Guidelines for Pressure Relief Design


5.0      OSHA Regulations
OSHA regulations for hazardous materials (29(CFR 1910.119) contains specific requirements for pressure relief system design and operation in addition to general requirements for storage and handling of selected materials.  This is the controlling regulation in the USA.  Designers and operators should possess knowledge of tis regulation to ensure compliance.  Reviewing current editions of the NFPA, CGA, and API documents to check for current practices and utilizing them is a prudent practice.

This regulation deals with maintaining proper documentation showing that the design and design basis for pressure relief devices complies with Recognized and Generally Accepted Good Engineering Practices (RAGAGEP)

API Standard 521 defines the minimum documentation that must be developed and maintained.  The documentation required includes:

1.      Recommended minimum relief system design content
          i.      Relief system information

                  1.       Relief Device Identification number

         ii.      Description of protected components

                  1.       List of equipment and design conditions

                  2.       P&ID’s, Mechanical drawings, etc.

        iii.      Design codes and Standards followed

                  1.       Pressure design code

                   2.       MAWP

        iv.      Analysis of causes of system overpressure

                   1.       Required rate/or area for each cause

                   2.       Supporting calculations and assumptions

                   3.       Consider all causes of overpressure

                   4.       Credit for safety instrumented systems

         v.      System operating conditions

        vi.      System relieving conditions

       vii.      Relief device selection / configuration

      viii.      Pressure relief valve / rupture disc combination capacity factor (if applicable)

        ix.      Relief system required area

        x.      Relief system capacity if rupture disc only configuration

       xi.      Pressure relief valve rated capacity

      xii.      Spring loaded pressure relief valve cold differential test pressure

     xiii.      Pressure relief valve capacity correction for maximum back-pressure      

                 1.       Built-up back-pressure

                 2.       Maximum and minimum superimposed back-pressure

                 3.       Back-pressure capacity correction factor

     xiv.      Rupture disk specified burst pressure and manufacturing design range selection

      xv.      Rupture disc specified disk temperature

     xvi.      Relieving Fluid disposal requirements (closed or atmospheric)

                  1.       Flare

                  2.       Thermal radiation

                  3.       Dispersion (toxic or flammable vapor)

                  4.       Vapor cloud explosion

                  5.       Environmental considerations

    xvii.      Relief device physical installation

                 1.       Pipe stress analysis

                 2.       Heat tracing

                 3.       Pressure relief valve inlet / outlet line pressure drop

                 4.       Relief device specification sheets

                 5.       Criteria for vacuum protection

In general, the broader design methods for non-reactive systems are contained in publications of the American Petroleum Institute (API 520/521/20000)

Caution!!!!  Chemically reactive systems and multiphase venting are not adequately covered by the API recommended practices.  The design of these systems requires the use of DIERS methodology.


6.0    General Provisions ASME Code
The ASME BPV Code requires that all pressure vessels be provided with certified overpressure protection systems.  The National Board of Pressure Vessel Inspectors has been designated by the ASME as an organization that can test and certify pressure relief devices.   They publish a list of pressure relief devices that have been certified.  National Board NB-18 ( can be downloaded at no charge.

1)      For definitions and terminology used such as set pressure, over-pressure, blowdown, operating pressure, accumulation pressure, as it relates to the vessel maximum allowable working pressure (MAWP) see Section 2 of ASME PTC 25.

2)    UG-125 General
a.        Other than unfired steam boilers [see UG-125(b)] all pressure vessels should be provided with the overpressure                protection in accordance with the requirements of UG-125 through UG-138 overprotection by system design                      per UG-140.

              i.      It is the user or his / her designated agent’s responsibility to identify all potential overpressure scenarios                              and the method of overpressure protection used to mitigate each scenario.

             ii.      It is the responsibility of the user to ensure that the required overpressure protection system is properly                               installed prior to the initial operation.

            iii.      If a pressure relief device(s) is to be installed, it is the responsibility of the user or his  / her agent to size                              and select the pressure relief device(s) based on its intended service.  Intended service considerations                                    should include, but not necessarily be limited to the following:

                     1.       Normal operating and upset conditions

                     2.       Fluids

                     3.       Fluid phases

            iv.      The overpressure protection system need not be supplied by the vessel manufacturer.

3)      For ASME Section 1 valves used for fired equipment (boilers) the maximum accumulation of pressure is 3% above              MAWP for one valve and 6% when using multiple valves.

