Enhancing Risk Management Using an EquipmentBased Pressure Relief System Analysis
Kaushal Shah and Abdul Aldeeb
Siemens Energy, Inc.
Houston, Texas
kaushal.shah@siemens.com
ABSTRACT
A classical risk management strategy is to control a hazard by providing layers of
protection between the hazard and the surroundings. These layers of protection may
include operator supervision, control systems, alarms, interlocks, physical protection
devices, and emergency response systems. For pressurized systems, physical protection
devices, such as pressure relief devices, are classified as the last layer of protection
against overpressure scenarios which may result in loss of containment. The significance
of each layer of protection, including pressure relief valves, is determined through the
implementation of a risk assessment program. Overall, the success of a risk management
system depends on the quality of the inputs to the assessment process, including the
quality of information about pressure relief valves.
Information about pressure relief devices is frequently managed using a data sheet
system that documents the relief device installed on the process. Generally, a relief
device data sheet includes the overpressure scenario for the protected system that is the
basis for the pressure relief device design requirements. Overpressure scenarios are
directly associated with individual pieces of equipment and systems and exist whether a
relief device is present or not. Therefore, to rely solely on pressure relief device data
sheets does not address unprotected equipment in a unit or plant. In addition, relying
solely on pressure relief device data sheets further threatens the quality of a risk
management plan because a data sheet generally documents only a single overpressure
scenario per device. The design scenario for a single relief device may not necessarily be
the same as the design scenario for a disposal system, such as a relief header. Therefore,
a comprehensive pressure relief systems management program cannot be effectively
achieved using a traditional pressure relief device data sheet approach. Without a
comprehensive and effective pressure relief systems management program, a successful
risk assessment program cannot be considered complete.
A comprehensive, equipment-based pressure relief system analysis approach is therefore
presented to address the inadequacies of a strategy that relies solely on relief device data
sheets. This equipment-based pressure relief system analysis will result in a best practice
to analyze and document any pressure relief system. Through extensive experience, this
best practice approach has proven to generate reliable input to the overall risk
assessment process for pressure relief devices. In addition, the equipment-based best
practice approach will be demonstrated to be a systematic methodology focused on
ensuring all equipment are protected, and to be the most efficient workflow for pressure
relief system design and analysis.
© 2009 Siemens Energy, Inc.
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1 REGULATORY MANDATES
The design and installation of pressure relief systems has long been a part of plant design
and operation; however, the same degree of care has not been given to the development
of methods to properly and completely document these systems. The regulatory mandates
contained in Occupational Safety and Health Administration (OSHA) 1910.119 address
the deficiencies of current pressure relief system documentation methods. This regulation
states that all chemical operating facilities governed by OSHA 1910.119 shall maintain
information pertaining to the "relief system design and design basis."
In addition, the American Society of Mechanical Engineers (ASME) Boiler and Pressure
Vessel Code Section VIII Division 1 (referred to as Section VIII from here on) states:
UG-125(a) All pressure vessels within the Scope of this Division, irrespective of
size or pressure, shall be provided with pressure relief devices in accordance
with the requirements of UG-125 through UG-137. It is the responsibility of the
user to ensure that the required pressure relief devices are properly installed
prior to initial operation.
Paragraph UG-125(c) of the states that
UG-125(c) All pressure vessels other than unfired steam boilers shall be
protected by a pressure relief device that shall prevent the pressure from rising
more than 10% or 3 psi, whichever is greater, above the MAWP except as
permitted in (1) and (2) below. (See UG-134 for pressure settings).
(1) When multiple pressure relief devices are provided and set in accordance
with UG-134(a), they shall prevent the pressure from rising more than 16% or 4
psi, whichever is greater, above the MAWP.
(2) When an additional hazard can be created by exposure of a pressure vessel
to fire or some other unexpected source of external heat, supplemental pressure
relief shall be installed to protect against excessive pressure. Such supplemental
relief devices shall be capable of preventing the pressure from rising more than
21% above the MAWP. The same pressure relief devices may be used to satisfy
the capacity requirements of (c) or (c)(1) above and this paragraph provided the
pressure setting requirements of UG-134 are met.
