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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

Asif Raza

RE: PSV Backpressure

Depends on your type of relief valve. See something like this for some basic descriptions http://wermac.org/valves/valves_pressure_relief.ht...

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

2
I'd recommend looking at the wording used to specify the backpressure. Does it mention the words superimposed, constant, or built-up? One possibility is that when the piping and relief scenario was previously reviewed it was found that a 6 psi pressure will build during relief conditions. This can happen if the discharge piping runs for a long length or it doesn't immediately increase in diameter on the outlet of the relief valve.

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

(OP)
I've checked the model number is Farris 26HA10-120. Per the vendor's catalogue it is a conventional spring loaded PSV without bellows
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

Is there any explanation of the original 6 psi backpressure? Out if curiosity, what is the inlet size, outlet size, and outlet pipe length from the RV to the atmosphere?

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