Hello All,

I am a junior engineer and I am having trouble with calculating the relieving temperature for a blocked flow psv scenario.

If the normal operating temperature is T1 at Pressure P1.

Then how do I calculate the relieving temperature T2 ant P2, where P2 is the Set point + overpressure.

We had previously used the following equation

T2 = T1 * (P2/P1)

Which is Boyle's/Charles law if I remember correctly, but this is assuming an ideal gas with a constant volume and number of moles which isn't true. I believe the equation was taken out of an older version of GPSA.

If your operating temperature P1 is far smaller than your Set Pressure, then you end up with a huge temperature that is unrealistic.

I believe the we should be using isentropic compression with the following equation

T2 = T1 * (P2/P1)^(k-1/k) Where k = Cp/Cv

And this would determine our relieving temperature T2 at our relieving pressure P2.

Is this correct?

Pauwl,
First of all, welcome...

Both equations are applicable when they are in gas/vapor at both stages (Normal & relieving condition). If either condition contains condensate, then it will starts to deviate. Both equations may not suitable.

T2 = T1 * (P2/P1) assuming constant volume and mass, i think for a gas/vapor (compression) involve volume change as pressure increase. Those my opinion is this approximation may not be suitable.

T2 = T1 * (P2/P1)^(k-1/k) Where k = Cp/Cv is an isentropic compression which assumed adiabatic and reversible. In my opinion, bring pressure from normal to relieving condition would be adiabatic and irreversible (real world). Polytropic exponent (n) may be used instead of isentropic exponent. However, it is hard to get this polytropic exponent (n). Those in most cases, we consider n=k and isentropic compression stands.

JoeWong