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.