Guidelines for Pressure Relief Design
2.0 Approach to Design
The following outline is predicated on the basis that adequate attention has been given to the application of inherently safer design, including selection of process chemistry and or the process scheme and conditions before detailed design of the pressure relief system is initiated.
1. Identify the equipment being protected
2. Obtain physical, thermal, and reactive properties of materials at set and relieving conditions. Also obtain equipment, piping, and instrument data.
3. Identify possible pressure producing emergency events known as over-pressure or vacuum contingencies analysis.
4. Analyze the behavior of the vessel contents during the emergency venting scenario in order to establish the history of the condition in the vessel.
i. equipment configuration
ii. potential foaminess of liquids
iii. Two-phase vs. single-phase fluid in vent system.
iv. difference in evaporative cooling effect of condensable vapor vs non-condensable gas system
v. reactive vs non-reactive
vi. Location of vent opening (above vs below; liquid or two-phase interface).
5. Select the appropriate types of relief devices. Determine the required size of the device and associated piping.
6. Determine reaction force loading on relief system components. Assume mechanical integrity of the system
7. Choose an appropriate effluent handling strategy and establish design values of flows. (see chapters 6 & 7 Guidelines for Pressure Relief & Effluent Handling)
8. A review of the design work is recommended at this stage. This audit should be performed by reviewers who have not materially participated in the design work and have equal or greater experience and expertise than the designer.
9. Document the design basis in a pressure relief management system. See Section 2.3.1 for OSHA, API, & ASME requirements and CCPS Guidelines for Documentation (1995).
10. Perform installation inspection as required to assure proper installation.
11. Assure that recommended, on-line, and off-line testing and maintenance procedures are implemented in the appropriate management system.
Note: Engineering Judgment should be used to match the extent of the relief protection effort to the severity of the consequences. A risk assessment that considers both the consequences and likelihood of the over-pressure event can be used to assist in this engineering judgment.
3.0 Limitations of Systems Actuated by Pressure
Providing adequate protection with pressure-actuated devices for certain classes of systems can be difficult. Examples of such systems include:
A. Reactions that are very fast at lowest possible relieving pressures required large relief devices. The recourse is to provide reliable and redundant instrumentation to reduce the likelihood of a loss of temperature control or to reduce the stored energy. May want to safety direct forceps using blast panels.
B. Reactions that propagate from hot spots or ignitions sources (fires, decompositions, etc.) Propagation may become so fast as to reach sonic velocity and cause detonation thus destroying the equipment before the pressure reaches the relief device.
C. Reactions that accelerate (exotherm) to uncontrollable rates before the process pressure begins to rise (gas evolution or vaporization will not increase the pressure until the venting capacity of the process pressure control system is exceeded). This condition can occur with high-boiling reaction mixtures and with material that decomposes below the normal boiling point.
D. Mixing of low-boiling liquid into a hot high-boiling fluid. Lack of nucleating sites can lead to appreciable super-heating of the fluid, which de-superheats very rapidly. The only recourse is to recognize the possible hazard in a given operation and to establish procedures to prevent such conditions from occurring.
E. Systems that generate excessive wall temperatures even if the relief system maintains the pressure below the allowable limits (fire exposure to gas-filled vessels, i.e. a BLEVE, for example). Typical provisions to avoid metal failures are to reduce the exposure time with improved fire-control systems, to slow the heating of the metal with insulation, to cool the metal with water spray, or to depressurize the vessels.
F. Systems that have more than one reaction where one reaction can lead to a secondary reaction is more vigorous, gas generating and not ventable.
Caution!!!: Many of these propagating and very rapid events belong more properly in the scope of explosion venting rather than pressure relief.
4.0 Codes, Standards, and Guidelines
1. Occupational Health and Safety Administration (OSHA)
i. OSHA 29CFR 1910.119
2. American Society of Chemical Engineers (AIChE)
i. Emergency Relief System Design Using DIERS Methodology
ii. Guidelines for Pressure Relief and Effluent Handling 2nd edition 2017
iii. Guidelines for Engineering Design for Process Safety 2nd edition
iv. Guidelines for HAZOP Evaluation Procedures 3rd edition (2008)
v. Guidelines for Safe and Reliable Instrumented Protective Systems (CCPS 2007)
vi. Layer of Protection Analysis- Simplified Process Risk Assessment
vii. Safe Design and Operation of Process Vents and Emission Control Systems.
3. American Petroleum Institute (API)
i. API 520 Part I Sizing & Selection
ii. API 520 Part II Installation
iii. API 521 Guideline for Pressure Relieving and Depressurizing Systems
iv. Standard 526 Flanged Steel Pressure-Relief Valves
v. Standard 527 Seat Tightness of PRV
vi. Standard 2000 Venting Atmospheric and Low-Pressure Storage Tanks
4. American Society of Mechanical Engineers (ASME)
i. Boiler and Pressure Vessel Code (BPVC)
ii. Section I Power Boilers
iii. Section II Nuclear Power
iv. Section IV Heating boilers
v. Section VIII Pressure Vessel (15-3000 psi)
vi. B31.1 Power Piping
vii. B31.3 Process Piping
viii. PTC 25: Performance Test Codes, Safety and Relief valves (ASME PTC25)
5. National Board of Boiler and Pressure Vessel Inspectors, Columbus, OH
i. NB-18: Pressure Relieving Device Certification
6. National Fire Protection Association (NFPA), Quincy, MA
i. NFPA15: Standard for Water Spray Fixed Systems for Fire Protection
ii. NFPA 30: Flammable and Combustible Liquids Code
iii. NFPA 68: Standard on Explosion Protection by Deflagration
iv. NFPA 69: Standard on Explosion Prevention Systems
7. International Standards
i. ISO 4126-1: Safety Devices for Protection Against Excessive Pressure-Part 1: Safety Valves
ii. ISO 4126-2: Safety Devices for Protection Against Excessive Pressure-Part 2: Bursting Disc Safety Devices
iii. ISO 4126-3: Safety Devices for Protection Against Excessive Pressure-Part 3: Safety Valves and Bursting Disc Safety Devices in Combination
iv. ISO 4126-4: Safety Devices for Protection Against Excessive Pressure-Part 4: Pilot Operated Safety Valves
v. ISO 4126-5: Safety Devices for Protection Against Excessive Pressure-Part 5: Controlled Pressure Relief Systems (CPRS)
vi. ISO 4126-6: Safety Devices for Protection Against Excessive Pressure-Part 6: Application, and Installation of Safety Devices Excluding Stand-Alone Bursting Disc Safety Devices
vii. ISO 4126-7: Safety Devices for Protection Against Excessive Pressure-Part 7: Common Data
viii. ISO 4126-9: Safety Devices for Protection Against Excessive Pressure-Part 9: Application, Selection and Installation of Bursting Disc Safety Devices
ix. ISO 4126-10: Safety Devices for Protection Against Excessive Pressure-Part 10: Sizing of Safety Valves for Gas/Liquid, Two-Phase Flow.