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Design and Analysis of Firewater Network for a Typical Onshore Gas
Processing Plant
Chapter · January 2018
DOI: 10.1007/978-981-10-7281-9_17
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DESIGN AND ANALYSIS OF FIREWATER NETWORK FOR
A TYPICAL ONSHORE GAS PROCESSING PLANT
Razeen J1, V. Venkata Krishnakanth2 and Shagufta Ejaz3
1 &3
2
Trainee Engineer-HSE Design, Technip India Ltd,
Asst. Professor-Dept. of HSE, University of Petroleum & Energy Studies, Dehradun
Abstract: Gas-processing plant is having many hazards, which are inherent to the facility.
Even though gas-processing installations are generally located in remote areas, experience
shows that residential/industrial units come up in close proximity with the passage of time.
Hence, these installations, which store, process and handle large quantity of flammable
materials, pose threat to surroundings as well, in addition to their own safety. Such conditions,
therefore, necessitate the introduction of in-built fire protection facilities. It is impossible to
design the fire protection facility to control catastrophic fires. Normally fire protection systems
will prevent the spread of fire and prevent emergencies to the installations. So designing an
effective firewater network plays an important role in controlling the spread of fire. This paper
deals about the design and analysis of firewater network for a typical onshore gas processing
plant based on OISD - 116.
Keywords: Firewater network, Fire zone, Gas processing plant, Hydraulics
1. INTRODUCTION:
Firewater network is the most common and economical fire suppression system and water
is the best medium for controlling the spread of fire. Firewater network usually contains
three major parts. They are Firewater storage tank, Piping distribution system and Fire
pump. Hydrants, monitors and fixed systems like foam system, deluge systems and fixed
monitors.
Generally, availability of continuous supply of water is of prime importance for operation,
which is either met by provision of a storage system and pumps or by nearby natural source.
Firewater supply can be derived from process systems like water injection system, if the
pressure and flow can be maintained under emergency conditions and if possibility of
hydrocarbon contamination can be ruled out. As firewater is a very critical aspect of oil and
gas industries, sizing and routing of firewater network is of prime importance.
The firewater system should be designed in such a way that the total demand calculated will
be maintained at the outlets with a minimum required pressure. The firewater distribution
piping shall be designed in such a way that water flows evenly to all parts of the network
and shall be able to isolate any section of a network without affecting other loops
1.1.
GAS PROCESSING PLANT
Gas processing plant belongs to the upstream activity to describe the production unit
performing the first transformation of the crude oil/gas after the production wells. The gas
processing plant is situated as closed as possible to the production wells or offshore platform.
The crude oil/gas just collected from the wellheads can be directed by the shortest way to
the central processing facility. Hydrocarbons produced from the wells are transported to
onshore gas processing facility through pipeline. Hydrocarbons produced from the well
contain natural gas, natural gas liquids and several contaminants like H2S, CO2 etc.
Hydrocarbon is passes through several unit operations to produce clean dry natural gas free
of contaminants to meet end user requirements. The natural gas liquids which come as a
byproduct is used as it is or fractionated to different lighter hydrocarbons like ethane,
propane etc.
2. METHODOLOGY
The methodology to design the firewater network involves the following steps.
2.1. FIRE ZONE DELIANETION
Fire zone is a geographical area of the plant, where certain minimum requirements shall
be respected so that in case of fire occurring within a fire zone, the potential for fire
spread to other fire zone is limited.
Typically, a Fire Zone shall be an area segregated by road, access way, pipe racks or
pipe sleepers, and clear spaces of 30 m width minimum between equipment to
equipment.
2.2. FIREWATER DEMAND CALCULATION
Fire Water Demand for an installation is the total water requirement to fight the major
two fire scenario in the installation (It may be either Process area or Tank farm area or
in Transformer area).
Firewater demand for this case study is calculated based on OISD-116.
Three alternative methods are used to calculate the demand.
