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Psv Discharge Line Sizing


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#1 chem_buddy

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Posted 01 October 2014 - 10:33 PM

Hi all

 

Presently I am doing the PSV sizing and Inlet/outlet line sizing.

 

For PSV inlet line sizing we use the 3% pressure drop criteria at rated flow of PSV and properties like density, viscosity, Cp/Cv are taken at relieving conditions.

 

For PSV outlet line we use the mach no. and backpressure criteria according to type of valve. 

 

I would like to know that for PSV outlet line sizing, properties should be taken at which pressure and temperature condition.  

 

If the PSV is discharing to Flare header then can we consider the properties at flare header pressure and corresponding temperature at that pressure or we should use the properties at inlet relieving conditions.

 

Your guidance / inputs will be great help on this matter.

 

Thanks 



#2 Bobby Strain

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Posted 01 October 2014 - 11:02 PM

I always use isothermal compressible flow model. It's a bit conservative, but much easier than the adaibatic calculation. If you are working on a real installation, you best confer with one of your associates or your boss.

 

Bobby



#3 latexman

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Posted 02 October 2014 - 07:13 AM

Give us the details on your Mach number criteria.  That is the key to the answer.



#4 fallah

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Posted 02 October 2014 - 11:51 AM

chem_buddy,

 

Using the method based on Fanno Lines would provide the results which are much close to actual conditions...



#5 flarenuf

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Posted 03 October 2014 - 01:53 AM

chem buddy

 

you have to be careful here as there are many possibilities.

 

if you are usin one of the commecial simulators to model the discscharge line then the calc will be taken care of automatically

 

the line will build up back pressure which depending on the flow,diameter,length, header discharge pressure could range from a few 1/10ths of a bar to several bar.

.

you may also be approaching sonic flow at the discharge point , again it depends on so many factors.

 

if you are trying to do a hand calc then start at the exit point pressure ( flare header)

if the tailpipe is long split it into several segments, calc backwards to the PSV the dp for the first segment

repeat for each segment using the properties at the pressure just calculted

 

as fallah says if you are running at low pressures with a simple gas then Fanno lines , or a simple calc using an isotheral equ for the entire length may be adequate.

flarenuf






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