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RJB32482 (Chemical)
20 Aug 06 20:43
When do I select the balanced bellows valve over the conventional relief valve.  Is it when the constant backpressure on the valve (like discharging into a lower pressure system) is over 10%, when the sumperimposed backpressure is >10%, when the sum of those 2 are over 10%?

Thanks.
Helpful Member!  Latexman (Chemical)
21 Aug 06 8:30
RJB,

Compensation for superimposed back pressure which is CONSTANT may be provided by reducing the set pressure of a conventional relief valve.  A balanced bellows is recommended when built-up back pressure is expected to exceed 10% of set pressure.

When superimposed back pressure is VARIABLE a balanced bellows is preferred.

Discussion on a pilot-operated relief valve was omitted.

Good luck,
Latexman

Helpful Member!  JimCasey (Mechanical)
23 Aug 06 11:41
Well said, Latexman.

RJB32482: The important thing to remember is that the opening force on the valve is caused by the differential pressure from upstream to the discharge, applied against the area of the seat.  The spring load is equal to the opening force at the set pressure.  
If you superimpose a back pressure, that will raise the set pressure by the same amount.  It is then possible for the "protected" vessel to exceed its MAWP if the superimposed backpressure is high at the time of a process upset.  

The Bellows effective cross-sectional area is the same as the area of the seat.  The bellows is vented to atmosphere, which is relatively more constant-and besides, the MAWP of the protected vessel is measured in gage pressure units.  So with the bellows referenced to atmosphere, there is no effect upon the setpoint with variable backpressure.  There IS STILL an effect upon capcity that should be evaluated.  With a lot of backpressure during discharge you may experience enough flow restriction so that the pressure continues to increase above the "BOOM" value.  
Ashereng (Petroleum)
24 Aug 06 10:33
Well said both of you, Latexman and JimCasey.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

npf (Chemical)
1 Dec 06 17:33
All:

Could anyone explain me how to determine the latent heat for supercritical fluids?
Also if its not a supercritical fluid how would you determine the latent heat in HYSYS?

Regards,

-sp77
Montemayor (Chemical)
2 Dec 06 10:50
sp77:

Start your own, new thread with this important subject.  I can assure you this post won't last long without getting "red flagged" for being out of place and "pirating" RJB32482's thread.

Your concern/question is valid; however you communicate wrongly or without knowledge of phase phenomena.  By definition, it is impossible for a supercritical fluid (SCF) to have a latent heat.  So your query doesn't make sense literally.  Additionally, HySys will go "bonkers" trying to follow your instruction to determine the latent heat of an SCF simply because there isn't any to be found.  Bear in mind this is not meant to get on your case; rather, it is to put an important subject in a clear and correct Thermo light.  An SCF is neither a gas or a liquid; therefore it cannot have a latent heat.  Graphically stated: it ain't under the dome.


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