Alternative-1: Fire Water Demand for a Fire Zone
Fire Water Demand for Fire Zone = Overall Area of Fire Zone x Application rate
Application rate: As required by OISD-116, application rate to be considered for
each fire zone area is 1 lit/min/m²
Fire zone surface: The surface of each fire zone is the sum of the surfaces of all sub
areas included in this fire zone
Allowance: An allowance of 372 m3/hr shall be considered for supplementary hose
stream protection and/or use of mobile fire-fighting equipment to protect adjacent
equipment
Alternative-2: Fire water demand for 10 m*10 m area
Fire water demand is calculated by considering 10 m*10 m area of process unit on
fire & provide cover on area of 30 m x 30 m area
Application rate: As required by OISD-116, application rate to be considered for
each fire zone area is 10.2 lit/min/m²
Allowance: An allowance of 372 m3/hr shall be considered for supplementary hose
stream protection and/or use of mobile fire-fighting equipment to protect adjacent
equipment
Alternative-3: Fire water demand for Water Spray system (Deluge system)
Fire water demand for the deluge protection for each equipment is assessed
Allowance: An allowance of 372 m3/h shall be considered for supplementary hose
stream protection and/or use of mobile fire-fighting equipment to protect adjacent
equipment
2.3. HYDRANT & MONITOR LAYOUT
2.3.1.
Hydrants
a. Hydrants shall be located in different areas of the facility to provide complete
protection for the plant
b. For hazardous area provide hydrant post for every 30 m around the plant and for
building and utility area provide hydrant post for every 45 m
c. Hydrants should be placed at 15m from the edge of storage tank and hazardous
equipment
d. For process plants location of hydrants shall be decided based on coverage of all
areas
2.3.2.
Monitors
a. Monitors shall be located at strategic locations for protection of cluster of columns,
heaters, gasifiers, etc., and where it is not possible to approach the higher levels
b. A minimum of 2 monitors shall be provided for the protection of each such area
c. Water monitors for protection of heaters shall be installed so that the heater can be
isolated from the remainder of the plant in an emergency
d. Monitors shall provide protection to firemen in case of fire and it is also placed in
such a direction to direct water on the object
e.
Monitors should not be installed less than 15 M from hazardous equipment
f. There should be proper planning for the placement of HVLRs so that it delivers its
intended purpose
g. The maximum distance of monitors from equipment protected should be 45m
2.4. FIREWATER NETWORK HYDRAULICS
A detailed analysis of the firewater network has been carried out and critical
parameters like supply pressure, available pressure at the remotest point, velocity and
head loss were analyzed. Firewater Network must be sized for 120% of the total water
demand. Proper selection of flow rate allows sufficient water availability in
emergencies. Several combinations of flow requirements must be assumed for design
of network. The firewater system shall be designed to provide a minimum residual
pressure of 7.0 kg/cm²g for the most hydraulically remote point of the fire water ring
main.
Pipe network problems are usually solved by numerical methods using software since
any analytical solution requires the use of many simultaneous equations. Simple
methods used to solve pipe network problems are by using Hazen William equation.
PIPENET Standard/Spray Module is used for hydraulic analysis of firewater systems
in compliance with NFPA13, NFPA15 and NFPA16 rules. This addresses the hydraulic
analysis requirements of virtually all national and international standards.
Hazen-Williams equation:
Pressure drop inside the pipe can be calculated using Hazen – William’s formula,
(4.25 ∗ 𝑄1.85 )
(𝐶 1.85 )𝑑 4.87
𝑃=
where;
P is the pressure drop or friction loss (per 100feet) inside the pipe (psi),
Q is the volumetric flow rate (gpm),
C is the Hazen – William’s friction loss coefficient,
d is the pipes internal diameter (inches),
(or)
in SI units,
𝑃𝑚 = 6.05 𝑥
𝑄𝑚 1.85
𝐶 1.85 𝑑𝑚 4.87
where;
Pm velocity pressure in psi,
Q is the volumetric flow rate (gpm),
D is the pipes inside diameter (inches)
𝑥 105
2.5. DELUGE SYSTEM HYDRAULICS
The hydraulics of deluge system lines from main header to the equipment connected
through deluge valves is done using PIPENET VISION Spray/Sprinkler module.
