Guidelines for Pressure Relief Design

The purpose of this document is to provide criteria for designing adequate relief systems and required design documentation based on ASME Code, API 520/521 Recommended Practices, NFPA, CGA-S, and DIERS methodology in order to meet the requirements of OSHA 29CFR 1910.119 Process Safety Management of Highly Hazardous Chemicals. 

Process Safety Management (PSM) is the proactive identification, evaluation, and mitigation or prevention of chemical releases that could occur as a result of failures in process, procedures or equioment.  OSHA 29CFR 1910.119

This guide is based on the book, Pressure Relief and Effluent Handling System 2nd Edition 2017, published by CCPS and DIERS, and the Mach II CDS Relief System Software Design Manual.  Order the Book from CCPS to get a more indept understanding and more detailed guidelines.

1.0  General Pressure and Relief System Design Criteria

A chemical process and plant facility should be analyzed for all possible causes of over-pressure to determine the worst credible scenario.

    A.    Process Hazard Analysis (PHA)
            1.  Methods for conducting PHA (CCPS 2003 &2008a)

            2.  Re validate HAZOP (CCPS2001 & Chadwell 1997)

            3.  Inherent Safety Concept (CCPS 2009)

                  a.      Use less hazardous material

                  b.      Avoid extreme temperatures or pressures

                  c.      Design for total containment

                            i.   Increase MAWP

            4.  OSHA Standard & EPA, RMP regulations

                    a.      Collect Process Safety Information (PSI)

                              1.  PHA should include a complete compilation of process safety information before conducting                                                      any required process hazard analysis (PHA)

            5.  The first step in the design of an emergency pressure relief system is to identify the source of the highest                            potential pressure and emergency relief load.

            6.  The scrutiny of sources of emergency pressure and of circumstances under which the pressure gets out of                          control falls under the Process Hazard Evaluation discipline.  This evaluation is a team effort among safety                          specialist, system designers, process people, subject matter experts, and the operating groups.

            7.  The goal is to agree upon a worst-case credible emergency scenario.  Rough relief sizing may be required                              for different scenarios to determine which is the worst case, and which one will be the design case if the                              worst-case requires impractical relief sizes.

            8.   Of course, the potential sources of fire should be studied with thought as to how to minimize or eliminate                           fire exposure.  Relief designs will be expected to handle the fire exposure cases unless there is no source of                      flammable liquids in the immediate area.

            9.   Very rapid or triggered events are excluded from the outset.  Examples range from water hammer as a purely                       physical effect, to dust or vapor explosions as very fast chemical effects. Sizing of relief systems for such                            non-uniform pressure cases is done by empirical scaling of test data and is beyond the scope of this guide.

          10.  The emergency fluid or energy input for an inert system can be determined readily as heat input from a fire or                     other source or flow of fluid through a line to or from a vessel. This gives the basis for determining the flow                       requirements.

          11.   Reactive systems are beyond the scope of this guide because the situation is more complex due to heat of                         reaction and change in volume between reactants and products.  Refer to Subject Matter Experts (SME)                                   utilizing DIERS methodology.

         12.  After determining the flow requirements, it is important to determine if any phase change will occur.  If there                       is no phase change then the relief flow is single-phase and the relief sizing is straight-forward.   If                                           equilibrium  phase changes occur in the vessel, relief flow and sizing calculations will still be straight forward                     only if one phase flows through the emergency vent.

         13.  After sizing for single-phase flow perform the two-phase flow test utilizing the Mach II Digital Engineer to                         determine the likelihood of two-phase flow based on the Alpha (α) vs. PSI (Ψ) curve.

        14.  If two-phase flow occurs in the vent, the relief flow rate requirements can be determined by Subject Matter                         Experts (SME).  The pipe sizing calculations for two-phase flow are discussed in G(max) piping tab.  Again,                            refer to Subject Matter Experts (SME) utilizing DIERS methodology and the Mach II CDS, PSM Network System.

     B.      OSHA Process Safety Management requires, Operating & Maintenance Training For:

               1.  Start-up

               2.  Shut-down

               3.  Upset and;

               4.  Normal operating conditions

     C.      Caution!!!!!   Before performing relief device calculations, a Process Safety Audit should be performed to                             include:
               1.  Over-view of initial calculations if any

               2.  Review Over-pressure scenarios considered from PHA

               3.  Review of mechanical design & specification

               4.  Review of current process conditions against initial design conditions

               5.  Review of mechanical changes and operating records (see CCPS 2008)

               6.  Review of robust and redundant process control and emergency shutdown systems installed

                     a.    Safety instrumented systems (CCPS 2007a)

                             i.    If the frequency of a particular event can be reduced to an extremely low level that event might be                                           considered not credible.

