Problems with Online PSV test

[Guibar]

Hi everyone.

We are facing a strange problem in our plant.
We have started to calibrate ours PSV online, but since we started, we have found the same problem. We have always found the set above the right open set. We have checked the equipment in our workshop, testing first in the workshop and then, comparing with the online machine. We use the temperature factor when necessary. No problem. Then, we install the PSV back in the plant and when we gonna do the online test, we always find the relief set 10, 15 or 20% above the right set.
Did anyone face a similar problem?

Thanks

 

[don1980]

When installed, does the valve see any backpressure, or is it routed to the atm?

 

[Guibar]

there is no backpressure. the valve is atm!

 

[don1980]

There's probably a simple explanation. Check the test value (CDTP) used by the shop. Possibly they're using an incorrect value. There might not be any CDTP adjustment necessary for this valve. That just depend on your service temperature.

 

[Guibar]

The valves we are having this problem are ASME I and ASME VIII. They are tested in the shop with air and they operate with steam. I checked the pressure tested in the shop. There is no incorrect value.Is any relation between testing with air and then operate with steam?

 

[Latexman]

Is the same pressure measuring device used for both tests? Could be calibration issues.

Good luck,
Latexman

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[don1980]

For this valve to open at a significant higher pressure, when hot, doesn't make any sense to me. I think there's a piece to this puzzle that missing. Talk to the supplier and I bet they'll figure it out pretty quickly.

 

[chicopee]

Are capacity tests being done when the PSV's are air tested on the test bench and when placed back on line?

 

[ppawel]

It looks starange for me. Are you sure that you compare set (valve start to open) pressure from shop test and online test? This pressure 10% 15% or 20% higher than set pressure looks like relieving pressure which includes pressure accumulation in ptotected equipment, usualy 10% or more depends on method ASME, API...

 

[Guibar]

Hi everyone.
I understand what you guys have been saying about this issue. However, everything you have said we have tested and checked.
We made a test last week. We received a PSV from supplier , calibrated. We tested on shop and tested it online in shop, to check the acuracity. Worked just perfect. the test was performed with air.
This week we checked the valve in the plant, online, and we had the same problem, the valve opened 15% above the open set. The valve as I said is a ASME VIII, working with steam at 525°C (977 °F).

We simply don't know what is happening.

But I Appreciate the help you have given.

Regards

 

[Latexman]

Have you called the supplier and PSV manufacturer into the plant to witness the test? Let them help you too.

Good luck,
Latexman

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[sjrfc2]

What method are you using to check the setpoint (in shop and online)? Years ago I read about inaccuracies associated an aux lift assist device on a valve with a thermal lip disc. The disc seat was flexible and was designed to seat better at higher temps but it created inaccuracies regarding what the mean seating areas (at least that is the crux how I remember the inaccuracies explanation). If you're using an assist device to test online, I'd check to see if there was something impacting how you're calculating the mean seat area.