4)      (c) For ASME Section VIII non-fire case relief valves the maximum accumulation of pressure above MAWP is 10% or            3 psi, whichever is greater except for:

         a.       When multiple pressure relief valves are provided and set in accordance with UG-134(a), they should                                      prevent the pressure from rising more than16% above MAWP or 4 psi, whichever is greater.

         b.       When a pressure vessel can be exposed to fire or other unexpected sources of heat, the pressure relief                                device(s) should be capable of preventing the pressure from rising more than 21% above the MAWP.                                        Supplemental pressure relief devices shall be installed to protect against this source of excessive pressure if                      the pressure relief devices used to satisfy the capacity requirements of (c) and (1) have insufficient capacity                        to provide the required protection.

         c.       Pressure relief devices intended primarily for protection against exposure of a pressure vessel to fire or                             other unexpected sources of external heat installed on vessels having no permanent supply connection and                       used for storage at ambient temperatures (100 oF max) of non-refrigerated liquified compressed gases are                           excluded from the requirements of (a) and (b) above, provided:

            i.    The pressure relief devices are capable of preventing the pressure from rising more                                                                         than 20% above the MAWP of the vessel.

           ii.     The set pressure marked on these devices shall not exceed the MAWP of the vessel

          iii.      The vessel has sufficient ullage to avoid a liquid full condition

         iv.      The maximum allowable working pressure of the vessel on which these pressure                                                                              relief devices are installed is greater than the vapor pressure of the stored liquified                                                                        compressed gas at the maximum anticipated temperature (<115oF or 45 oC) that                                                                              the gas will reach under conditions; and

          v.      Pressure relief valves used to satisfy these provisions also comply with the                                                                                       requirements of UG-129(a)(5), UG-131(c)(2), and UG-134(d)2)

5)   Other ASME and Design Requirements
         a.       For ASME Section VIII relief valves sized for fire exposure the maximum accumulation above MAWP is 21% no                     matter how many valves are being used.

         b.       For ASME Section I valves when using multiple valves, the 2nd and all remaining valves can be set at 103% of                       set pressure.

         c.       For ASME Section VIII valves when using multiple valves, the 2nd and all remaining valves can be set at 105%                        of set pressure.

         d.       For initial API sizing use a discharge coefficient (Kd) of 0.95 for vapor or gas sizing and Kd of 0.65 for liquid                        sizing. 

         e.       For API sizing for two-phase flow use a discharge coefficient (Kd) of 0.95 for choked flow and 0.65 for non-                      choked flow.

         f.       For initial ASME sizing use actual orifice size and 90% of API discharge coefficient(Kd)

        g.       For final sizing after selection check manufactures catalogues to determine actual Kd’s and check                                             performance charts to see if there are any negative effects of performance due to back-pressure (Kb).  (See                      Vendor Charts of Kb vs Valve Capacity Performance)

        h.       Use rupture disc instead of a relief device or in conjunction with a relief device to relieve possible runaway                         reaction scenarios because rupture disc responds more quickly than a relief valve.

         i.       When using a rupture disc in combination with a relief valve use a combination capacity factor (Kc) of 0.9 for                      initial sizing.  After selection you may consult manufacture’s catalogue for the rupture disc resistance                                    coefficient (KR) to use as a piping resistance.

         j.       Pay close attention to the rupture disc operating margin to prevent premature bursting due to operating close                    to disc operating margin.  Within 5 psi for < 70 psi set pressure with set tolerance of +2 psi.   Within 10%                              for 70  psi > set pressure < 1000 psi with set tolerance at +3 psi, within 7%  for > 1000 psi set pressure with                        set tolerance + 3 psi.

       k.       For gas, vapor, and two-phase flow the fluid can choke if the relieving pressure is twice the discharge                                    pressure.  Use choke pressure and flowrate instead of back-pressure when sizing.

        l.       For Two-phase flow use Homogeneous Equilibrium Model (HEM) to determine Maximum fluid flux (Gmax)                               entering the relief nozzle and for the inlet and outlet pressure drop determination.

      m.       API recommends using the Omega method for calculating the maximum flux (Gmax) through relief nozzle                              however, it does not give a method for calculating the inlet and outlet pressure drop.  Use homogeneous diret                    integration (HDI) method instead.  Which can be used for flashing flow, hybrid, frozen, sub-cooled flow for                          nozzle and inlet /outlet piping.

      n.       Perform two-phase test to determine likelihood of two-phase flow.  Design for two-phase venting unless                          vapor-liquid disengagement correlations support all vapor designs. 

      o.       Problems involving two-phase relief and piping flow should be referred to a process safety expert that can                          apply DIERS methodology.

      p.        Problems involving runaway reactions should also b e referred to a process safety expert that can apply                               DIERS methodology when self-heat rate is known or will be determined calorimetry testing.