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# Relief Valves in Series

## Relief Valves in Series

(OP)
If I have 3 total relief valves installed in series in a liquid filled system, how do I calculate the pressure required to start flowing out of that relief line (i.e. all 3 valves open)???

I know the Cv equation is 1/Cvt^2 = 1/Cv1^2 + 1/Cv2^2 + 1/Cv3^2, but is there another way to calculate the upstream pressure at which flow will start?

### RE: Relief Valves in Series

I'm sure someone had what seemed like really good reasons for putting relief valves in series, but the reasoning escapes me.

For conventional PSV (not pilot), when the first PSV opens into a closed system, the dP across it quickly goes to zero and the valve shuts. The second valve never sees system pressure and doesn't lift. Basically this scenario removes overpressure protection from the system.

If they are all pilot operated PSV (connected to the process upstream of the first PSV), with the 3rd set lower than the 2nd which is set lower than the first, then you don't compromise the overpressure protection of the system, but you have added two failure points that don't add commiserate value. Flow would begin when the closest PSV to the process opens. If the valves are set at progressively higher set-pressures instead of progressively lower set pressures then the PSV will start flowing when the highest pressure valve opens.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: Relief Valves in Series

(OP)
Thanks for your reply. Some clarification...the relief valves aren't pilot operated, they are simply spring closed poppet type relief/check valves.

They are in series becuase they each segregate a separate liquid filled chamber. So, the upstream most chamber relieves to the next, and the next to the downstream most chamber, and the downstream most out of the system.

Does this help clarify the arrangement. What would be true of the pressures in the chambers if the liquid in the system were expanding at a fast enough continuous rate for constant flow to be occurring for a period of time?

Thanks,
PK

### RE: Relief Valves in Series

It does clarify things, but I would be very concerned about operating this system without a huge difference in set points. For example if all of the components had a MAWP of 600 psig.
If the first PSV is set to operate at 300 psig, then the second chamber PSV could not be set higher than 300 psig (because the first one would not see a 300 psig dP until it was above MAWP at the middle PSV setpoint) and the third chamber couldn't be set more then about 30 psig (taking advantage of allowable overpressure).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

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