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Two-Phase Relief. Api 520 Vs Iso 4126-10

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#1 shvet1


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Posted 13 December 2018 - 11:26 PM

Good day, forum
I have an issue concerning backpressure (pardon my assumption concerning discharge coefficient).
API 520-1-2014 stated in equations C.43 and C.44 that mass flux is calculated with upstream pressure and backpressure difference or backpressure ratio. By this way backpressure affects PRV reqired capacity.
ISO 4126-10-2010 stated in equations 35 that mass flux mass flux is calculated with only upstream pressure. By this way backpressure does not affect PRV required capacity.
In case of high back pressure ratio this situation leads to required capacity calculated with API 520-1-2014 is much less than ISO 4126-10-2010. Can anyone comment this?

Edited by shvet1, 13 December 2018 - 11:40 PM.

#2 breizh


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Posted 14 December 2018 - 01:00 AM

Hi ,

you may consider the resources attached ,with examples .


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#3 PaoloPemi


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Posted 14 December 2018 - 02:04 AM

from a point of view of PSV sizing (not piping) with critical flow condition (at vena contracta) backpressure has no influence,

so the first step would be to verify vena contracta condition...

take care there are many different methods for two phase flow (assuming, for example, homegeneous vs non homegeneous equilibrium, boiling delay etc.) ,

both ISO and API include discussions about simplified HEM methods as Omega (Leung) which neglects boiling delay and slip,

API  also includes a rigorous solution of HEM (with similar limits),

there are Excel pages at cheresources showing how to solve HEM, see for example,




for subcritical flow (dP < dP critical) there are simplified solutions based on conservation of energy principle  or you may ask the manufacturer for specific parameters ...

#4 marjan3612


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Posted 17 March 2019 - 12:09 PM

Hello every body,


I need to size a PSV for a Thermal Fluid called Syltherm 800 which does not have a lot of known properties. It will flash in the PSV. i am trying to use one-point Omega method. API 520 says one-point Omega estimation can be used but doesn't provide a formula for that. Can any one tell me what the formula is?


Thank you!

#5 breizh


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Posted 17 March 2019 - 09:56 PM

hello marjan 3612,

Consider reading my reply to this post .

I've added a link with thermal fluids comparison , you may find pointers there




Good luck .


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