check valve is
installed and except on the drain connection. A master emergency switch shall be
provided to shut off the power to the pumps and dispensers and to close off all the
emergency shutoff valves installed on the tank connections and at the dispensers. This
emergency switch shall be so positioned as to be readily visible to the public and within
easy reach for quick operation in cases of emergency. It shall be clearly identified by
signage.
11.1.1.2
Layout
LPG tanks shall be located such that the minimum separation distances shown in Table
11.1.1.2-a are not exceeded.
Storage
Minimum Separation Distance (meters)
From Site Boundary, Buildings,
Fixed Sources of Ignition, etc.
Between Tanks
Underground
Water
Capacity of
3
Tank ( m )
Above
Ground
Buried
Portion
Valve Assembly,
Filling Point, etc.
Aboveground
Above
Ground
Under
Ground
0.5 to 2.5
3
3
3
1
1.5
2.5 to 10
7.5
3
7.5
1
1.5
10 to 150
7.5
3
7.5
1.5
1.5
Table 11.1.1.2-a: Minimum separation distances for LPG tanks
Separation distances of LPG facilities from each other and from other features of the
service station shall not be less than that given in Table 11.1.1.2-b. LPG dispensers may
be installed adjacent to other LPG, petrol or Diesel fuel dispensers as long as they are of
flameproof construction.
Pumps other than submersible pumps shall be installed as close to the tank liquid outlet
valve as possible, but not underneath an aboveground LPG tank.
11-2
AUTOMOTIVE LPG
Safety in LPG Design
Safety relief valves shall be fitted with vents with outlets at least 1.8 m above the top of
the tank and not less than 3m above ground level. The vent outlets shall be at least 4.5
m away from the property line or any fixed source of ignition.
LPG dispensing facilities shall not be permitted in service stations built underneath
buildings.
LPG Tank
LPG
Tank Fill
Conn.
LPG
Pump
LPG
Dispen
ser
M Vehi.
LPG Fill
Conn.
1
diameter
Nil
Not below
TK
3m
3m
Nil
-
Nil
3m
3m
Not below
TK
Nil
-
Nil
Nil
LPG Dispenser
3m
3m
Nil
-
Nil
Motor Vehicle LPG
Fill Connection
3m
3m
Nil
Nil
-
Undergr. petrol TK,
manhl. or fill conn.
1.5 m
3m
3m
3m
3m
Aboveground petrol
tank
6m
6m
6m
6m
6m
Petrol tank vent
3m
3m
3m
3m
3m
Flameproof fuel
pump / dispenser
3m
3m
Nil
Nil
Nil
4.5 m
4.5 m
4.5 m
1.5 m
1.5 m
1.5 m
4.5 m
4.5 m
4.5 m
LPG Tank
LPG Tank Fill
Connection
LPG Pump
Non-flameproof fuel
pump / dispenser
Parked cars
As Table 11.1.1.2-a
for fix. source of ign.
1.5 m
Site bound. buildgs,
fix. sources of ign.
3m
As Table 11.1.1.2-a
Table 11.1.1.2-b: Separation distances of LPG facilities from each other and from other features of
the service station
11.1.1.3
Overfill Protection
All tanks shall be equipped with an overfill protection device. A variety of overfill
protection devices are available. The available systems range from high-tech electronic
capillary measuring systems to simple mechanical systems with a floater or a
mechanical meter.
When the maximum filling level is reached, the system shall transmit a signal to the
filling valve and then close the valve within a predetermined time lapse (approx. 15
seconds).
11.1.1.4
Piping System
To avoid excessive pressure in the liquid lines, a thermal relieve valve shall be installed
between two block valves.
Safety in LPG Design
AUTOMOTIVE LPG
11-3
Excess flow valves or non-return valves shall be installed in all LPG lines from/to the
tank. The type of valve to be used depends on the flow direction of the LPG. Install
excess flow valves in both underground liquid and vapor lines at the dispenser.
A vapor release valve shall be installed at the filling point to enable the tank truck driver
to release the vapor in the filling hose before and after the filling operation. The
maximum quantity to be released is 1 kg. The quantity sets limits for the size/length of
the filling hose. The liquid capacity of the filling line shall not exceed a volume of 200
liters.
Underground pipes conveying liquid LPG shall be either:
1.
Installed in a concrete lined duct which is subsequently filled with clean sand,
or
2.
Buried at a depth below ground of at least 1 m. The route shall be indicated
by markers on the ground surface.
Underground LPG vapor or liquid pipes shall not be embedded in concrete.
Figure 11.1.1: Piping and instrument diagram of automotive LPG installation
11.1.1.5 Dispensing Equipment
The dispensing system shall consist of the following essential components:
11-4
1.
A vapor separator to separate vapor from the liquid before metering.
2.
A meter to measure the volume of liquid delivered.
AUTOMOTIVE LPG
Safety in LPG Design
3.
A differential valve to prevent the formation of vapor beyond the vapor
separator and in the meter.
4.
A flexible hose and filling nozzle. An excess flow valve as near as practicable
before the inlet of the flexible hose.
