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PSV discharging temperature (valve outlet) 6

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PaoloPemi

PaoloPemi

Mechanical
Mar 23, 2009
269
The isentropic nozzle model (see the attached Excel file nozzle.xls) available in Prode Properties (sizing of relief valves with critical and two-phase flow with isentropic nozzle model HEM) calculates required area and estimates the discharging temperature in two steps, a isentropic flash to choked pressure and then adiabatic flash to outlet pressure,
according Prode this rigorous method is applicable to both gas and gas+liquid (HEM method) flows.
I have tested this method for many years comparing results against single isentropic and adiabatic flash (with specific corrections) and the constant energy flash (available in Prode Properties) and the results seem more accurate than the other options that I have investigated.
I have been informed that different software applications (as Aspen Flarenet) utilize similar procedures.
Since the determination of discharging temperature can have a large impact in plant's design (selection of materials etc.) and different methods may produce very different estimates it would be interesting to know if a specific method or procedure has been discussed in standards,
does anyone know if some standard (API etc.) do include a discussion about this specific topic (determination of discharge temperature) ?
 
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Paolo,
to answer your question, I do not know of any point in API std. discussing this topic,
perhaps you may contact the API Subcommittee on Pressure-Relieving Systems


I have read they have been discussing that model (numerical solution of isentropic model) but I do not know the details,
perhaps some reader can provide additional information,
by the way I have used the nozzle model (included in Prode Properties) for several years and I can confirm it gives consistent results.
 
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Paolo.

I do not recall having seen or heard of these calculation method details being discussed in any public standard.

I have a simple nozzle spreadsheet which is discussed in the FAQs in this forum. The calculations in that spreadsheet terminate at the end of the nozzle and do not get into estimating conditions at the PSV outlet flange. So, I have never really thought in detail about what happens in a PSV between the flow nozzle exit and the outlet flange. My company has it's own PSV sizing software, so hopefully our triple PhD's in charge of that have thought it out.

From my understanding of compressible flow where Mach 1, or at least transonic flow, is achieved at the end of the flow nozzle, there will be a corresponding shock wave at the flow nozzle exit. The classical approach to this is for the stagnation enthalpy (h0) to be the same just upstream of the discontinuity (x) and just downstream of the discontinuity (y). This is represented in the form:

hx + Vx2/2 = hy + Vy2/2 = h0
h = enthalpy
V = velocity

Is this what you mean by ”then adiabatic flash to outlet pressure"? Somehow, I think not.


Good luck,
Latexman
 
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Latexman,
thanks you for the comment,
yes, the question is about the method for estimating the temperature at PSV flange outlet,
for the sake of discussion there are some numerical examples here

from what I have heard it seems that this specific point is not mentioned in API (and other public standars)
Paolo
 
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Yes, I have been following the post over there too.

Good luck,
Latexman
 
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thanks for the comments,
I have created a excel page (see the file psvcompare.xls) to compare the numerical solution of isentropic nozzle with several variants of API formula for gas/vapor,
it is interesting to notice that different methods can produce large differences in results (10-30% and more)
I have posted several test cases in above mentioned thread,
the Excel page requires Prode Properties for calculating cp,cv,zv etc. for non commercial purposes a free copy is available from Paolo
 
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thank you for the page Paolo,
I had created a similar one but this is more easy to use.
 
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