Pressure drop inside the pipe can be calculated using Hazen – William’s formula.
The following considerations have been taken into account:

The fire water pressure in the range of 1.4 to 3.5 bar (g) is to be achieved for all
the water spray nozzles in the system

The maximum allowable velocity in the header shall be 5.0 m/s for the water
spray pipes & ring pipes

Hazen -William co-efficient is considered as 120
Nozzle Discharge Formula The pressure drop at the nozzle discharge can be calculated
by the following formula,
𝑄 = 𝐾√𝑃
where: Q is the volumetric flow rate from the nozzle (gpm),
K is the nozzle K-factor,
P is the pressure drop across the nozzle.
Deluge valve modelling equation
The pressure drop across the deluge valve can be calculated by the following formula,
P = QX / K
where: P is the pressure drop across the deluge valve,
Q is the volumetric flow rate through the valve,
K is a constant for the valve,
X is a constant for the valve (with typical values being 1 and 2).
3. RESULTS AND DISCUSSIONS
3.1. Fire Zone Delineation: In this case study, the onshore gas processing plant is delineated
into 15 fire zones based on OISD – 116. The 15 fire zones are:
1. Slug catcher area
2. Caustic dosing area
3. Gas Compressor area
4. Scrubber area
5. Gas dehydration area
6. Refrigeration area
7. MEG Refrigeration area
8. MEG storage area
9. Produced water storage area
10. Methanol storage area
11. Diesel storage area
12. Hot oil heater area
13. Air & Nitrogen area
14. Substation
15. Liquid propane area
Fig.2. Fire Zone Delineation
3.2. Firewater Demand Calculation
 In this case study the firewater demand was calculated for two fire scenario
 Largest two firewater demand are in Fire zone-01(Slug catcher area) and Fire zone03(Gas compressor area)
 Thus Fire Water Demand Calculation for the selected case study is as follows:
Demand for slug catcher area = 550.8 + 372(supplementary stream) = 922.8 m3/hr
Demand for Gas compressor area = 600 + 372(supplementary stream) = 972 m3/hr
Total firewater demand = 922.8 + 972 = 1894.8 = 1895 m3/hr
Therefore, Firewater demand is 1895 m3/hr.
Firewater pump capacity = 680 m3/hr
No of pumps = 3
50% of standby pumps shall be provided; so no. of standby pumps = 2
Total pumping capacity = 2040 m3/hr
Firewater storage capacity = 2040 x 4 = 8160 m3
3.3. Hydrant and Monitor Layout
 For hazardous area provide hydrant post for every 30 m around the plant and for
building and utility area provide hydrant post for every 45 m
 The total no. of hydrants required to protect the onshore gas processing plant is 81
 Monitors shall be located at strategic locations for protection of cluster of columns,
heaters, gasifiers, etc., and where it is not possible to approach the higher levels. A
minimum of 2 monitors shall be provided for the protection of each such area
 The total no. of monitors required to protect the onshore gas processing plant is 22
Fig.3. Hydrant and Monitor Layout
3.4. Firewater Network Hydraulics
The hydraulics is done by taking in to account that;
 The length of pipes is considered as 1.5 times the layout dimensions to cater the fitting
losses
 Maximum velocity in the firewater network should not exceed 5 m/s
 The minimum available pressure at the remotest point shall not fall below 7 kg/cm2(g)
 Hazen -William co-efficient is considered as 120
In this case study, the firewater demand calculated was 1895 m3/hr. The firewater
network is sized 120% of the required water demand i.e. 2274 m3/hr. The network
hydraulics is done for two major fire scenario. The two major demand are in Gas
compressor area and Slug catcher area.
3.5. Deluge System Hydraulics
3.5.1. Deluge System Hydraulics for Gas Compressor
 The theoretical firewater demand for gas compressor was found out to be 600 m3/hr
and length of pipes is considered as 1.5 times the actual layout dimensions to cater
the fitting losses.