Hydrostatic relief valves.
5.
6.
7.
8.
A pump switch to control any remotely located electric pump.
features to prevent unauthorized use or tampering.
A driveaway protection coupling on the hose.
Security
An emergency shutoff valve at the base of the dispenser that will close off the
liquid supply upon being hit, in addition to the normal means of closure as an
emergency shutoff valve.
The driveaway protection “beak-away” coupling shall be shall be able to disconnect in
the event of a force of no greater than 600 N. It shall be capable of being re-assembled
without the need for draining the hose, the use of special tools, or the replacement of
parts.
The filling nozzle shall be of the low emission transfer type. It shall mate with the
filling connection on the receiving container on the vehicle such that, when they are
disconnected after refilling, no more than 4 milliliters (cm3) of liquid shall be released to
the atmosphere. It shall not be possible to discharge LPG unless connected to a fill
connection on the vehicle. It shall not have any latching device.
11.1.1.6
Hose Requirements
All hoses for tank filling shall be approved for LPG services. The responsibility for the
filling hose stays with the transportation company.
The flexible delivery hose (dispenser) shall be manufactured to a recognized standard
such as BS 4089, AS 1869, UL 21 or equivalent. It shall be of stainless steel wire braid
or nylon reinforced synthetic rubber and shall have a design working pressure of not less
than 25 bar and a burst pressure of not less than 100 bar.
The length of a delivery hose may vary from 3 to 5 meters. In the delivery hose a
“break away” coupling shall be installed. It is important to follow manufacturers’
installation requirements for the break away installation. It is recommended to perform
an initial test to ensure that the break away connection works as installed. The delivery
hose shall be so secured on the dispenser that it cannot lie on the ground with potentials
of being run over by vehicles. Damaged hoses shall be replaced. Authorized contractor
firm may only carry out replacement.
11.1.1.7
Bulk Filling System
Prior to starting any discharge operation, an earth connection shall be made between the
tank truck and the LPG installation at the Service Station. The earth connection shall be
independent of the hose connection. A 24 volt signal from the tank truck will open the
filling valve. When the maximum filling level is reached this 24 Volt signal will be
interrupted by the overfill protection system and will close the filling valve. This means
that the filling valve is always in a closed position during idle times. The Emergency
Shutdown System (ESS) on the tank truck is part of this 24 volt signal system.
Activating the push button of the truck ESS will also close the filling valve. In cases
where the bulk truck cannot come close to the LPG tank, a remote unloading point may
be installed.
11.1.1.8
Fire Protection
Fire protection shall be designed in accordance local authority which has to be consulted
for final approval.
Safety in LPG Design
AUTOMOTIVE LPG
11-5
Warning signs with the words such as “STOP MOTOR”, “NO SMOKING”,
“FLAMMABLE GAS” shall be posted at all LPG handling areas. The locations of the
signs shall be determined by local conditions, but the lettering shall be large enough to
be visible and legible from each point of transfer.
Emergency controls, if provided, shall be conspicuously marked, and the controls shall
be located so as to be readily accessible in emergencies.
At least one 20 mm hose reel shall be provided. Water supply shall be at least from
hydrants not more than 100 m away. Firewater piping system shall be constructed of
metallic material. Plastic material is not allowed. Fire-fighting foam shall not be used
for LPG fire.
At least one 9 kg portable dry chemical with a B: C rating fire extinguisher shall be
available at strategic locations around the station premises. Minimum shall be one at
each LPG dispenser and one in the sales office attendants’ kiosk. These are in addition
to whatever is required for the fuel dispensers and fuel storage at the same station.
Figure 11.1.1.7: Remote unloading point
11.1.1.9
Crash Barriers
The filling point shall be protected by means of crash barriers. The crash barriers can be
made of steel pipes filled with concrete. The minimum diameter of the steel pipes is 100
mm and the pipes shall extend at least 0.6 m above ground level. The customer
dispenser shall also be protected by means of crash barriers. Valves and Instruments on
Automotive LPG Tank
11.1.1.10 Security Around Tank
The LPG storage installation shall be fenced in. The minimum distance from the fence
to the LPG installation is 3 meters. The fence shall be equipped with two exit doors, the
doors shall be placed opposite of each other. The doors shall be kept closed and only be
opened by authorized persons. The area inside the fence and at a suitable distance from
11-6
AUTOMOTIVE LPG
Safety in LPG Design
LPG tank shall be kept free of vegetation. The following text or pictograms shall be
placed on the fence:
Entry by unauthorized
persons is prohibited.
Smoking & open fire
prohibited
Safety in LPG Design
AUTOMOTIVE LPG
11-7
12 LPG PROPERTIES
12.1
Product Properties
LPG consists of light hydrocarbons, including Propane, Propylene, normal Butane,
Isobutane, and Butylenes. The most common LPG components are Propane and normal
Butane or mixtures of these. At ambient temperature and atmospheric pressure, LPG is
a gas. It can be liquefied under moderate pressure or by cooling to temperatures below
its atmospheric boiling point, but will readily vaporize upon release to normal
atmospheric conditions. This property permits LPG to be transported as a liquid, and
used in the vapor form.
