Allowable pressure drop when testing valves per API 6D 1

[Mohsen579]

I am witnessing class 600 valve hydrotests (per API 6D) at around 2500 psi (customer required pressure) at the manufacturer shops. The valve size is normally between 4 to 12 inches so the test duration is between 2 to 15 minutes. There is a 2-10 psi pressure drop most of the times with no leakage. I was wondering if anyone is aware of any API 6D acceptable tolerance for such pressure drops. Thank you all.

 

[SPDL310]

It is my understanding that the acceptance criteria for a 6D shell test is no visible leakage with the minimum pressure maintained for the duration of the test. As long as the pressure drop did not fall below the minimum pressure the test should be acceptable. There are no listed allowances within 6D for pressure drop during the shell test. If the pressure fell below the minimum required pressure you would need to apply pressure above the minimum required pressure, and begin a new hold period. You could have possible leakage and containment behind a secondary seal or some other blead through that is not readily detectable.

 

[LittleInch]

That's probably about two drops...

Or less than 1C change in water temp.

Is there any difference in temp between the casting and the water?

Try equalising the temperature for 30 mins or longer before the test?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Mohsen579]

Thank you both for quick feedback

@SPDL310, I didn't find anything addressing this question on the 6D either. So assuming we start at 2551 psi and dropping to 2501 (still above the minimum required) would it be acceptable despite the 50psi drop?

@LittleInch, the valve remains under pressure to stabilize for at least 20 minutes prior to the test start. The testing fluid tank, pump, and the valve are all in a 12 feet/4m distance and there is not much of temperature increase/decrease at the shop.

 

[jmec87]

The acceptance criteria is no visible leakage and maintaining the minimum pressure for the duration of the test, as stated by SPDL310. Check 6D yourself to be sure (10.1.3, 10.3.1, and 10.4.1 - note that there is an exception for metal-seated valves to allow some leakage).

See the interpretation from API at the bottom of the first page at the below link:

https://mycommittees.api.org/standards/techinterp/epequip/Shared Documents/6D.pdf

Question: I have a vendor, who believes that pressurizing the valve above this recommended pressure, testing to the table 6 duration, and having pressure drop is acceptable as long as it doesn't go below the 1.1 times the rating pressure. Acceptance criteria: no visible detectable leakage for the duration of the test at test pressure. It is my position that the pressure must be stabilized at pressure, then tested with no loss of pressure. That overpressurization and pressure loss is unacceptable, that pressure must remain stable and not drop. Bypass or unstabilized pressure is a failure of the test. Is this correct on my part?

Answer: No, the acceptance criteria for soft seated valves is no visible detectable leakage.

This interpretation was for the 24th edition, but the relevant requirements have not changed in the 25th edition.

 

[LittleInch]

"Not much" temp difference is enough to cause this sort of pressure drop.

The two need to be identical.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Mohsen579]

@jmec87 thank you very much for detailed answer and addressing the exact parts of API 6D. I am now confident about my disposition on the test. Really appreciate your advice.

@LittleInch, thank a lot for the advice. I will work with the vendor to see the possibility of insulating the lines, so we have less possible temperature difference.