constant enthalpy or entropy in PSV sizing?

[wormhhh]

Hi,there:

Would you handle the prossure relief process through the valve as an isenthalpy or an isentropy process? Please give some reference if possible. Same questions to control valve. Many thanks.

 

[dezirak]

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.

 

[rbcoulter]

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.

 

[EGT01]

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.

 

[JimCasey]

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

 

[wormhhh]

thanks, guys. I appreciate it.