Property
Commercial Commercial
Propane
Butane
Molecular Weight
44
58
Liquid Density, (kg/m3), 15 °C @ Vapor
pressure
505
580
Vapor Density, (kg/m3), 15 °C @ Vapor
pressure
15.3
5.62
Liquid Specific Volume, (m3/ton), 15 °C @
Vapor pressure
1.96
1.73
Vapor Specific Volume, (m3/ton), 15 °C @
Vapor pressure
65.4
178
Vapor Density, (kg/m3), 15 °C @ Atmospheric
pressure
2.0
2.6
Vapor Specific Volume, (m3/ton), 15 °C @
Atmospheric press.
500
400
0.510
0.575
1.5
2.0
- 42
-2
Liquid Specific Gravity 15/15 °C
Specific gravity of vapor (air = 1.0)
Atmospheric boiling point, (°C ), @ Atmospheric
pressure
Table 12.1-a: Properties of Commercial Propane and Commercial Butane
Safety in LPG Design
LPG PROPERTIES
12-1
Commercial
Propane
Commercial
Butane
Ignition energy, (mJ)
0.1
0.1
Flash point, (°C )
-104
-60
450 - 580
420 - 550
1970
1975
Combustion air, (m3/m3 gas) @ stochiometric
24
30
Lower flammable limit(LFL), % in air
2.0
1.8
Upper flammable limit(UFL), % in air
10
9
Motor Octane Number
100
95
Expansion factor at transition from liquid to
vapor at 15 °C
270
240
0.003
0.002
Property
Auto-ignition temperature range, (°C)
Flame temperature in air, (°C )
Cubic Expansion Coefficient of Liquid per °C
Table 12.1-b: Properties of Commercial Propane and Commercial Butane
Common properties for Propane, Isobutane and Normal-Butane are shown in Table
12.1a and 12.1-b. Selected properties as a function of temperature for Propane,
Isobutane and Normal Butane, and commercial mixtures are shown in Figures 12.1-a,-e.
Figure 12.1-a: Vapor Pressures for Butane-Propane Mixtures
12-2
LPG PROPERTIES
Safety in LPG Design
650
N - Butane
Density, kg/m3
600
ISO - Butane
550
500
Propane
450
400
-40 -30 -20 -10 0
10 20 30 40 50 60
Temperature, C
Figure 12.1-b: Liquid density at vapor pressure as a function of temperature
2.90
Density, kg/m3
2.70
2.50
Comm. Butane
2.30
2.10
1.90
1.70
1.50
Commercial
-40 -30 -20 -10
0
10
20 30
40 50
60
Temperature, C
Figure 12.1-c: Vapor density at atmospheric pressure as a function of temperature
Safety in LPG Design
LPG PROPERTIES
12-3
60
Density, kg/m3
50
Propane
ISO - Butane
30
20
0
-30 -20
0
10
30 40
60
Temperature, C
Figure 12.1-d: Vapor density at saturation pressure as a function of temperature
450
Latent Heat, kJ/kg
430
390
370
Propane
330
310
Butane
270
250
3
5
9
11
15
17
Vapor Pressure, bar
12-4
LPG PROPERTIES
Safety in LPG Design
12.2
LPG Hazards
LPG produces certain hazards unique to this fuel. Concentrated LPG vapor is heavier
than air and tends to stay close to the ground. It drifts downwind and collects in low
spots, and disperses less readily than lighter-than-air gases. Since LPG is mostly stored
under pressure and vaporizes readily, it is difficult to control leaks once they occur.
Once LPG is released, it mixes with air to form a flammable mixture. Leakage of a
small amount of liquid produces a much larger volume of vapor, as shown in Table 12.2.
This makes it important to prevent leaks, keep ignition sources at a safe distance, and
disperse vapor from leaks before it is ignited.
Property
Liquid Volume
Resultant Vapor
Volume
Volume of
Flammable Mixture
at LFL
Propane
Butane
1
1
270
230
12600
12200
Table 12.2: Vapor volumes for Propane and Butane
A leak ignited near its source produces a jet flame, which can endanger nearby
equipment and enlarge the incident. If ignition is delayed and the release is large,
enough flammable mixture can accumulate to produce a large Vapor Cloud Explosion
(VCE).
The magnitude of a VCE incident depends on the amount of LPG involved. It has been
conservatively estimated that release of as little as 500 kg of LPG may produce
conditions under which a VCE might occur. Blast damage from a large VCE can extend
beyond 200 meters and is almost certain to cause secondary effects as tank fragments
impact additional piping and equipment.
Fireball diameter, m
1000
100
10
1
0,001
0,01
0,1
1
10
100
1000
Mass in Tank, tons
Figure 12.2-a: BLEVE fireball diameter
Safety in LPG Design
LPG PROPERTIES
12-5
Potentially more serious is a Boiling Liquid Expanding Vapor Cloud Explosion
(BLEVE). Such an event typically starts with a fire near an LPG pressure storage or
transportation tank. The fire heats the tank contents and the internal pressure rises. If
there is direct flame impingement on the top (vapor) part of the tank at the initial stage
of the fire, BLEVE may happen before the PRV activates. If the tank is still intact, the
pressure relief valve may open and continue to discharge.
