The gas is being throttled.  The enthalpy is constant, but the internal energy is not necessarily constant.  This may result in a temperature change through the valve.  Look up information on the Joule-Thompson effect to better explain this.  

Most of what I have read on the subject is to prefer treating it as an isentropic process although the isenthalpic approach is condidered acceptable.   I think the isentropic process assumes all the energy of expansion goes into to kinetic energy.  The isenthalpic process implies a some transfer of "expansion energy" to random heat.  So, I would think that the isenthalpic process underpredicts the theoretical maximum flux.    

The isentropic process, in theory, occurs in an ideal nozzle.
However, we generally assume that a gas expanding through an orifice in a pipeline is isenthalpic.   PSVs seemed to be a combination of both.

Wormhhh,

I was involved with a similar discussion some time past.  The discussion started about the outlet piping but also covers what happens in the PSV.  Have a look at this thread...
thread408-63173

Hope this helps.

Everything I have ever seen on the topic indicates that the process is isenthalpic and essentially adiabatic.  

thanks, guys. I appreciate it.