Cp/Cv for PSV calculation

[MMII]

Hi everybody. I’m sizing some PSVs and I have read the article “using the ideal gas Cp/Cv ratio for RV sizing” in which at the end the author concludes “In no case should the real gas specific heat ratio be used as an estimate of the isentropic expansion coefficient, as this can lead to significant under-prediction of the required discharge area for the RV”. As it is said in the article I have verify that Hysys gives the real Cp/Cv, not the ideal one. Anyone know how can I calculate the ideal Cp/Cv to be used to size the RV? Changing the fluid conditions to Normal Conditions 0 atm and T = 0K? what are the "Ideal Conditions"?

 

[CJKruger]

MMII, instead of the ideal Cp/Cv, I have in the past used Cp/(Cp-R), where Cp was actual Cp from a simulator.

 

[dcasto]

by definition, Cv = Cp-R, it not something you use , it fact.

 

[lizking]

API 520 states in the definition of Coefficient "C" that "k" should be taken at "inlet relieving conditions".

 

[CJKruger]

dcasto, as far as I know, Cv=Cp-R is only true for an ideal gas.

lizking, good point. But to confuse us, they tell us to base the critical pres ratio on the ideal k value. Theoretically I do think the ideal k value is more correct, but given the other inaccuracies in our relief load calcs, it probably makes no difference.

 

[dcasto]

thats why I use a empherical cubic equation form of Cp. The constant R is truely a constant.