The lower part of the tank is “wetted” and cooled by boiling product inside, but the
metal temperature of the upper “dry” area rises. The “dry” steel may be overheated and
weakened until it can no longer withstand the internal pressure, and the tank fails
catastrophically. Depending on conditions, failure of a tank may happen from five
minutes to one hour after the fire begins.
If the tank fails, the boiling LPG in the tank is released resulting in massive
vaporization. The vapors will ignite immediately, though there is little or no mixing
with air, and burning occurs at the surface in the form of a large fireball. The size of the
fireball can be taken from Figure 12.2-a “BLEVE fireball diameter” above. A 1 kg
disposable LPG container will create a fireball diameter of about 5 m diameter, whereas
a 1000 ton sphere BLEVE will result in a fireball of 500 m diameter.
Heat radiation from the fireball will cause serious damage within a sizable radius.
Wood and other combustible items will ignite spontaneously at considerable distances.
The damage produced by the blast wave of a BLEVE is less serious because its radius is
usually smaller than that for radiant heat from the fireball. Another hazard is fragments
produced from the tank failure. This is most serious with horizontal tanks, which tend to
behave as rockets and have traveled over 1000 meters in actual incidents (Mexico City).
A BLEVE is possible for a pressurized container of any size. Failure of a large tank will
be destructive for hundreds of meters. Even a small domestic cylinder can BLEVE;
initially, the affected area is limited, but other nearby cylinders and facilities are likely
to become involved. The probability of a BLEVE or VCE is very low, but because of
the potential severity, it is important to provide sufficient fire protection.
In addition to fire and explosion hazards, LPG also presents personnel hazards including
the potential for asphyxiation and cold burns. These are described in the LPG Safe
Operations Guidelines manual.
Release Rate, kg/s
100
10
Liquid
1
Vapor
0,1
0,01
1
10
100
Pipe Diameter, mm
Figure 12.2-b: Propane release rates from guillotine pipe failure
12-6
LPG PROPERTIES
Safety in LPG Design
1000
Distance, m
Very Stable, 2 m/s
100
Neutral, 5 m/s
10
1
1
10
100
Release Rate, kg/s
Figure 12.2-c: Propane cloud dispersion to lower flammable limit
Release Rate, kg/s
100
10
Liquid
1
0,1
Vapor
0,01
1
10
100
Pipe Diameter, mm
Figure 12.2-d: Butane release rates from guillotine pipe failure
1000
Distance, m
Very Stable, 2 m/s
100
Neutral, 5 m/s
10
1
1
10
100
Release, kg/s
Figure 12.2-e: Butane cloud dispersion to lower flammable limit
Safety in LPG Design
LPG PROPERTIES
12-7
13 Glossary of Terms
ACTIVE FIRE PROTECTION
Active fire protection is provided by firewater, sprays, monitors, dry powder, fire
brigade etc. Passive fire protection is provided by spacing, fireproofing etc.
ANSI
American National Standards Institute.
API
American Petroleum Institute.
ASME
American Society of Mechanical Engineers.
ASTM
American Society for Testing and Materials.
AUTO-REFRIGERATION
The chilling effect from vaporization of LPG when it is released or vented to a lower
pressure.
BAR GAUGE
Since bar is always absolute the term bar gauge is used to indicate pressures above
atmospheric inside systems i.e. the pressure read on the gauge. It is often impractical to
use absolute pressures. In this manual some pressures are in bar (absolute), some in bar
gauge (bar gauge is bar minus one).
BLEVE
Boiling Liquid-Expanding Vapor Explosion. A BLEVE occurs if a tank that is exposed
to fire disintegrates suddenly because the tank metal was overheated. This usually
happens if cooling by firewater is insufficient.
Safety in LPG Design
Glossary of Terms
13-1
BONDING
Bonding means electrically connecting two pieces of equipment that do not have metal
to metal contact. Typically this is done during truck loading/unloading. Bonding straps
around flanges are not needed as the flange studs and nuts provide adequate
conductivity.
BULK PLANT
A facility that receives LPG by tank car, tank truck, marine vessel (barge), or piping. It
distributes this gas to the end user by cylinder (package) delivery, by tank truck, or
marine vessel (barge). Such plants have bulk storage of 7.6 m3 water capacity or more
and usually have either container filling or truck loading facilities or both on the
premises.
BULK STORAGE
Bulk storage includes LPG storage in plants or at large industrial customer sites.
BULLET
A bullet is a large above ground horizontal tank containing LPG. It is typically used for
bulk storage at plants and industrial customers. Fireproofing may protect it.
CAR SEALED OPEN
Term used to define a position of a valve during normal operating condition. Car seal
wires or other devices should indicate that this position has been maintained at all times.
