Predicting Flashing Flow Through Relief Valve

[RJB32482]

Is there a good procedure for predicting flashing flow through a pressure relief valve? Would you just predict pressure drop through the valve and see if the pressure drops enough to get the liquid to boil? Or do you need the critical temperatures and pressures?

Thanks.

 

[rbcoulter]

In most cases, one knows the downstream pressure which will indicate if you have a flashing flow. Compare the vapor pressure of your fluid to the downstream pressure. If it is greater than the downstream pressure then it would probably be best to assume that the flow will flash. In that case, using a method like API RP 520 D.2.2. is probably advised.

 

[JimCasey]

you know it is flashing if the downstream pressure (usually atmospheric) is less than the vapor pressure.

The process is adiabatic and isenthalpic.

h1=x*hg2+(1-x)hl2

h1 is the inlet enthalpy
hg2 is the enthalpy of the gas phase at the discharge pressure.
hl2 is the enthalpy of the liquid phase at the discharge pressure.

Percent flash is usually pretty small. But if you convert to density, with the expansion as the fluid flashes to gas, the outlet mist is pretty light and goes very fast.