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# True or Pseudo Critical Pressure for Sizing Control Valves

## True or Pseudo Critical Pressure for Sizing Control Valves

(OP)
For an assignment I was working on, I needed to reference the critical temperature and pressure for a hydrocarbon mix. For the particular stream, I used the "critical properties" utility, which displayed "true" and "pseudo" temperatures and pressures.

What do you recommend for sizing control valves, true or pseudo critical pressure? What do you think for hydrocarbons (diesel, fuel oil, HVGO...)?
The difference between both values may be significant in some cases, which affects the control sizing calculations.

If anybody is able to shed some light on this issue, i'd be greatly appreciative :)

Thanks,
Replies continue below

### RE: True or Pseudo Critical Pressure for Sizing Control Valves

it affects the criteria for valve sizing when cavitation is likely. It is not exact, but a basis for design that is usually found to be acceptable.

The API is a good source

### RE: True or Pseudo Critical Pressure for Sizing Control Valves

for two phase flow with wide boiling mixtures I do not use ISA formulation but HEM or HNE models,
ISA requires critical pressure to estimate the pressure drop (i.e. the maximum allowable dp) in case of flashing as hacksaw noted,
but the method doesn't work reliably for wide boling mixtures,
note that for pure fluids there is no difference between true and pseudo values and the standard do not discuss that possibility.
I calculate true critical pressure (or temperature or other properties) with a library (Prode) and macros in Excel as =StrPc() this allows to create pages to compare results from different methods,
according some comments and papers it would seem that a more accurate way to evaluate the maximum allowable dp (i.e. single phase liquid flashing) is to model the valve for example solving an adiabatic flash operation (in my case =HPF() in Excel) to identify the point where the liquid begins to flash, then adopt that dp instead of ISA value,
you may try this solution and compare values.

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