Car sealed valves may be closed for maintenance purposes but then re-opened for
operations.
CATHODIC PROTECTION
Cathodic protection is a technique to reduce corrosion of a metal surface by passing
sufficient protective Direct Current to cause the anodic dissolution rate (corrosion
caused by electrolytic properties of the soil) to become negligible.
CAVITATION
If the Net Positive Suction Head at the suction of the pump falls below its (correct)
design value the liquid starts boiling and forms bubbles. This occurs in the high velocity
zones of the pump, especially at the rotating impeller. The bubbles are unstable and
collapse in zones of higher pressure. The collapsing bubbles impact the metal and erode
metal surfaces, ultimately damaging the pump.
CENTRIFUGAL PUMP
In a centrifugal pump the fluid flows through a rotating impeller where centrifugal force
increases kinetic energy, which is later converted to a static pressure rise as the fluid
velocity is reduced in a diffuser.
CET
See Critical Exposure Temperature.
13-2
Glossary of Terms
Safety in LPG Design
CGA
Compressed Gas Association.
CHATTERING
Pressure relief valve chattering is a condition in which the valve opens and closes
rapidly. This can lead to leakage or valve destruction. It may be caused by too large a
PRV capacity, too high a pressure drop in the connection to the PRV or by a faulty
setting of the PRV.
CHECK VALVE
Valve designed to permit flow only into one direction.
COMMERCIAL BUTANE (OR PROPANE)
Have typical specifications that are available from the manufacturing plants.
Commercial Butane or Propane contains percentages of other petroleum fractions of
about the same volatility, such as propylene and Isobutane.
COMMISSIONING, DECOMMISSIONING
Taking a tank, container or cylinder into LPG service or out of service. This happens
during initial filling or before scrapping and before and after any maintenance or test
activity.
COMPRESSED GAS
Any material or mixture having, when in its container, an absolute pressure either
exceeding 276 kPa (2.76 bar) at 21.1 °C, or exceeding 717 kPa (7.17 bar) at 54.4 °C.
CONTAINER
A container is a small tank containing LPG. Such containers are typically used at
customer sites for heating, cooking or small industry consumption. They may be
installed above ground, mounded or below ground.
CONTAINER APPURTENANCES
Items connected to container openings needed to make a container a gas-tight entity.
These include, but are not limited to, pressure relief devices; shutoff, back-flow check,
excess-flow check, and internal valves; liquid level gauges; pressure gauges; and plugs.
CONTINGENCY
A contingency is a normal or abnormal event during plant operation that could lead to
overpressuring. The magnitude of the contingency will have a direct impact on the
sizing of the pressure relief valve.
Safety in LPG Design
Glossary of Terms
13-3
CRITICAL EXPOSURE TEMPERATURE
The Critical Exposure Temperature (CET) is the minimum steel temperature at which a
component will be subjected to a pressure greater than 25% of the design pressure. If a
component is exposed to lower temperatures it may fail through brittle fracture. The
possibility of auto-refrigeration and the lowest one day mean temperature will be of
influence when determining the CET for LPG tanks.
CSO
See Car Sealed Open.
CYLINDER
A cylindercontains LPG for customer consumption. It is normally portable, may be
reusable or disposable. Cylinders are also often called “Bottles.”
CYLINDER FILLING
Cylinder filling is a manual, semi-automatic or fully automatic process to fill cylinders
with LPG. Most important in this process are prevention of overfilling and leak control.
CYLINDER FILLING SHED
A well ventilated building in which the filling of cylinders takes place.
CYLINDER VALVE
A valve, used on LPG cylinders, which normally incorporates a shutoff valve and a
pressure relief valve in one unit. A fusible plug, incorporated in the same valve may be
required while PRV may not be allowed in some countries.
DESIGN PRESSURE
The design pressure is the maximum internal (or external) pressure used to calculate the
minimum permissible wall thickness of tanks, drums, containers or cylinders. It is
always higher than the operating pressure. For vacuum conditions the external pressure
is the maximum difference in pressure between the atmosphere and the inside of the
equipment.
DESIGN TEMPERATURE
The design temperature is the temperature corresponding to the most severe condition of
coincident pressure and temperature. The design temperature may include both a
maximum and a minimum (CET) condition.
DFT
Dry Film Thickness is the thickness of a surface coating once the coating has dried or
cured.
13-4
Glossary of Terms
Safety in LPG Design
DIFFERENTIAL SETTLEMENT
Differential settlement occurs after a structure has been erected. As a result of structure
size and uneven compressibility of the ground, parts of the foundation may sink into the
ground to different depths and cause stress in the structure.
DIFFERENTIAL VALVE
A special valve actuated by the difference between two pressures. It is usually used to
control the higher pressure at a desired amount over the lower pressure.
DIP PIPE, DIP TUBE
A pipe extension from a tank opening or from a valve which extends into either the
liquid or vapor space of the tank. It is used to locate the tank liquid level.
DOT
Department of Transportation, US government.