Thus, ASME Section VIII requires that each equipment covered by this code have
overpressure protection to prevent loss of containment. The consequences of not meeting
this requirement may jeopardize the safety of the operation facility. The equipment-based
approach presented herein, which covers each equipment in the operating facility
regardless of the presence of a relief device on equipment, is the best practice for pressure
relief system design and analysis.
© 2009 Siemens Energy, Inc.
All Rights Reserved
1.1 THE BEST PRACTICE AND METHODOLOGY
A best practice is defined as “a technique or methodology which, through experience has
proven to lead reliably to a desired result.” The desired results of this best practice are:
•
•
•
Most comprehensive pressure relief system analysis,
Systematic way to ensure all equipment is protected, and
Most efficient workflow for pressure relief system design and analysis.
The objective of the equipment-based pressure relief analysis is to ensure that all
equipment have the required overpressure protection. Overpressure scenarios are directly
associated with each pieces of equipment and systems and exist event if no relief devices
are present. Therefore, every piece of equipment must be analyzed for potential
overpressure regardless of the presence of a relief device. A relief device list cannot serve
as the primary checklist for confirming overpressure protection for reasons previously
identified. Typically, pressure relief device documentation consists of a relief device
specification sheet that contains physical data of the device as well as information on the
overpressure scenario for which the device was sized. The relief device specification
sheet does not capture all of the overpressure scenarios that may apply to the associated
equipment. The relationship among equipment, overpressure scenarios, and relief devices
must be established. The figure below illustrates the patented relational database structure.
Causes of
Overpressure
Pressure Relief
Devices
Equipment
Fig 1: Equipment-Overpressure Scenarios-Relief Devices Relationship
1.2 SYSTEMATIC AND OPTIMIZED WORK FLOW
In order to accomplish the best practice of the
equipment-based
pressure
relief
evaluation outlined above, a systematic and optimized workflow must be developed. A
systematic and optimized workflow ensures the integrity of the analysis. The pressure
relief system evaluation steps of the systematic work flow are outlined below.
© 2009 Siemens Energy, Inc.
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Step 1: Identification of all potential causes of overpressure associated with each piece of
equipment.
• Document rationale regarding scenario applicability and credibility
• Typical overpressure scenarios include closed outlets on vessel, external fire,
failure of automatic controls, split exchanger tube, cooling failure to
condenser, top-tower reflux failure, power failure, instrument air failure, etc.
API 521 Section 5.1, Table 2 lists common scenarios that may require
overpressure protection.
Step 2: Quantification of required relief rates for each credible scenario expected to result
in overpressure as identified in Step 1. API 521 Section 5.1, Table 2 provides
guidance regarding required relief rates for various scenarios.
• Determine appropriate calculation method
• Define stream properties
• Quantify required relief rates
• Document assumptions, methods, and clarifications
Step 3: Calculation of relief device capacities and comparison to required relief rates.
• Evaluate inlet and outlet pressure drops
• Determine scenario specific capacity
Step 4: Evaluation of the associated vapor disposal system (i.e., flare header).
• Define global overpressure scenarios – relief devices that relieve
simultaneously to a common relief header in the event such as total power
failure, cooling water failure, instrument air failure, steam failure, etc.
• Construct hydraulic model of the vapor disposal system
• Evaluate system hydraulic performance
• Evaluate knockout drum liquid separation and retention time
• Evaluate flare radiation levels
The equipment-based analysis approach and systematic work flow will be able to identify,
at minimum, the following:
1. No relief device on equipment with one or more potential overpressure scenarios
2. Undersized relief device on equipment with one or more potential overpressure
scenarios
3. Improperly installed relief device (including excessive inlet and/or outlet pressure
drops, excessive relief device set points, low-temperature releases)
4. Process Safety Information (PSI) deficiencies (including P&ID inaccuracies,
incomplete equipment and/or relief device data)
5. Vapor disposal system deficiencies (i.e., excessive back pressure, velocity, radiation,
inadequate retention time of knock out drum, etc.)