Operating pressure range for Nozzle: Medium velocity water spray nozzle: 1.4 - 3.5
kg/cm2g
Nozzle Specification
Type
K-factor
Flow rate at 1.4 barg (lpm)
Flow-rate at 3.5 barg (lpm)
Nozzle
Angle
(Degrees)
MVWS
70
Theoretical Flow Rate
Equipment Protected
82.82
Length (m)
Gas compressor (7)
Installed Flow Rate
Equipment Protected
130.95
Diameter (m)
14
5
Surface
Application
Area
Rate
(m2)
(lpm/m2)
70(7)
20.4
Average Flow rate
per Nozzle (lpm)
Numbers of Nozzles
(Nos.)
113
112
Gas compressor
120
Theoretical
Flow Rate
lpm
m3/hr
9996
600
Installed flow rate
lpm
m3/hr
12600
756
3.5.2.Deluge System Hydraulics for Flare KO Drum
The theoretical firewater demand for gas compressor found to be 137.28 m3/hr.
Assumptions: same as mentioned in 3.5.1.
Operating pressure range for Nozzle: Medium velocity water spray nozzle: 1.4 3.5 kg/cm2g
Nozzle Specification
Type
K-factor
MVWS
Flow rate at 1.4 barg (lpm)
Flow rate at 3.5 barg
(lpm)
Nozzle Angle
(Degrees)
49.69
78.57
120
42
Theoretical Flow Rate
Equipment Protected
Flare KO drum
Length
(m)
17
Breadth
/ Dia.
(m)
Surface Area (m2)
4.2
224.32
Application
Rate
(lpm/m2)
Theoretical
Flow-Rate
lpm
m3/hr
10.2
2288
137.28
Installed Flow Rate
Equipment
Protected
Flare KO drum
Average Flow Rate per
Nozzle
(lpm)
68.03
Numbers
of Nozzles
(Nos.)
42
Installed Flow Rate
lpm
m3/hr
2857.66
171.46
4. CONCLUSION
The firewater network for a typical onshore gas processing plant is designed as per OISD
-116 “FIRE PROTECTION FACILITIES FOR PETROLEUM REFINERIES &
OIL/GAS PROCESSING PLANTS”. Effective operation of the ring main is modelled to
acquire adequate flow, velocity and pressure for the smooth operation during emergency
conditions like fire breakout. The simulation and hydraulic design of the firewater ring
main system of an onshore processing terminal to determine the pipe sizes is
accomplished using PIPENET software. The assessment of the fire protection system
meeting the OISD standards is analyzed by using PIPENET Spray/sprinkler module. The
pipe sizing (diameter) are modelled in the software. The typical output illustrates the
flow, velocity and direction in each pipe segment and pressures at each node.
5. REFERENCES
1) Mannan, Sam, ed. Lees' Loss prevention in the process industries: Hazard
identification, assessment and control. Butterworth-Heinemann, 2004.
2) OISD 116 Fire Protection Facilities for Petroleum Refineries and Oil/Gas Processing
Plants – Second Edition, 2012
3) Gordan P. Mckinnon, Keith Tower, Fire Prevention Handbook, National Fire
Protection Association Harry E. Hickey, Hydraulics for Fire Protection, National Fire
Protection Association
4) Cunha, M.C., and Sousa, J., (1999), “Fire Water Network Design Optimization:
Simulated Annealing Approach”, Journal of Water Resources Planning and
Management, Vol. 125, No. 4, pp. 215-221.
5) Lansey, K.E., and Mays, L.W., (1989), “Optimization Model for fire Water Distribution
Design”, Journal of Hydraulic Engineering, Vol. 115, No. 10, pp. 1401-1418.
6) Dragan A. Savic, Godfrey, “A. Walters An Evolution Program For Pressure Regulation
In Water Distribution Networks”, Centre for Systems and Control Engineering, UK
Larry W Mays, “Water Distribution System Handbook”, Mc Graw Hill Handbooks
7) NFPA 15
Edition
Standard for Water Spray Fixed Systems for Fire Protection 2001
8) NFPA 20
Standard for the Installation of Stationary Pumps for Fire Protection
2003 Edition
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