4 replies to this topic

[Abdessamed1961]

Hello everyone

I need your help about line sizing for PSV's, i'm sizing PSV's for a pipeline transporting a gas with the following data:

MW=                        20.53 (mostly methane 71%)

Flowrate=                 320 000 kg/hr

Density upstream = 184 kg/m³

Temperature=          100 °C

Pressure=                240 bar

Z factor =                 0.89

Cp/Cv=                    1.55

i used HYSYS safety analysis for sizing, i choosed blocked outlet scenario as the worst case scenario, so i discharged all the flowrate, HYSYS suggested 3 pilot PSV's, the simulation selected 2 J 3 (2500 x 300)  for each PSV for a flowrate of 106 666 kg/hr, the problem i'm facing now is sizing the downstream lines, i took a mach number of 0.7, density of 2 kg/m3 at relieving.

hand calculation gave me a 10" pipe and the flare header around 20", these results seem  weird to me, i would like to know if these are reasonable or my calculation are missing something.

thank you.  

[chocobang]

Pipeline are usually protected by PSV at the upstream compressor station which has a set point equal or lower than the pipeline MAOP. Hence, I guess your PSVs that you mentioned will be located at the upstream compressor facility.

 

There will be backpressure exerted on the flare header at peak flow which will increase your density and lower your mach number. This might help lower your tailpipe size.

 

Flare header are size at Mach of 0.5. This will increase your flare header size.

Edited by chocobang, 04 March 2026 - 02:38 AM.

[snickster]

I checked for 0.5 mach in header (maximum for lighted flare and recommended for maximum piping velocity) and PSV discharge and for the flowrates indicated you need a 19.6" ID discharge and 34" ID header calculations attached.

 

Your PSV sizing appears to be way off as normally PSV discharge line is only a few diameters larger than PSV outlet flange.  What is your PSV orifice size?  I assume you have a reason for using 3 smaller valves in place of one large one.

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Edited by snickster, 04 March 2026 - 05:33 PM.

[Abdessamed1961]

Pipeline are usually protected by PSV at the upstream compressor station which has a set point equal or lower than the pipeline MAOP. Hence, I guess your PSVs that you mentioned will be located at the upstream compressor facility.

 

There will be backpressure exerted on the flare header at peak flow which will increase your density and lower your mach number. This might help lower your tailpipe size.

 

Flare header are size at Mach of 0.5. This will increase your flare header size.

the PSV is located downstream and yes the set pressure is equal the MAOP, thanks for your help.

[Abdessamed1961]

I checked for 0.5 mach in header (maximum for lighted flare and recommended for maximum piping velocity) and PSV discharge and for the flowrates indicated you need a 19.6" ID discharge and 34" ID header calculations attached.

 

Your PSV sizing appears to be way off as normally PSV discharge line is only a few diameters larger than PSV outlet flange.  What is your PSV orifice size?  I assume you have a reason for using 3 smaller valves in place of one large one.

Thanks for your efforts i really appreciate that.

I agree with you that's why my calculation seemed weird to me, the PSV orifice is J (1.287 in²) 2" x 3", the reason why i used 3 PSV's because the inlet class is #2500 and there is no larger orifice after "J" having this class as per API 526.