PSV Backpressure
PSV Backpressure
(OP)
Hi All:
I have a non-code pressure vessel with a design pressure of 15 psig. The vessel has PSV set at 14.99 psig with a specified backpressure of 6 psig. The backpressure is wrongly specified since the outlet of the PSV is routed to atmosphere. The maximum backpressue the PSV can see is maximum 0.2 psig
Questions -
1) What are the effects of backpressure on the PSV set pressure?
2) Since there is no backpressure on this PSV; what is the pressure when PSV will start opening? it is 14.99 psig?
3) If in reality, 6 psig backpressure do exists then what pressure the PSV would start relieving, is it at 14.99 + 6 = 20.99 psig. Is this correct?
Thanks Araza
Asif Raza
I have a non-code pressure vessel with a design pressure of 15 psig. The vessel has PSV set at 14.99 psig with a specified backpressure of 6 psig. The backpressure is wrongly specified since the outlet of the PSV is routed to atmosphere. The maximum backpressue the PSV can see is maximum 0.2 psig
Questions -
1) What are the effects of backpressure on the PSV set pressure?
2) Since there is no backpressure on this PSV; what is the pressure when PSV will start opening? it is 14.99 psig?
3) If in reality, 6 psig backpressure do exists then what pressure the PSV would start relieving, is it at 14.99 + 6 = 20.99 psig. Is this correct?
Thanks Araza
Asif Raza
Asif Raza





RE: PSV Backpressure
A basic spring relief valve will (or should) take the back pressure into consideration if you think it is a fixed pressure. Then the set pressure on the bench with no back pressure is your set pressure inside the vessel minus the back pressures, so in your case 9 psig.
BUT if no one has taken account of the back pressure then your question 3 is correct.
Only way to find out is either lock it off and test it or remove it and put it on a bench test.
However if it is a balanced bellows relief valve or the pressure is set by a separate pilot valve, then back pressure should be irrelevant.
Note that accuracy of set pressure is commonly up to 10% below the set pressure and can go to 10% above the set pressure at max flow.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: PSV Backpressure
From a design standpoint, the above scenario should not have happened if you have a conventional valve because that is excessively high backpressure. Do you happen to have a model number from the relief device? The information from any tag could be fairly informative depending on the brand.
I wouldn't discredit the 6 psi backpressure yet as it is a possibility during relief condition although it would require the use of a bellows RV. I believe typically these are limited to 30% so 6 psi would still be excessive.
RE: PSV Backpressure
Recently we've send out the PSV out for repairs. Per the inspection and repair report, they've tested on a stand with a set pressure of 8.9 ( PSV set pressure = 14.99 - 6) and the PSV discharged at 9 psig three times and the PSV was passed @ 90% per API 527
As I mentioned there is no backpressure on the PSV since it is discharging to atmosphere. Considering the present set-up, I think we have to change the PSV spring for a lower backpressure of 0.1 psig to make it work at 14.99 psig
Asif Raza
RE: PSV Backpressure