cbadia

I have found several papers discussing this topic, one is "Improved control valve sizing for multiphase flow" from Hydrocarbon Processing March 200, the authors say "HNE-DS method basically assumes that the flow pattern in the control valve is homogeneous but gas and liquid are not in equilibrium – a non-equilibrium factor N represents the boiling delay"
your comment agrees with this statement.
By the way I found also an interesting thread comparing API PSV formulation for gas flow here

'http://www.cheresources.com/invision/topic/16104-psv-discharging-temperature'

the author has provided a Excel page to compare different procedures

Discharging temperature (PSV outlet)
1) calculated with isentropic + adiabatic flash (the default in nozzle.xls)
2) calculated with adiabatic flash
3) calculated with isentropic flash

Calculated Area
1) rigorous numerical solution of isentropic nozzle
2) API formulation for gas and vapors, ideal cp/cv
3) API formulation for gas and vapors, real cp/cv
4) API formulation for gas and vapors, real cp/cv, Zv = 1

all the required properties are calculated by Prode library , for personal/academic use you can download a free -with limited number of components- copy from 'www.prode.com'

I recommend to read this thread to everyone interested in PSV sizing.
psvcompare1.xls