Calculating Back Pressure on PRV/SRV

[carltogr]

How does one calculate the back pressure imposed on a PRV/SRV by downstream components (valves, fittings, etc.) in a vent-header, including the vent header? I imagine one would use equivalent lengths for the compenents....? I have a high pressure application (2400 psig venting/relieving to the atmosphere (constant back pressure).

 

[Latexman]

carltogr,

The equivalent length method (and the Darcy-Weisbach formula) is probably not the preferred method for your application. Notice I said probably, because not enough detail has been given for me to be certain. You probably have high speed gas or vapor that should be characterized using compressible flow methods.

You may be able to break your system down into a small number of pipes and fittings and use simplified compressible flow hand calculations to figure out what is going on. Or, you may need a software package that handles high speed, compressible flow in a complex piping network.

Either way, you need to use the flow the PRV/SRV is rated at. Do not use the scenario sizing rates. Since you said there is a "vent header", you also have to figure out how many and which PRV/SRVs are going off simultaneously in your "overall worst case scenario" if there is more than one PRV/SRV discharging into the "vent header".

Good luck,
Latexman

 

[carltogr]

Thanks Latex,

where could one get easy access to these "hand calculations" without reinventing the wheel?

carltogr

 

[MortenA]

I agree with latexman:

Either:

Set up a flow diagram showing the entire flare header system system. Print flow from each source on the diagram while identifying simulaneous reliefs and then add flow rates up in the header. This will most likely cut the header system up into fairly small segments. Identify pipe sizes, fittings, bends etc for each segment and calculate equivalent length and enter this into a spreadsheet. Then start from the flare tip and work backwards calculating dP for each segment in the spreadsheet. Remember to check that the dP for any segment is not "too large".´Use the fact that pressure is allways "thw same" at junctions - but remember that temperature isnt - so you must also set up a system to allow for different gas temperatures and calculate the mixed gas temperature. You can play it simple and assume that the temperature does not drop as a result of JT in the ehader - if the max dP is low this is not unreasonable (but remeber the temperature drop for each source when it goes through the BDV/PSV or whwatever)

or use a software package to do allmost the same.

Best regards

Morten

 

[Latexman]

Three I use are:

Perry's Chemical Engineers' Handbook
Crane TP 410 (Google it and buy it on the web)
"The Dynamics and Thermodynamics of Compressible Flow" by Ascher H. Shapiro

There's dozens of others.

Good luck,
Latexman