PSV set point.

[deerun75]

I have a starting air process with a series of vessels in series. Each vessel is equipped with a PSV. The first PSV is set at 100 psi. At what pressure can I set the other PSVs? Setting them at 100 psi seems redundant since the first PSV will trigger at that pressure. Should I lower/increase the setting on subsequent PSVs? Any assistance is appreciated (especially those with references).

 

[Latexman]

I don't think we have enough information. If we had a P&ID of the process and MAWP's of the vessels, you'd probably get an answer.

Good luck,
Latexman

 

[deerun75]

The air tanks/vessels are rated for 101 psi. Also, P&ID is attached.

Thanks for your assistance.

 

[MortenA]

oyedee, im almost tempted to just wrtie: Read the manual )e.g. API 521) - but i would also like to elaborate a bit.

You must determine the relief case for each vessel independently. If e.g. there are no way that you could sectionalise the vessels so that one (downstream) vessel is blocked in from the upstream (with the PSV) then one PSV may be sufficient. By systematically going through the relief cases for each vessel then you will be able to determine the size, setpoint, relief rate etc for each valve.

Best regards

Morten

 

[Latexman]

What's the deadband on the air pressure controls?

Is there anything to be gained by reducing the downstream PSVs setpoint? I doubt it. Each vessel can be valved in, so they each need a PSV.

Good luck,
Latexman

 

[SeanB]

What MortenA is saying is right. Your compressor is the pressure source. If the PSV on the first wet air tank is sized for the blocked discharge case of the compressor then there should not be a blocked discharge case for the downstream PSVs. The other other two would probably have the same set pressure but would be sized for thermal or fire relief case. You will have to do that analysis to determine your reliefing cases.

 

[Latexman]

MortenA, I'm confused by the two seemingly opposing statements:

"If e.g. there are no way that you could sectionalise the vessels so that one (downstream) vessel is blocked in from the upstream (with the PSV) then one PSV may be sufficient."

"By systematically going through the relief cases for each vessel then you will be able to determine the size, setpoint, relief rate etc for each valve."

The way I'm looking at this is, the 3 vessels downstream of the compressor are "air receivers", right? Air receivers require PSVs by Code. Of course, size and setpoint are up to the Engineer's evaluation.

Good luck,
Latexman

 

[knapee]

Dear Sir/Madam

I wonder could you use breather valve sizing software to sizing this air receiver valve. You may check the software in the web.(GROTH's sizing program)

Please forgive me if my suggestion is silly.

 

[MortenA]

Latexman i can see what you mean. I was speaking in general amd havnt looked at the attached schematic. IF there were no valves (at all) between the vessels then as far as i know one PSV might be enough (but i dont know if there are any US rules specifically about air receivers). But the drawing clearly shows valves and i therefore also believe that there should be PSV's. The first valve sized for blocked flow if max P>design P. The rest can be thermal/fire.

Best regards

Morten