Hydrostatic Test and Air Entrapement

[Filippo54US]

Could anybody explain why in the hydrostatic test of a pressure vessel the allowed percentage of air in volume in the vessel is not allowed to be more than 0.5%-1%?

What would happen if the pressure vessel contains 90% water and 10% air?

Many thanks

 

[Francis M]

Air compresses a lot more than water. IF there were a catastrophic failure of the cylinder under test with gas there would be a big shrapnel causing explosion. Using water mitigates that problem to a large extent. Also since air and water compress differently a cylinder with a 90/10% water to air ratio in it would throw off the accuracy of the test.

 

[stewartcs]

Also since air and water compress differently a cylinder with a 90/10% water to air ratio in it would throw off the accuracy of the test.

Pressure is pressure.

Air in the liquid would only require that the hydrostatic pressure test have a longer settling time before an accurate reading could be obtained.

Additionaly, the air can leak past the seals easier and possibly fail the test.

CS

 

[Mech_Engineer]

Air compresses a lot more than water. IF there were a catastrophic failure of the cylinder under test with gas there would be a big shrapnel causing explosion. Using water mitigates that problem to a large extent. Also since air and water compress differently a cylinder with a 90/10% water to air ratio in it would throw off the accuracy of the test.

Pressure is pressure.

Air in the liquid would only require that the hydrostatic pressure test have a longer settling time before an accurate reading could be obtained.

The main difference between pressurized gas and pressurized liquid in a tank is the stored energy. Pressurized gas holds a lot more potential energy than a pressurized liquid, and a failure in a gas pressure vessel will be far more chatastrophic than that of a liquid vessel.

 

[FredGarvin]

There are multiple reasons, but it is difficult to say what was at the forefront of the authors when the spec was authored. It could be possible that it pertains to a very particular application and thus the test must be done in that fashion. Does it say in the spec anywhere? Usually, if there is a safety concern, there will be applicable notes stating safety concerns. What spec are you using? Chances are that the safety factor is what is driving it, but understand that there are other possibilities that only someone very familiar with the spec may be aware of.

 

[stewartcs]

Just to clarify, I wasn't suggesting that there isn't a danger involved with air in the liquid in regards to the potential energy. I was pointing out that the compressibility of air wouldn't throw the test off since x psi of air is equal to x psi of liquid in a hydrostatic test.

CS

 

[Filippo54US]

Thanks to all.

The many specifications for hydrostatic tests of pipelines that I have read limit the air content to 0.5-1.0%. My understanding is that the issue is test accuracy, not safety, even I understand the safety concerns mentioned by some of the people that replied to my poste.

I was thinking that the air content limit was related to the need for a much longer stabilization period (as it has been suggested) or the air would make the pressure more susceptible to temperature variations or the air dissolving in the water could cause changes in the volume of the water (!?), but I really don't know.