2 replies to this topic

[flarenuf]

I read with interest recently various postings on this site about whether a PSV should be modelled as isenthalpic,isentropic or a combination of both. Various people expressed various views and the concensus seemed to be an isentropic flash from inlet pressure to sonic velocity & pressure at the Vena Cava , ie a nozzle. Then an isenthalpic flash from the VC to the valve outlet. Little or no loss in the outlet due to friction.

Being a user of AspenHYSYS and Flarenet I did some comparison runs at various inlet pressures, temps and MW's using a combined flash and the good old fashioned isenthalpic flash ( ie as a control valve). In HYSYS assumed a perfect isentropic flash with 99% of heat recovery in the kinetic energy recovery zone

summary of results
a simple isenthalpic flash gives exit temperatures 1-2% higher than the complex model.
at high pressure drops across the valve this can increase to 5%.
the complex model does predict the throat temperature nicely which could impact on the choice of metallurgy, but this is down to the valve vendor surely.

In Aspen Flarenet selecting the option to compensate for the change in kinetic energy from inlet to outlet gives results closer to the complex model values. The use of this option is encouraged.
The optional isentropic method in Flarenet gives ridiculously low exit temps and should not be used as it could give incorrect metallurgy selection. ( you only need an isentropic efficiency of 0.005 to match the complex model). In my opinion its not worth using it.

Conclusions

I agree with the thinking that a combined isentropic/isenthalpic model is the absolute correct way to model a PSV, given the tools available this is possible but...
..to all extents and in the real engineering world it doesnt make any difference. We are talking tenths of a degree difference in the outlet temps.
Given all the other errors in data prediction taht could occur when sizing a PSV the old fashioned method of simulating as a valve with an isenthalpic flash seems not too bad away to do it after all.

comments welcome

[djack77494]

flarenuf,
Thank you for your efforts to provide poor process engineers with this valuable background information on this important topic. Perhaps I had gotten it wrong, but my impression was that the preferred method (more accurate?) was to consider the entire valve to be isentropic and the tail pipe to be isenthalpic. Could you comment on this? Thanks.

[JoeWong]

See some response in http://www.eng-tips....d=241218&page=1