     D.      Process Safety Information (PSI)
               The design basis and description of all pressure relief systems must be retained and available for review.  All                        other process safety information should be stored into a reliable and accessible database.

               1.  During each yearly review and during PHA the following relief design information must be identified,                                       evaluated, stored and maintained:

                     a.    Identification

                     b.    Description

                     c.    Design basis

                     d.    Equipment protected

                     e.    Required flow capacity

                     f.     Specification of device

                    g.     Operating parameters

                             i.    Flow

                            ii.    Set pressure

                           iii.    Relieving pressure

                           iv.   Physical properties

                  h.       Inspection, testing, maintenance history

                           1.   See CCPS 1995d and Howell 2009

                   i.      OSHA Standard & EPA, RMP regulations

                          1.  Process safety Information (PSI) - should include a complete compilation of process safety                                                          information before conducting any required process hazard analysis (PHA)

                          2.  Provide information on Hazards of highly hazardous chemicals- may use material safety data sheets                                        (MSDS) but must include required data as per 29CFR 1910.1200(g)

                          3.  Process Equipment should include documentation such as:

                                 a.       Relief system design and design basis

                                 b.      Design codes, standards, and recommended practices.

                          4.  Safety Systems

                                a.       Interlocks

                                b.      Detection

                                c.       Suppression systems

                         5.  Documentation that equipment complies with Generally Accepted and Recognized                                                                           Good  Engineering Practices (GARGEP)

                                a.       Codes

                                b.      Standards

                                c.       Technical Publications

                                           i.      DIERS

                                          ii.      CCPS

                         6.  Equipment based on outdated Codes must be proven fit for use

                         7.  Mechanical integrity documentation must be complied on:

                               a.      Vessels

                               b.      Piping

                               c.      Valves

                               d.      Relief vent systems

                               e.      Emergency shut-down systems

                        8.   EPA Risk Management Program 40 CFR Part 68

                               a.       Aligns with OSHA

                               b.      Concerned with health of off-site public and environment

    E.      Problems Inherent in Pressure Relief and Effluent Handling
             1.  Uncertainties due to variable flowrates and fluid conditions

                     a.      Flowrate

                     b.      Temperature

                     c.      Pressure

                     d.      Vapor-liquid ratio

                     e.      Physical properties

                     f.      Material composition

DIERS  - Super Chems Software Simulation and Mach II CDS, PSM Network System, and others can predict vessel dynamics and estimate effluent rate, composition and flow through relief device.

              2.  High viscosity inlet of flashing liquid mixture can generate hard to separate small liquid droplets which                                   cannot be separated by cyclone or gravity separators.

                     a.      Quench Pools-uncertainty when handling vapors with large quantities of non-condensable gases                                              (toxic release to atmosphere)

                      b.      Spargers- generate vibration and large shock loading that are difficult to predict and control.

     F.      Relief Design Criteria and Strategy

An audit of existing installations may reveal possible shortcomings of the design in light of advances in relief system technology.  (See Chadwell 1997)

Note: Failure to implement proper relief system management procedures is an unacceptable practice.

    G.       Limitations of the Technology
               The technology does not apply to situations in which the pressure-producing process propagates through the                   equipment from a point of initiation (such as the ignition and burning of flammable vapors and runaway                                 reactions that propagate through liquids).

   Relief of vapor deflagrations and other propagating event are treated in publication such as the National Fire                      Protection Association (NFPA 68 and 69) and also see API 521 for additional guidance.

    H.      General Pressure Relief Strategy

               Since the Emergency Relief System (ERS) is normally the last line of defense, its functioning must not depend                       upon the proper operation of other devices.         

      I.      Mechanism of Pressure Relief
              With few exceptions emergency pressure relief is an unsteady state process.  Conditions will change if there                      are changes in pressure, temperature, heat input rates, inventory, composition, etc.  Steady-state methodology                  suffices to obtain an adequate vent size if applied to conditions at the worst-case instant of the transient                            event.