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Not Getting True Critical Properties In Unisim

unisim critical properties choke valves

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#1 Laxpro


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Posted 14 January 2019 - 12:41 AM

Dear All,


i am doing some Unisim simulations to check the adequacy of Choke Valve / Control valves (CV) on the Gas Well flowlines. I am using 'Critical Property' Utility for this as in the latest Unisim version R451.3.


I notice that, I am not getting True Critical properties for some of the streams (GOR adjusted) coming from some wells. It shows only Pseudo properties but as i know they are not realisitc so cant be used for choke valve / CVs sizing. Whereas if i separate the stream in a separator, i can get True criical properties for induvidual stream but agian not for all.


1. What is the reason? Can there be a stream without True Critical Properties?

2. What data to use for control valves sizing in this case?


Refer attached snapshot for your ref. Request for your expert opinions.



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#2 PaoloPemi


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Posted 14 January 2019 - 02:08 AM

There are some cases (as the presence of multiple phases, selection of wrong thermo models, etc.)  where a procedure can't calculate fluid critical properties.  AFAIK Hysys (and the similar Unisim) should include a modification of the Heidemann / Khalil procedure, see "the calculation of critical points", 1980.  It is a stable procedure, and well tested.


The software that I use (Prode Properties) adopts a different procedure (based on Gibbs minimization).  There are some advantages (in terms of speed) but in terms of reliability, the results are equivalent in most cases.


You may find several threads discussing the calculation of critical points with different tools at ChE Resources.


About your specific application on CV sizing, make sure you know which kind of properties you really need.

For choke valves, I adopt a procedure included in Prode (similar to PSV sizing) which integrates real values as density etc..  However, some generic procedures do simply require you to enter values at some specified conditions.

#3 Technical Bard

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Posted 15 January 2019 - 08:52 AM

Especially for a wide boiling mixture, what you seek is not necessarily the critical point, but the cricondenbar (the maximum pressure at which two phases exist.  Plot the phase envelope (P-T or P-H with isotherms) to figure out if you are in the dense phase (beyond the cricondenbar).

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