API STD 521
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This document summarizes API STD 521 Part-I, which provides guidance on overpressure protection for refinery equipment. It discusses overpressure causes and protection philosophies. It also lists the minimum recommended contents for relief system designs and flare header calculations. These include analyzing overpressure causes, operating conditions, relief device sizing, and documentation of simulation inputs and outputs. Various overpressure causes are outlined, such as closed outlets, absorbent or cooling failures, accumulation of non-condensables, abnormal heat input, explosions, and depressurizing. Protection measures against these causes like relief valves, rupture disks, and explosion prevention are also mentioned.
API STD 521
- 1. API STD 521 Part-I
- 2. Section I Overpressure Overpressure Protection Philosophy Potentials for Overpressure Recommended minimum relief system design content List of items required in flare header calculation documentation Guidance on vacuum relief
- 3. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 4. Section I Overpressure Overpressure Protection Philosophy Potentials for Overpressure Recommended minimum relief system design content List of items required in flare header calculation documentation Guidance on vacuum relief
- 5. Overpressure Overpressure is the result of disruption of the normal flow of materials and energy that causes the material or energy to build up in some part of the system. Pressure relief systems and depressuring system are employed to keep the system within MAWP .
- 6. Section I Overpressure Overpressure Protection Philosophy Potentials for Overpressure Recommended minimum relief system design content List of items required in flare header calculation documentation Guidance on vacuum relief
- 7. Overpressure Protection Philosophy Double or multiple jeopardy Latent failure Operator error Role of instrumentation Contd…
- 8. Overpressure Protection Philosophy Double or multiple jeopardy The simultaneous occurrence of two or more unrelated causes of overpressure. Operator error It is a potential source of overpressure. Latent failure Latent conditions should be considered as an existing condition and not as a cause of overpressure Role of Instrumentation Pressure relief devices are significant but there might be situations where their use is impractical. At these scenarios, the reliance on conventional instrumentation such as fail safe devices are justified.
- 9. Section I Overpressure Overpressure Protection Philosophy Potentials for Overpressure Recommended minimum relief system design content List of items required in flare header calculation documentation Guidance on vacuum relief
- 10. Potentials for Overpressure Closed Outlets on the vessels Inadvertent valve opening Check-valve leakage/failure Utility failure Electrical or mechanical failure Loss of fans Loss of heat Reflux failure Heat exchanger tube failure Transient pressure surges Plant fires Process changes/Chemical reactions
- 11. Section I Overpressure Overpressure Protection Philosophy Potentials for Overpressure Recommended minimum relief system design content List of items required in flare header calculation documentation Guidance on vacuum relief
- 12. Recommended minimum relief system design content Relief system information Description of protected components Design codes/standards Analysis of causes of system overpressure System operating conditions System relieving conditions Relief device selection/configuration Pressure-relief valve/rupture disk combination capacity factor Relief system required area Relief system capacity for rupture disk Contd…
- 13. Recommended minimum relief system design content Pressure relief valve rated capacity Spring loaded pressure relief valve cold differential test pressure Pressure relief valve capacity correction for maximum back pressure Rupture disk specified burst pressure and manufacturing design range selection Rupture disk specified disk temperature Relieving fluid disposal requirements Relief device physical installation Pressure relief valve inlet line pressure drop Relief device specification sheets Criteria for vacuum protection Contd…
- 14. Recommended minimum relief system design content- Example Recommended minimum relief system design content.pdf Flowline-PSV-latest.pdf exPORT oIL TANK PSV.pdf Fuel gas kod PSV - 21.12.12.pdf Separator PSV-revised size- 19-12-12.pdf PVRV for API 2000 Tanks-latest.pdf
- 15. Section I Overpressure Overpressure Protection Philosophy Potentials for Overpressure Recommended minimum relief system design content List of items required in flare header calculation documentation Guidance on vacuum relief
- 16. List of items required in flare header calculation documentation Description of the initiating event and the intermediate consequence that leads to relief flow Documentation of the basis for the network flow simulation Schematic diagram of the flare system Inputs for network flow simulation Relief valve size selection datasheets List of disposal system loads List all credits taken to reduce disposal system peak load List the instrumentation Back pressure limit for each source Acceptance criteria for flare system capacity Contd…
- 17. List of items required in flare header calculation documentation-Example Flare header150-FL-A-40015-25PP ,REV-0.pdf Flare headerGas lines.pdf Flare headerFlare header length.xlsx
- 18. Section I Overpressure Overpressure Protection Philosophy Potentials for Overpressure Recommended minimum relief system design content List of items required in flare header calculation documentation Guidance on vacuum relief
- 19. Guidance on vacuum relief • The volumetric outflow of material from the protected system exceeds the inflow. • The energy outflow from the protected system exceeds the energy inflow Causes • Operating procedures • Mechanical design • Relief system design • Instrumented protective system Protection Sometimes the heat and material balance factors can lead to fall in operating pressure. Therefore the use of vacuum relief becomes important.
