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What Is A Two-Phase Flow For Pressure Relief Device Sizing?

prd rupture disk pressure relief

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#1 rajbhar_s


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Posted 28 November 2023 - 05:44 AM

Hi all,


I am rating a rupture disk for a flashed two-phase flow with 0.1% vapor manly consisting of methane and some CO2. Per API 520-1 Annex C, if I have 'some' non-condensable gas, I can use Direct Integration Method or Omega Method for calculation of two-phase relief area.


I  used both Liquid-only flow (API 520-1 eqn 33) method and two-phase flow method (API 520 Part 1 eqn C.10), and as per the results, the calculated liquid-only area is 10% higher (i.e. more conservative) than two-phase calculated area. 


My question is,

  1. Is it Direct Integration Method or Omega Method is a correct choice for this application (i.e. 99.9% liquid or 0.1% vapor)?
  2. What does API mean by 'some noncondensable gas, is it 0.1%, 1%, 5% or 10%? Do they have any guideline?




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#2 latexman


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Posted 28 November 2023 - 07:42 AM

Is that 0.1% by weight or volume?


  1. Either is applicable.
  2. From a practical standpoint, when a component, or a group of similar components, are in the 1-10% by weight concentration range, I start to wonder if they will truly affect my estimates, or not.  Sometimes they do, and sometimes they don't.  Until you have the experience, you have to calculate.  At 0.1% concentration, I wonder if that is so low it can just be ignored?  Your identification that the area of the liquid case > 2-phase case seems to prove this out.

My $0.02.

#3 PaoloPemi


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Posted 28 November 2023 - 03:01 PM

the main difference (Omega vs. Direct Integration) is that Omega adopts a simplified model (see Leung ""A Generalized Correlation for One-component Homogeneous Equilibrium...") , as general rule the Direct Integration procedure should be more accurate / reliable  ...
I would verify your result  "the calculated liquid-only area is 10% higher (i.e. more conservative) than two-phase calculated area"... normally the opposite is true (at least  for common compositions /  operating conditions)
there are many  models available for flashing chocked flow (to mention a few : homogeneous equilibrium, homogeneous non equilibrium, models including corrections as boiling delay etc. ) and, as you may imagine, each model has some specific range of application,  
while I utilize preferably HEM (the version included in Prode Properties) which gives normally conservative results, it is certainly useful to compare different models when the value of some parameter / condition is outside the normal range... 

as example see the different results obtained with a small amount of vapor at PSV / RD inlet...


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