Difference PRV Opening Pressure On Test Bench and in Line 2

[sebestie]

Hi

I am Commissioning Engineer on a Gas Plant Project. A piloted PRV is installed at the discharge of a firewater diesel pump. We tested the PRV (equipped with a pilot) on a certified test bench with air so that the opening pressure was set to 13.7 barg as specified in the PRV's datasheet.
When we installed the PRV and did a online test with the diesel firewater running increasing engine RPM but the PRV had still not opened at 15 barg.
The PRV's opening during inline test with firewater pump running was witnessed through an inline side-glass connected to the discharge flange of the PRV.
We decided to set the pilot in line with the pump running so that the PRV would open at 13.7 barg.
Then we dismantled the PRV to bring it back to the bench to test it again with air. The PRV opened at 12.1 barg instead of 13.7 barg.

The PRV discharge line goes back to the top of the Firewater tank, but The PRV is piloted so we can rule out any back pressure effect on the opening set pressure.

I am trying to find an explanation as why there is a difference in the opening pressure between the bench and inline?

Thanks

Seb

 

[don1980]

Unlike a conventionsl PSV, a pilot operated valve should open at the same pressure in the field and on the test bench. A plugged sensor tube could cause the valve to open low, but there's no logical explanation for opening high, other than a mechanical problem in the pilot itself.

I wouold present this to the manufacturer and ask them to give you a valve that works.

Several years ago one of the major manufacturers had a significant quality control problem with their pilot operated valves. That problem has sense been corrected. If this is a new valve, that's probably not the explanation. Regardless, this ought not be happening - I'd let the manufacturer figure it out.

 

[Duwe6]

Why did you use a gas to test a liquid relief valve. They behave a LOT differently. I'll bet that the mfr will not guarantee setpoints established using air on one of his liquid, incompressible flow, PRV's.

 

[BenThayer]

stupid question but was the gauge in the field a calibrated/certified gauge? or even a new gauge? or was it a gauge that had been out on the firewater loop for several years? if the field gauge is not accurate, there may not be anything wrong.......

 

[sebestie]

Thanks for answering my question.

I forgot to mention that the gauge in the field was calibrated.

I wrote to the manufacturer of the valve. He answered a that "the pressure relief valve is preset only with water. Air is prohibited to adjust valves. The priming and depriming of the loading chamber will cause huge delay in valve reaction"

 

[AzriHms]

@Duwe6
Can you expalin further ->"They behave a LOT differently. I'll bet that the mfr will not guarantee setpoints established using air on one of his liquid, incompressible flow, PRV's"

 

[AzriHms]

I forgot to mention that we have similar problem with our pilot rv which is running on LNG. If we test it with water, the only thing is how to clear the water after testing at internal chamber of the pilot?

Previously we test it using air/gas.

Thank you