- 20. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 21. Causes of Overpressure Closed outlets Cooling or reflux failure Absorbent flow failure Accumulation of non-condensables Entrance of volatile material into the system Failure of process stream automatic controls Abnormal process heat input Internal explosion Chemical reaction Hydraulic expansion External pool fires Jet fires Opening manual valves Electric power failure Heat transfer equipment failure
- 22. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 23. Closed Outlets ► Capacity of the relief device must be at least as great as the capacity of the sources of pressure. ► Sources of overpressure including pumps, compressors and process heat. ► In the case of heat exchangers, a closed outlet can cause thermal expansion or possibly vapour generation. Separator Closed Oil Storage PW Degasser From wellhead
- 24. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 25. Absorbent Flow Failure ► Even partial absorbent failure produces a rapid temperature rise in the methanator causing the SDV to close and open the vent ► If the vent fails to open, it leads to OVERPRESSURE Absorber Methanator MDEA Feed With CO2 Excess CO2 Absorbent failure
- 26. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 27. Accumulation of non-condensables ► With certain piping configurations, non-condensables can accumulate to the point that the overhead condenser is blocked. ► This effect is equal to a total loss of cooling. ADU Condenser Accumulation of non- condensables
- 28. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 29. Abnormal process heat input ► In shell and tube heat exchangers, the design shot be based on clean conditions and not on fouled conditions. ► Otherwise, it will lead to abnormal heat input Shell and tube Heat Exchanger Design based on clean condition Design based on Fouled condition
- 30. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 31. Internal explosion Internal explosion is caused by ignition of air-vapor mixtures. Relief valves should not be used as protection. Relief valves are too slow to react to the rapid pressure built up of the internal explosion. Rupture disks, explosion relief systems, explosion containment and explosion suppression can be used as protection measures. Also, explosion preventive measures such as inert gas purging can be used.
- 32. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 33. Failure of process stream automatic controls Separator Oil Storage PW Degasser From wellhead ► If one or more of the inlet valves are opened by the same failure that caused the outlet valve to close, pressure-relieving devices are required to prevent overpressure. ► The required relieving rate is the difference between the maximum inlet and maximum outlet flows.
