entrapped air during pressure test

[scomil]

I'm seeking good technical info regarding how entrapped air effects pressure water or pressure sewer pipelines. In particular, assuming not all air is evacuated, what could happen during a static leakage test? How will gaskets behave versus the pipe wall? Since air is compressible, will the actual pressure in the pipeline be what is showing on a pressure gauge? Any experience to share or good texts to recommend?

 

[JDJACKSON]

You can eliminate your concerns about entrapped air in your pipeline by opening high-point vents on the line until you see the water coming out. If you do not have any high-point vents it is advisable to install them.

J. D. Jackson B.S. M.E. & B.S. E.E.
TransContinental Engineering

 

[zdas04]

The biggest problem with air in the line during a pressurized test is you don't know what you're seeing. If you have an air bubble, then a smallish leak will bleed off pressure more slowly than with a liquid-full system. Pressure changes associated with temperature changes happend more slowly as well. If a leak is in a gas-full section, then you could easily miss the hissing while looking for a wet spot.

I will always invalidate a short test (less than 4 hours) that behaves like it has air in the system. I will use judgement on invalidating a 24 hour test because of air, but the decision will usually be a re-test.

The best technique I've found is for my test procedures to include a 24-48 hour soak time (where it is reasonable to add test liquid) before the test begins. This lets evolving air get vented out high-point vents during the soak period (like JDJackson says, if you don't have them, install them) and most systems can pass a test the first time.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[MortenA]

is it possible to pig the line?

Best regards

Morten

 

[scomil]

No mandrel test is required as it is a pressure line. I guess if a mandrel were to get hung up it could show drastic changes in direction at the joints (or ovalization). If this were so in the vertical direction, then air pockets could be expected. Is this what you are thinking Morten?

 

[MortenA]

not really i just though that if you could run a pig through the line (perpahps a couple in series) then it would push out air in front of the pigs.

Best regards

Morten

 

[zdas04]

Morten,
I've seen this tried with pretty poor success. There are 2 problems here: (1)any pig will have some bypassed flow and you really can't predict the mix of fluids ahead of the pig and (2) most of the problem with tests is disolved air in the liquid that evolves out during the soak period. Just filling the line is usually pretty effective at shifting the air that was in the pipe prior to the test.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[StephenA]

Pigging with the correct type of pig will usually take all the air out of the line. We have used polycoated foam pigs for 10 kilometres of 56" sewer with a differential head from inlet to lowest point of 20m without significant bypass. It does also depend on pipe lining. Is the lining cement mortar or epoxy. CML may bypass more.

Also foam pigs will negotiate quite tight bends.

Stephen A