2 replies to this topic

[Ghasem.Bashiri]

Dear Process Experts
I need to define critical pressure for control valve within datasheet for liquid services. But my mixture contain some dissolved solid. Actually my mixture is caustic, water, mercaptans dissolved as solid within caustic.
I have two problem: If I use some software to such prediction software will give me two critical pressure: Pseduo and True! which one should be reported in CV Datasheet. Another issue: If Software can not predict such data how can I predict it? Which data is safe for CV, maximum critical pressure or minimum critical pressure. Becasue with this knowledge I can neglect some component in the mixture and then calculate Critical pressure.
GB

[Zauberberg]

For defining critical pressure ratio factor in control valve sizing applications, you can refer to:

1. C.Branan, "Rules of thumb for chemical engineers";
2. Emerson - Control valve handbook.

The latter can be downloaded from: http://www.documenta.../book/cvh99.pdf pages 110-118 in the document.

[paulhorth]

Ghasem,

I believe that Critical Pressure is only relevant for control valve sizing, in liquid service, if there is a possibility of flashing in the valve. That is, if the vapour pressure of the liquid is at or close to the operating pressure, so that a drop in pressure within the valve could cause local vapour formation.
I would guess from your description of the fluid that you are dealing with a Merox unit, somewhere in the regeneration circuit, which I believe does not run very hot.. If the caustic solution is not very hot then it will not be at risk of flashing in the valve. In this case the critical pressure is irrelevant, so you can enter the critical pressure for pure water and ignore the caustic and the mercaptans. Even if there is a risk of vaporisation, I would say that the critical pressure for pure water will be good enough for this application which will be a small valve. Water will not have a pseudocritical pressure - this value is just for hydrocarbon mixtures. Just use the true critical pressure which can be found in the steam tables.

Paul