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Relief valve design cases

  • 1.  Relief valve design cases

    Posted 18 days ago
    Good day,

    I was wondering when do you choose the accumulation for blocked in scenarios versus fire scenarios. I am aware that API 520 states that for any cases that can result in over pressure, an accumulation of 10% be used (barring  that one relief valve is installed). For a fire scenario, the accumulation shall be 21%. My question is when shall the fire case versus the other be considered? Is it that a risk analysis shall be done which indicates that a fire is more likely to occur than a blocked in condition? Or is it basis the physical properties of the fluid, e.g. handling flammable liquids? Hop I can get some clarity.

    Shantal Ramdeo PE
    Senior Process Engineer
    Heritage Petroleum Company Limited
    Princes Town

  • 2.  RE: Relief valve design cases

    Posted 18 days ago
    Hi Shantal,

    My understanding is that you have to consider all scenarios described in API 520. After reviewing each scenario, you will have to select the case which gives you the highest relieving capacity as the basis for pressure relief valve (PRV) sizing. This is also commonly known as the controlling case.

    The unwritten understanding for allowing a higher accumulation of 21% during fire scenario is that the probability of fire scenario is low, as compared to a blocked in scenario.

    In my experience, many calculations done in the past do not adequately document all the cases considered by the design engineer. Instead, only the controlling case is documented. If you are designing or reviewing existing PRV calculation, it is best to keep full record of all the scenarios described for future reference.

    Jia Ming Ngai
    Senior Engineer

  • 3.  RE: Relief valve design cases

    Posted 18 days ago
    You have consider all the possible cases, fire, blocked in, etc.  Calculate the required flow rates then calculate the required orifice area.  You choose a device that has an orifice to handle the worst required orifice area.

    Good luck

    Elizabeth Robin PE
    Consulting Engineer
    Eli Lilly & Company
    Indianapolis IN

  • 4.  RE: Relief valve design cases

    Posted 17 days ago

    A fire case is to be considered if it is possible to have a sustainable fire in the area. Also, the surface considered is the first 30 feet above that surface. I see you work for a petroleum company, so fire cases are always important. A surface that can sustain a fire is also important. If you have an overhead receiver for a distillation column in a structure, it frequently is more than 30 feet above ground. So the question is if the deck below it will hold enough liquid to sustain a fire. You also have the question of wetted surface. I assume the high level case even if that level is not the normal level. That is a conservative but the safest approach. Also note that for pressure vessels there is a case for vapor filled vessels in a fire. That scenario is pertinent for gas plants. The other consideration is whether the vessel is directly in a fire or if the primary heat flux is by radiation. (The equations differ.)
    I hope this helps. 

    Robert Clay, PhD, MBA, PE
    Sr. Associate Engineer
    ECI, Inc.
    Lenexa, Kansas

  • 5.  RE: Relief valve design cases

    Posted 17 days ago

    Each system must be analyzed to see which cases apply.  For many pieces of equipment, fire applies as a case as well as many other overpressure cases.  The different cases are calculated, with their varied accumulated pressures of 110, 121% etc. and the governing case will determine the required relief valve size (orifice area).



    Typical cases to be examined:       1.  Auto Control Failure

    1. Blocked Vapor Outlet
    2. Blocked Liquid Outlet
    3. Abnormal Heat Input
    4. Tube Rupture

                                                                  6.  Reflux Failure

                                                                  7. Loss of Feed

                                                                  8. Fire

                                                                  9. Cooling Water Failure

                                                                 10.      Power Failure

                                                                11.       Instrument Air Failure

                                                                   12. Steam Failure

                                                                13.       Other: Back Flow



    Best Regards,


    Zelka S. Sostaric

    Senior Training Specialist

    Honeywell UOP

    25 E. Algonquin Rd. Des Plaines, IL 60017

    P: (847) 391-2666 | F: (847) 391-2758





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  • 6.  RE: Relief valve design cases

    Posted 17 days ago

    Thank you, you all have confirmed my thoughts.  I always like to practice due diligence with my designs

  • 7.  RE: Relief valve design cases

    Posted 17 days ago


    Many years ago I asked the same question to a member of the ASME Section VIII (pressure vessel) committee. The answer was the accumulation is a function of the frequency of the event.  So I found some data on event failures and yes, fires have a lower frequency than the majority of other events.  Two codes commonly used by process engineers, ASME Section VIII and ASME B31.3 (process piping), both have frequency based accumulations.  Please understand that this does not consider the consequence and the risk due to an event is a function of both consequence and frequency.

    Freeman Self

    Bechtel Oil, Gas & Chemicals, Houston

    Level 3 - Bechtel Internal and Selected External Distribution

  • 8.  RE: Relief valve design cases

    Posted 17 days ago
    Shantal,  I'm not sure I understand your question correctly, but both cases and others as appropriate should be calculated for each relief valve or rupture disk.  So in your situation, calculate the fire case relieving load and the blocked in relieving load.  Whichever gives you the largest relief device is the determining case.   Hope this helps.

    Barry Juran PE
    Philadelphia PA
    Chemical Process Engineer and Sr. Biopharm Specialist

  • 9.  RE: Relief valve design cases

    Posted 14 days ago
    Interested in this topic since I am associated with safety instrumented systems upgrade program with Chevron and safety relief valve is an important safety device for HAZOP/LOPA protocols

    Rohit Korde MSc,CEng
    Program Manager
    Thane (W)