Vacuum test vs Pressure test

[SAK9]

Guys,

I would like to know whethet we get same leakage rates if I were to test vessel/pipe under absolute vaccum and 1 bar (g) pressure.In both cases the differential pressure across the wall is the same ie 1 bar.I have been told the vacuum test is more difficult to do and would give a greater leakage rate over time.

 

[StoneCold]

SAK9
I have pressure tested vacuum piping for leaks. Just because it is easier to find the leaks when the pipe is under pressure than when it is under vacuum. However there are some leaks that leak when they are under vacuum but that do not leak when they are under pressure. The forces exerted on the joints (the most common leak points) are different when you are under vaccum vs pressure. Is the vacuum leak rate higher? Maybe. If the end service is as a vaccum line then I would say the vacuum leak rate is more important.

 

[notnats]

Vacuum leaks are VERY hard to find.

Jeff

 

[unclesyd]

We have some very large and intricate jacketed piping systems with Ring Joint flanges for vaporized Therminol that we test with a vacuum as a preliminary test, as we charge Therminol into this vacuum. We allow only 1/2# pressure drop within a one hour period. If the leak is more than 1/2# drop, we at one time used ultrasonic leak detectors but went to He at 20 psig with a portable He leak detection system that quickly finds the leak. If the vacuum holds the Therminol is charged into the system. On individual components we use He alone.
The original test procedure was to use NH3 gas with the NH3 detection sticks.