Can Mach Number Formula from API 521 be used for PSV and BDV Inlet Line Sizing

[DoraeS]

Hi, I would like to know if API Std 521 (Year 2007) Mach number formula (27) and (28) can also be used for PSV lead line / BDV inlet line etc, and not only limited to PSV outlet line.
Is there any limitation in the mach number formula derivation for the above?

Thanks.

 

[madithak]

DoraemonS

My experience I just don't take a formula used elsewhere for another scenario and apply I to other cases. There maybe examples you may have to do this just to get a quick and dirty answer. But not for applications.

basic principles of hydraulics of gas and liquids or multiphase and client's spec usually drives the problem. I have evaluated flare header with mach# close to 1.0 with adequate support for cases like an emergency sut-down due to fire or gas detection. Usually PSV upstream piping has only one limitation that is to have delta P less than 3% of set pressure. There is a reason for that and important to understand why.

BDV sometimes can reach close to sonic but remember it falls down immediately. If you see the BDV curve the flow falls down rapidly and so will the velocity.

hope above helps

Maditha

 

[PaoloPemi]

It is based on ideal gas equation so you should expect a relatively limited range of application (concerning P,T),
to estimate accurately speed of sound (and mach number) you can utilize a EOS (as Peng Robinson, Soave etc.),
out of curiosity I have evaluated the results calculated with API formulation against those calculated in EXCEL with Prode Properties library (and PR, SRK, BWR models) with macro
=EStrMSS(stream,t,p)
which returns in a cell the speed of sound at specified t,p
as expected there is little difference far from critical region (low pressures and temperatures),
while results may differ considerably at high t,p