I understand that valves in two-phase vapour / liquid were sometimes sized by considering the required vapour and liquid flow rates separately, and then adding the results together. But this is now considered an outdated practice.

I have also heard of the homogeneous equilibrium model (HEM), but I am under the impression that there are a number of other accepted sizing techniques.

My primary question is: what are the currently accepted practices / standards for sizing relief devices with two-phase vapour / liquid flow?

Otherwise, if the older techniques are still valid, under what conditions or assumptions behind each technique are they still valid?

  • $\begingroup$ So in order to clarify: How is the pressure of the two phase fluid measured? Can the fluid be in a single phase condition as well? $\endgroup$
    – jjack
    Commented Feb 13, 2015 at 20:41
  • $\begingroup$ I assume that you don't want to design a new safety valve. So if you want to find a suitable valve dimension, the first point to check is the specification sheet of the valve supplier. HEM would relate to CFD simulations. Which are only meaningful if you want to define the details of the geometry of the component. $\endgroup$
    – BerndGit
    Commented Mar 6, 2015 at 13:32

1 Answer 1


While designing a relief the valve flow rate is the deciding variable. To calculate its value in two phase flow different models can be used like HEM, drift flux, separated flow, etc. For this particular application (where two phase flow is because pressure drop across valve) HEM is best suited. I would also suggest to check for choking in the two-phase flow while designing relief valve.

Disclaimer: I have no practical experience in area.


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