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10 comments
Active Contributor

Join Date: Sep 2008
Posts: 13

adiabatic or isenthropic flash calculation

10/19/2008 4:17 AM

hi

i am calculating relief condition of PSVs in new project. my means from relieving condition is specially about releif temperature (to determine autorefrigeration effect in metallurgy selection). I do not assure should be either adabatic flash calculation or isenthropic flash calculation applied (during pressure change from upstream PSV to flare header back pressure)?

please guide me in this regards

thanx

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Power-User

Join Date: Jan 2007
Location: Houston,Texas
Posts: 378
Good Answers: 24
#1

Re: adiabatic or isenthropic flash calculation

10/19/2008 10:33 AM

Read and study API RP521--all about design of relief systems. To determine the minimum temperature achieved in flashing a hydrocarbon, one should assume there is no built up back pressure (atm pressure flash,) and no heat input to the boiling fluid. This is an 'adiabatic' situation.

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Keith E Bowers, PMP
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Active Contributor

Join Date: Sep 2008
Posts: 13
#2
In reply to #1

Re: adiabatic or isenthropic flash calculation

10/20/2008 2:56 AM

could you please to specify which section and page of API 521 this statement has been given in?

I could not find it in API 521.

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Power-User

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Location: Houston,Texas
Posts: 378
Good Answers: 24
#4
In reply to #2

Re: adiabatic or isenthropic flash calculation

10/20/2008 5:32 PM

The lack of specific knowledge demonstrated by the nature of your question indicates you ARE NOT QUALIFIED to make this calculation on a 'real' facility-either under design or in construction. Relief system design is a life safety field and must never be designed or evaluated by inexperienced engineers. Vessel Failure, large economic losses, and loss of human life are the certain results of an undersized or improperly constructed relief system.

Please find a qualified MENTOR to assist in your education on this complex subject. It IS NOT as simple as it seems to the un-iniated.

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Keith E Bowers, PMP
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Active Contributor

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Posts: 13
#5
In reply to #4

Re: adiabatic or isenthropic flash calculation

10/21/2008 3:24 AM

keith

I know I am jounior in PSV calculations but I like to learn this knowledge rapidly. you could accept our request and give section address of API.

ok, I will find it myself

thanx

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Participant

Join Date: Nov 2009
Location: Houston
Posts: 4
#9
In reply to #4

Re: adiabatic or isenthropic flash calculation

11/03/2009 10:11 AM

Mr. Bowers, Please review your previous comments for accuracy. The tone of this thread is hostile and should be rescinded in good taste, if not to save face.

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Justin Phillips, B.S. Chemical Engineering
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Join Date: Nov 2009
Location: Houston
Posts: 4
#8
In reply to #2

Re: adiabatic or isenthropic flash calculation

11/03/2009 10:07 AM

See API STD 520, Part I, sections 5, and Annexes C and D. API 521 is not a valid reference as far as this discussion is concerned.

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Justin Phillips, B.S. Chemical Engineering
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Participant

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Location: Houston
Posts: 4
#7
In reply to #1

Re: adiabatic or isenthropic flash calculation

11/03/2009 10:05 AM

The correct API reference is API STD 520, Part I, Section 5.5.1, and Annexes C and D. Also, in accordance with API STD 520, Part I, the fundamental basis for relief orifice sizing is the assumption of isentropic flow, not adiabatic.

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Justin Phillips, B.S. Chemical Engineering
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Commentator

Join Date: Nov 2007
Posts: 62
Good Answers: 1
#3

Re: adiabatic or isenthropic flash calculation

10/20/2008 6:03 AM

A couple of on line references about ideal gas behavior will set the stage for your calculation:

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/adiab.html#c1:
another discussion with online calculator

http://en.wikipedia.org/wiki/Adiabatic_process
discussion of ideal gas behavior

In the real world, you may need to use empirical activity coefficients, but the IDG estimates are often useful upper and lower bounds of what to expect.

HAVE FUN

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Active Contributor

Join Date: Sep 2008
Posts: 13
#6
In reply to #3

Re: adiabatic or isenthropic flash calculation

10/21/2008 3:55 AM

jcrock

thanx for introduced references. but I know adiabatic and isentropic process well. I don't know which of them are applicable for PSV calculations? anyway thanx for your fast reply

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Participant

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#10
In reply to #6

Re: adiabatic or isenthropic flash calculation

11/03/2009 10:17 AM

Hossein, Please take a look at API STD 520, Part I. The fundamental basis for sizing equations assumes isentropic flow through the relief orifice. Isentropic flow will typically result in much lower temperatures than isenthalpic (adiabatic) flow. However, in reality, the flow through a PSV will be somewhere inbetween pure isentropic and pure isenthalpic. Isentropic flow will give more conservative values as far as temperature is concerned, but isenthalpic flow will give more conservative results as far as outlet losses are concerned.

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Justin Phillips, B.S. Chemical Engineering
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Users who posted comments:

Hossein Deldari (3); jcrock (1); Keith E Bowers (2); paladrache (4)

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