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constant enthalpy or entropy in PSV sizing?

constant enthalpy or entropy in PSV sizing?

constant enthalpy or entropy in PSV sizing?

(OP)
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.

RE: constant enthalpy or entropy in PSV sizing?

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.  

RE: constant enthalpy or entropy in PSV sizing?

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.

RE: constant enthalpy or entropy in PSV sizing?

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.

RE: constant enthalpy or entropy in PSV sizing?

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

RE: constant enthalpy or entropy in PSV sizing?

(OP)
thanks, guys. I appreciate it.

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