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1.3 INFORMATION MANAGEMENT
The best-practice has been employed in various projects ranging from several relief
devices to complexes with over one thousand relief devices as well as complicated header
networks. The management of the vast amounts of data associated with these systems is
an integral part of this challenge. For example, a large petrochemical complex with 2,000
relief devices may have approximately 6,000 equipment, which can result in more than
20,000 potential overpressure scenarios. Implementation of an enterprise-level relief
system solution with a systematic approach to not just relief device lists but also
overpressure contingencies, related equipment, piping, global failures (power failure,
cooling water failure, etc.) is a crucial component to enhance the risk management
program.
1.4 DRIVERS
Typical drivers for equipment-based pressure relief analysis include:
•
•
•
•
Safety: As any petrochemical complex is dynamic and undergoes significant
changes over the years, there may be a concern on adequate overpressure
protection for each piece of equipment.
Compliance: Note that Pressure Relief Design Documentation needs to be
updated or recreated per regulatory needs.
Capital Projects: Such as capacity enhancement, debottlenecking studies,
throughput increase, etc.
Standardization: Implementing standard pressure relief design and documentation
methodologies.
1.5 CONCLUSION
In conclusion, an operating facility may not be able to comply with industry regulations
without a current set of documentation for each piece of equipment and relief device. To
ensure that a facility is operating safely, taking into consideration the uniqueness of each
equipment, a relief system design documentation program or system must be based on
equipment and use the equipment list as the primary checklist, not the relief device
checklist. A relational database which establishes links between overpressure scenarios,
pieces of equipment, and pressure relief devices is the key to ensure that every piece of
equipment in the operating facility is protected. With a relational electronic database, the
plant’s relief system information can be easily accessible, maintained, and updated. The
relief system information consists of at minimum, equipment information, relief device
information, overpressure scenario rationale, required relief rate calculations, etc. The
electronic information can also be integrated with other systems such as maintenance,
management of change, mechanical integrity, and P&ID database. Operating facilities are
dynamic and keeping pressure relief system documentation evergreen is a challenge in
this constantly changing environment.
© 2009 Siemens Energy, Inc.
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1.6 THE NEXT CHALLENGE
Once the best-practice methodology is established and implemented, the next crucial
challenge is to ensure pressure relief system design information remains “evergreen” (upto-date) as changes are made to the facility. The examples of incidents that could trigger a
review or revalidation of a pressure relief analysis include:
• Rate change or composition change
• Electrical Failure or Electrical Redistribution (New Transformer or MCC or
Substation Installation)
• Change in relief device set pressure or size or type or discharge location
• Change in unit or equipment operation mode
• Rotating or fixed equipment change
• Equipment failure or piping change
• Change in vessel or flange ratings
• Utility conditions change
• Vessel or relief device relocation or new service
• Changes in flare header size or Knock Out Drum or Flare Tip
• PHA or HAZOP Study
The implementation of the best-practice methodology and utilization of the electronic
relational database are the best way to ensure that pressure relief system design
information remains “evergreen”.
1.7 REFERENCES
1.
U.S. Occupational Safety and Health Administration, Labor 1910.119; 29 CFR Ch.
XVII, July 1, 1993.
2.
American Petroleum Institute, API Standard 520 - Part I, Sizing, Selection, and
Installation of Pressure-Relieving Devices in Refineries, Eighth Edition, 2008.
3.
American Petroleum Institute, API Standard 521, Pressure-Relieving and
Depressuring Systems, Fifth Edition, 2007.
4.
American Society of Mechanical Engineers, ASME Boiler and Pressure Vessel Code,
Section VIII, Division 1, 2007.
5.
Pressure Protection Manager System & Apparatus, United States Patent 5,774,372,
June 30, 1998
Disclaimer:
The information contained in this document represents the current view of the author at the time of
publication. Process safety management is complex and this document cannot embody all possible
scenarios or solutions related to compliance. This document is for informational purposes only. Siemens
makes no warranties, express or implied, in this paper.
© 2009 Siemens Energy, Inc.
All Rights Reserved