- 34. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 35. Vapour depressuring Depressuring systems is used to mitigate the consequences of a vessel leak by reducing the leakage rate and/or inventory within the vessel prior to a potential vessel failure They are most often used to prevent overheating The metal temperature rises such a level that stress rupture occurs This can be possible even though the system pressure does not exceed the maximum allowable accumulation. The following should be considered when designing/specifying the depressurization system: rupture pressure of vessels rupture pressure of pipes total release of flammables instantaneous release rate rupture time
- 36. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 37. Entrance of volatile material into the system Precautions to avoid this situations are maintaining minimum circulation of hot oil through equipment on stand-by in order to minimize collection of water avoiding water-collecting pockets installing proper steam condensate traps installing interlocks to trip sources of heat in the event of water-contaminated feedstock Hot Oil Water Vapour
- 38. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 39. External pool fires ► The effect of overheating ASTM A515 Grade 70 carbon steel ► At a stress of 138 MPa (20000 psi), an unwetted steel vessel ruptures in about 0.1 h at 649 °C ► X is time for rupure(hrs) ► Y is the stress (Mpa)
- 40. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 41. Hydraulic Expansion Piping or vessels are blocked in while they are filled with liquid at near-ambient temperatures and are heated by direct solar radiation. An exchanger is blocked in on the cold side with flow in the hot side. Heat Exchanger Cold Water
- 42. Section II Causes of Overpressure Closed Outlets Absorbent Flow Failure Accumulation of non-condensables Abnormal process heat input Internal explosion Failure of process stream automatic controls Vapour depressuring Entrance of volatile material into the system External pool fires Hydraulic Expansion Opening manual valves
- 43. Opening manual valves The following applies when manual valve is inadvertently opened, causing pressure build-up The manual valve should be considered the capacity at a full-open position at relieving condition. The required relief capacity=Max flow (inlet) - Max flow (outlet) Separator Oil Storage PW Degasser From wellhead Manual Valve
Editor's Notes
- #8: Double or multiple jeopardy The simultaneous occurrence of two or more unrelated causes of overpressure. Example- Operator error that leads to a blocked outlet coincident with power failure. International Standard describes single jeopardy scenarios that should be considered as a basis for design. Latent failure Latent conditions should be considered as an existing condition and not as a cause of overpressure. Example- Latent failures can exist in instrumentation that prevents it from functioning favourably during an overpressure condition. Operator error It is a potential source of overpressure. Role of instrumentation Pressure relief devices are significant but there might be situations where their use is impractical. At these scenarios, the reliance on conventional instrumentation such as fail safe devices are justified. If risk tolerance criteria are not available, the overall system is designed to provide Safety Integrity Level 3 (SIL-3) performance
- #9: Double or multiple jeopardy The simultaneous occurrence of two or more unrelated causes of overpressure. Example- Operator error that leads to a blocked outlet coincident with power failure. International Standard describes single jeopardy scenarios that should be considered as a basis for design. Latent failure Latent conditions should be considered as an existing condition and not as a cause of overpressure. Example- Latent failures can exist in instrumentation that prevents it from functioning favourably during an overpressure condition. Operator error It is a potential source of overpressure. Role of instrumentation Pressure relief devices are significant but there might be situations where their use is impractical. At these scenarios, the reliance on conventional instrumentation such as fail safe devices are justified. If risk tolerance criteria are not available, the overall system is designed to provide Safety Integrity Level 3 (SIL-3) performance
- #20: Operating procedures Operating procedures are sometimes relied upon for activities associated with preparation for maintenance (e.g. vessel draining and steam-out) and activities such as hydrotesting. However they are not fool proof. Mechanical design The simplest solution to a potential vacuum is to design the equipment to withstand full vacuum. Relief system design If a vacuum-relief device is used, it shall be rated at a pressure comparable to that of the protected equipment. Instrumented protective system An instrumented system or regulator (repressuring system) can be used to add gas to prevent the vacuum from exceeding a specified level. Air, nitrogen and fuel gas are commonly used.
- #26: Any quantity of carbon dioxide above design capability that enters the methanator, as occurs on even partial absorbent failure, produces a rapid temperature rise that usually closes a methanator feed shutoff valve and opens a vent to the atmosphere. If the vent to the atmosphere fails to open, the possibility of overpressure arises.
- #34: If one or more of the inlet valves are opened by the same failure that caused the outlet valve to close, pressure-relieving devices are required to prevent overpressure. The required relieving rate is the difference between the maximum inlet and maximum outlet flows.
- #38: Relief devices are inefficient because expansion in volume from liquid to vapour is so great (approximately 1:1 400 at atmospheric pressure)
- #42: Piping or vessels are blocked in while they are filled with liquid at near-ambient temperatures and are heated by direct solar radiation. An exchanger is blocked in on the cold side with flow in the hot side.