Pneumatic leak test vs. Hydrostatic leak test 1

[nasbee]

Is this correct? You can justify a leak pneumatic test at a lower pressure than a leak hydro test. Because the molecular weight of the gas is lower, thereby you are able to identify a leak faster. If so how can this relationship for a nitrogen leak test vs. a diesel leak test?

 

[TGS4]

No - it's a safety thing. Pressurized air holds SIGNIFICANTLY more energy than pressurized water. Therefore the Code writers deemed it prudent to have a lower pressure on the pneumatic test. As far as I am concerned, it has nothing to do with identifying leaks. In fact, I believe that in NBIC, one can perform a leak test with VERY low pressure air on items such as boiler tubes where hydrostatic test pressures would have a deliterious effeect.

 

[metengr]

The NBIC does not specify a pressure for pneumatic testing. This is between the owner/user and their Inspector.

 

[nasbee]

I appreciate the comments. I understand the inherent dangers of stored energy from the pneumatic test and thanks to a previous thread on this subject i was able to come up with the stored energy numbers. I guess my question was more in regards to how a fluid vs. a gas behaves through a small opening and if their molecular weight makes a difference as to how much flow rate you get out.

 

[TGS4]

I'm quite certain that molecular/atomic SIZE makes a huge difference. For example, if you were to look at standardized gasket testing, you would see that they use helium (second smallest atom). The helium leak rates will be more than, say, water or even air. However, helium leak detection requires a very sensitive "sniffer" whereas water is pretty easy to detect. You will also notice that any flanges in hydrogen service will not have insulation boxes around them because the hydrogen leaking through the gasket would accumulate in the insulation boxes and could lead to an explosion.

If it's a sensitive leak test that you're after, then a low pressure air test with soapy water (to detect bubbles from the air) is probably your best bet.