DRY BREAK COUPLING
A dry break couplingworks on the principle that only a negligible amount of product is
released to atmosphere when the coupling is disconnected. This is achieved by
displacing the product inside the coupling with two pistons.
ELECTRICAL AREA CLASSIFICATION
The concept of electrical area classification has been introduced to control electrical
ignition sources around equipment that contains flammable materials. Certain areas in a
plant require higher quality electrical equipment to take account of the fact that
flammable mixtures may be present at some time during plant operations.
ELT
See LDE below.
EMERGENCY BLOCK VALVE
An Emergency Block Valve (EBV) is a manual or remote operated valve that is installed
upstream of a source of potential leak and can provide tight shutoff. Sources of
potential leaks are pumps, compressors, loading arms, hoses, cylinder filling, etc.
Remote operated valves are activated by the Emergency Shutdown System (ESS).
EMERGENCY RELEASE SYSTEM
An Emergency Release System is provided at certain marine unloading piers. Under
emergency conditions it will release the loading arm automatically to prevent damage
and product leakage.
EMRE
ExxonMobil Research and Engineering Co., Fairfax, Virginia, USA.
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Glossary of Terms
13-5
ERE
Former Exxon Research and Engineering Co., Florham Park, New Jersey, USA
EXCESS FLOW VALVE
A device designated to close when the liquid or vapor passing through it exceeds a
prescribed flow rate as determined by pressure drop. An excess flow valve allows flow
to pass in either direction, but protects against excess flow in only one direction. The
valve reopens when the design pressure differential is restored.
FAIL SAFE
A device or system “fails safe” if on loss of energy it automatically moves to the safest
position. For valves on LPG tanks this would mean the closed position.
FILLING DENSITY
The percent ratio of the weight of liquefied gas to the weight of water at 15.6 °C, which
a container will safely hold at a specific temperature.
FIRE IMPINGEMENT
Fire impingement occurs if in a fire situation the flames come into direct contact with
tanks, containers, cylinders, piping etc. See also jet flames. Flame impingement is one
of the overpressure contingencies since the external heat leads to a temperature and
pressure increase inside equipment.
FLEXIBLE CONNECTOR
A short (not exceeding 1 m overall length) component of a piping system fabricated of
flexible material (such as hose) and equipped with suitable connections on both ends.
FLOAT (OR MAGNETIC) GAUGE
A gauge constructed with a float inside the container resting on the liquid surface that
transmits its position through suitable leverage to a pointer and dial outside the
container, indicating the liquid level. Normally the motion is transmitted magnetically
through a non-magnetic plate so that LPG cannot be released to the atmosphere by a seal
failure.
FUSIBLE LINK
A link or section in a support, rod, shaft, or cable which will melt at a predetermined
temperature and cause a valve or fire door to close, or will effect another action for fire
protection purposes.
FUSIBLE PLUG
A low melting temperature metal plug designed to melt and release the pressure in a
cylinder or piping system at a predetermined temperature. A fusible plug may be
included in the cylinder valve when specified.
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GP / BP
ExxonMobil Global Practices (previously International Practices and before that Basic
Practices). Engineering Practices for facility installation.
GROUNDING (EARTHING)
Grounding means electrically connecting major equipment parts (tanks, pumps) to the
ground. This is to provide quick dissipation of lightning or stray currents.
HARD ARM
A hard piping connection to unload or load LPG on trucks, rail cars or ships. Swivel
joints provide flexibility.
HAZARD
A threat which could cause an accident
HAZOP
Hazard and Operability Analysis is a hazard identification analysis used during the
design phase of a project. An independent HAZOP team studies all aspects of process
related hazards to verify that adequate controls are included in the design to control or
mitigate those hazards.
HEAT RADIATION
Heat radiation is the transfer of thermal energy from a fire by radiation to equipment. It
is one of the contingencies that create overpressure inside equipment.
HOSE TRACKING PROGRAM
A hose tracking program provides information on the origin of hoses, their age, their test
results, and their anticipated date of retirement.
HYDROCARBON
Organic compounds of hydrogen and carbon whose densities, boiling points, and
freezing points increase as their molecular weights increase.
ICC
US Interstate Commerce Commission.
INERT GAS
A noncombustible, non-reactive gas that renders the combustible material in a system
incapable of supporting combustion by virtue of displacing air.
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Glossary of Terms
13-7
INSULATING FLANGE
An insulating flange has to be installed to achieve electrical isolation of a piece of
equipment for cathodic protection purposes. Typical application of insulation flanges
are around mounded drums, at pipeline terminals and at marine piers. Insulating flanges
at piers also prevent any sparking upon disconnecting.
INTERNAL VALVE
A primary shutoff valve for containers that has adequate means of actuation and that is
constructed in such a manner that its seat is inside the container and that damage to parts
exterior to the container or mating flange will not prevent effective seating of the valve.
ISGOTT
International Safety Guide for Oil Tankers and Terminals.
JET FLAME
If fluid escapes at a leak under high pressure it usually takes the shape of a long plume.
If this plume ignites it forms a jet flame.
