Pneumatic testing

[Downhand]

When doing a pneumatic test, what is the purpose of doing the leak test at test pressure and then dropping the pressure down to design pressure and doing another leak test. One would think that if the piping system held at the higher test pressure that dropping down to design pressure and leak testing again would be redundant. This is typically what engineers call for. Any thoughts?

 

[israelkk]

I can think of two reasons:
1. When you pressure to test pressure some structural distortion may occur which may affect the test at design pressure.
2. Some systems are designed such that the pressure assist the sealing, therefore, in lower pressure the leak may be higher than at higher pressure.

 

[Duwe6]

Misguided procedure to keep from killing Inspectors. Pneumatics are usually lethal, it a major failure occurs while you are inspecting properly [closely]. First you pressurize to Test Press, and hold 10 minutes [while hiding behind somethisn sturdy. *Then* you bleed the pressure down to Design, and start 'soaping' the welds. Additionally, I resoap at 10 to 20-psi, after Design Press. Reason is that I have seen small, round holes that would blow the test solution away and never form a bubble at Design Press, but would bubble nicely at 10-20 psi.

 

[tab1999]

Downhand,

I'm not sure what Code or Standard you are testing to.
ASME B31.3 clearly states the procedure under 345.5. I see where it addresses achieving the test pressure incrementally and holding to relieve the stresses and strains at each increment.
Then reduce to design, "before" examining for leakage.
Pneumatic testing harnesses a tremendous stored energy that just doesn't release like a pinhole in a pipe being hydro tested. It reaps havoc.
If the redundancy saves lives (Even One!), its well worth the effort.


" There is no Shortcut to anyplace worth going "