air test or nitrogen test

Date 09-28-2008 04:43 Edited 09-28-2008 04:46

Hello qaqcmpm, I don't believe you will want to consider replacing the hydrotest for one with air/nitrogen. There is a considerable amount of difference in a couple of things concerning what you are proposing. A hydrotest can be conducted with a much greater degree of safety. If you were to experience a rupture of the pipe spool there is way less chance of personal injury. When a fluid under pressure is released in an event such as a rupture it will not act in the same manner because a fluid cannot be compressed, it can still exhibit violence but not nearly as much as a gas. If you pressurize your spool with a air/nitrogen you will be compressing many times the volume of gas that it would occupy at atmospheric pressure. So as an example and NOT A REAL ONE, if you had a tank or something that had an area of 10 cubic feet and you pressurized it to 450 bar it might now contain a 1000 cubic feet of air/nitrogen before it reached 450 bar, if there was a rupture, that 1000 cubic feet of air/nitrogen would immediately try to reoccupy it's original space and volume. It would result in an immense explosion. This would be the example of why they do hydrotesting instead of using air/gas. So please don't even think about substituting the hydrotest. In other words stick with the accepted method of testing. Best regards, aevald

By CWI555

Date 09-29-2008 11:56 Edited 09-29-2008 13:39

Good post, if I may add an explaination.. Picture a balloon filled with water and one with Air. Pin prick them both. The balloon with water will just leak. The one with air will explode.
Consider this in light of the vastely larger volumes of fluid medium as found in the typical piping spool.

By js55

Date 09-29-2008 12:43

The advantage to pneumatic testing is that it can find leaks that hydro can't. It is often used as an accompanying method after hydro on some pressure vessel assemblies. The usual method is to do 1.3 (1.5) times the MAWP with hydro and then do a lesser pressure with pneumatic to minimize risk.
But I'd agree, don't replace hydro with pneumatic.

By swsweld (****)

Date 09-29-2008 18:48

B31.1-2004
137.5.1 Except for preliminary testing in accordance w/137.5.4 pneumatic testing shall not be used unless the owner specifies it or permits its use as an alternative.
you can use it if.
(A) when piping system are so designed that they cannot be filled with water
(B) when piping systems are to be used in services where traces of the testing medium cannot be tolerated

If you use it instead of hydrostatic testing 137.5.4 says not to exceed 25psig [175kPa (gage)]may be applied ....as a means of locating major leaks.

I agree with the other posters. Avoid air if at all possible.
I was nearly killed when a 150 psig air test on an 18" diameter CS pipe went bad.
It is VERY dangerous. Air continued to violently exit the pipe for several minutes as it was compressed.

I have learned to respect pressure testing whether it is air or hydro.

By G.S.Crisi (****)

Date 09-29-2008 20:07

I can't but agree to what has been said so far.
Giovanni S. Crisi
Sao Paulo - Brasil