KNOCK OUT DRUM
A Knock Out Drum (KO Drum or Liquid Trap) is a container provided at the suction
side of a compressorto prevent intake of liquid, which could damage the compressor.
LEVEL HIGH CUT OFF
A level high cut off is a means to prevent the liquid level from rising too high. This is
provided on tankage or knock out drums on compressors.
LDE
Loan Delivery Equipment. Also called ELT in Mobil heritage, Equipment Loaned to
Trade. Tanks or cylinders owned and maintained by the Company but operated by third
parties (customers).
LOWER FLAMMABILITY LIMIT
Lower Flammability Limit (LFL) is the lowest concentration of flammable vapor in air
that will result in a mixture, which can be ignited. For Propane this is 2.0 volume % and
for n-Butane it is 1.9 volume % in air. If temperature or pressure of the mixture are
increased the LFL will decrease (i.e. will be more dangerous).
LIQUEFIED PETROLEUM GAS (LPG OR LP GAS)
Propane and Butane are called Liquefied Petroleum Gas (LPG) because they will liquefy
if subject to higher pressure at atmospheric temperature. They will also liquefy if
refrigerated at atmospheric pressure. They may be composed predominantly of any of
the following hydrocarbons or mixtures of them: Propane, Propylene, Butane (n Butane
or Isobutane), and Butylene.
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MAGNETIC GAUGE
See Float Gauge.
MAXIMUM ALLOWABLE WORKING PRESSURE
The maximum gauge pressure permissible in a tank or container during normal
operation at design temperature.
MERCAPTANS
A family of chemical compounds similar to alcohol in which sulfur replaces oxygen.
Many mercaptans have an offensive odor and are used for odorization of LPG.
MEP
Mobil Engineering Practices.
MINIMUM VAPOR SPACE
The minimum space in a tank, above the liquid level, which is necessary to ensure that
the tank does not become hydraulically full under normal operating conditions.
MOUNDED TANK
Large horizontal tank containing LPG which is installed above ground level but is
covered by an earth mound which protects it from fire. These tanks need a high quality
corrosion protection.
NET POSITIVE SUCTION HEAD
Net Positive Suction Head (NPSH) is the pressure at the impeller of a pump. In order
for the pump to function properly, the NPSH must always be above a certain level
specified by the pump manufacturer. The pumped product, the product temperature, the
product vapor pressure, and the suction piping arrangement will influence this.
NFPA
National Fire Protection Association.
NPGA
National Propane Gas Association.
NPSH
See Net Positive Suction Head.
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13-9
NPQC
Non Process Quality Control is used to ensure that materials of construction and
equipment will perform as required per specifications and design. NPQC is often done
at manufacturers sites and may involve tests.
OVERFILLING
LPG expands its volume with a temperature increase. If a tank, container or cylinderis
overfilled an increase in ambient temperature may lead to opening of the pressure relief
valve and a discharge of LPG. To avoid this, the filling level is limited. Normally
tanks, containers or cylinders are filled to 85%, however, this figure may vary depending
on conditions (e.g. if LPG is already at a higher temperature during filling, it cannot
expand much more).
OVERPRESSURE PROTECTION
Equipment containing pressurized fluids by design is limited to a maximum pressure.
This maximum pressure is determined by operating conditions and economical
considerations. Overpressure protection in form of automatic pressure relief should be
provided to ensure that the pressure inside the equipment remains within safe limits.
PASSIVE FIRE PROTECTION
Passive fire protection mitigates the impact of a fire without relying on rapid detection,
alarm, or response. Mechanical or electrical failures have no effect on its performance.
Passive protection includes spacing, mounding or burying a tank, or providing
fireproofing. These passive measures increase the time available to provide active
response, i.e. firefighting response.
POSITIVE DISPLACEMENT PUMP
In a positive displacement pump the fluid is aspirated into a cavity and than expelled by
decreasing the volume of the cavity. The pressure rise is created by displacement force.
For LPG, sliding vane, internal gear or crescent pumps are used. In case the discharge
side can be blocked, such pumps should be protected against overpressure by a relief
valve between the pump discharge and any block valve.
POST-WELD HEAT TREATMENT
Post-Weld Heat Treatment is heating of equipment to a temperature that will reduce the
stress that originates from welding on the equipment.
PRESENTATION FLANGE
Dock side flange which makes contact with the ship flange.
PRESSURE RELIEF VALVE
A pressure relief valve (PRV) is a mechanical device that is designed to open
automatically once a certain pressure upstream of the PRV is reached. By this feature it
provides overpressure protection for the equipment.
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PUSH-BUTTON
The Emergency Shutdown System may be activated by push-buttons placed in strategic
locations.
PWHT
See Post-Weld Heat Treatment.
REFRIGERATED STORAGE
A tank artificially maintained at a temperature below the nominal ambient temperature.
REGULATOR
Device to reduce and control pressure at a defined level. Usually regulators are installed
between tankage or cylinders and end users.
RELIEF VALVE MANIFOLD
A piping manifold that may hold two or more relief valves with only one connection to
the tank. Some manifolds, such as the Multi-port manifold manufactured by RegO, are
designed to permit the removal of one relief valve without disturbing the others or
impairing the relieving capacity required for the tank.
RISK
Probability of an accident occurring within a certain time, together with consequence for
people, property and environment.
ROLLOVER
The spontaneous and sudden movement of a large mass of liquid from the bottom to the
top surface of a storage reservoir as a result of an instability caused by an adverse
density gradient.
REMOTE IMPOUNDMENT
In the case of LPG leakage under a bullet or sphere a certain amount of LPG may
remain in the liquid phase. By inclining the ground surface and diverting the liquid to a
shallow pool, which is located away from the tank, it is possible to divert a part of the
liquid to a safer location. Should the vapors ignite the fire exposure to the tank will be
lower.
SET PRESSURE
The set pressure is the pressure at which a pressure relief valve opens. This is identical
to the design pressure of the equipment.
SELF SEALING COUPLING
A coupling that automatically closes when disconnected and opens when connected. It
is also known as a dry break coupling.
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13-11
SHUTOFF PRESSURE
Shutoff pressure is the highest pressure that a pump can reach, i.e. the pressure when the
discharge valve is closed.
SLIP TUBE GAUGE
A variable liquid level gauge in which a relatively small positive shutoff valve is located
at the outside end of a straight tube, normally installed vertically, that communicates
with the container interior. The installation fitting for the tube is designed so that the
tube can be slipped in and out of the container so that the liquid level at the inner end
can be determined by observing when the shutoff valve vents a liquid-vapor mixture.
SPACING
The concept of spacing has been adopted to prevent fire from quickly spreading in a
plant or at a customer site. By experience the industry has suggested minimum
distances between LPG containing equipment and a variety of other locations. Such
locations are the fence, the control house, firefighting equipment or other LPG
containing equipment (loading/unloading etc.). Spacing requirements are incorporated
in most codes.
SPHERE
Large above ground spherical LPG tank. It is typically used for bulk storage at refineries
and large terminals. It may be fireproofed.
TANK
In this Manual the term tank is used for bullets, spheres and mounded drums. The term
container is used for smaller customer tanks.
TANK IDENTIFICATION PLATE
Each tank, drum, or container needs to be permanently identified by a name plate
indicating all relevant data.
TARE WEIGHT
Weight of cylinder, or any container, before filling. Cylinders are stamped with their
empty tare weight plus weight of valve to be fitted when manufactured.
THERMAL EXPANSION RELIEF VALVE
LPG inside equipment expands under the influence of solar or other external heat. If
LPG is subject to elevated temperatures and it is enclosed inside piping sections that are
blocked on both sides, the pressure may rise above design and rupture the piping.
Therefore, Thermal Expansion Relief Valves (Hydrostatic Valves) should be provided.
THREAD
National Gas Taper (¾” NGT ) threads are used on cylinderconnections and National
Pipe Taper (NPT) threads are used in other LPG service. Some countries use DIN 477
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thread. Both NGT and DIN threads look similar.
recommended on cylinder bungs and valves.
Therefore, marking of DIN is
UL
Underwriters Laboratory. US organization for quality controls.
ULLAGE
The space in a tank or container above the liquid level. Ullage is necessary to allow a
minimum vapor space above the LPG.
UNDERGROUND TANK
A tank in which all parts are completely buried under the general grade of the facility.
VAPOR CLOUD EXPLOSION
A Vapor Cloud Explosion (VCE) occurs if a large amount of LPG vaporizes (usually
after a leak), mixes with air and finds an ignition source. In case all LPG is within the
flammable limits the VCE can be severe. The degree of LPG/air mixing is largely
governed by weather conditions.
VAPOR PRESSURE
The pressure developed over a liquid in a closed container. The vapor pressure of LPG
depends on the temperature of the liquid and the composition of the primary
hydrocarbons present.
VAPORIZER
A heat exchanger designed to supply the heat required to convert LPG from liquid phase
into vapor.
VESSEL
Ship. In the text of this manual the term vessel is not used for tanks, drums etc. but the
titles of codes use the word vessel for tanks, drums or towers.
VOLATILE LIQUID
A liquid that will readily vaporize at a relatively low temperature, such as normal
ambient temperature.
WATER CAPACITY
The amount of water, in either weight or volume, at 15.6 °C required to fill a container
full of liquid water.
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13-13
ZONE 0
Zone 0 is an electrical area classification. Zone 0 is an area where an explosive gas
atmosphere is continuously present, or present for a long period. There is no equivalent
US classification, although Zone 0 would fall under Class 1 Division 1.
ZONE 1
Zone 1 is an electrical area classification. Zone 1 areas are defined as locations where
an ignitable concentration of flammable gases or vapors are likely to occur in normal
operations. It is equivalent to the US Class 1 Division 1.
ZONE 2
Zone 2 is an electrical area classification. Zone 2 areas are defined as locations where
ignitable concentrations of flammable gases or vapors are not likely to occur in normal
operation, but may occur under abnormal operation. It is equivalent to the US Class 1
Division 2.
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