4 replies to this topic

[omnibus]

Can somebody give me the reference of any code or standard specifying the pressure limit for pneumatic testing of the gas lines? As per the ITB the line should be checked at 110% of the operating pressure but below 90% of the safety valve set pressure. Operating pressure is 9 Kg/cm2g therefore thye line should be leak tested at 9.9 Kg/cm2g but the available utility N2 pressure is 7.5 Kg/cm2g.

In my operational experience for leak checking we have never used air/ N2 above utility pressure.
Is there a code which specifies or restricts the pressure for pneumatic leak testing? If so please give me the reference.

Thanks as always are in advance.

I Think I made a mistake of posting the above on safety folrum and got the following reply.

'Buy portable compressor.'


therfore, let me rephrase the question.

N2 Line preoperation leak test is to be done. Is it safe enough to do it at 7.5 barg ( operating pressure 9.0 kg) or it should be insisted that the test be carried out at 9.9 bar(110% of operating pressure)?

Any straight answers with reference?

[Art Montemayor]

Omnibus:

You’re doing the correct thing. Common professional courtesy dictates that on an Engineering Forum you should either get no reply to your request or a sensible, logical reply to your direct query.

I don’t know what the ITB stands for and what it regulates – assuming it is a code or standard. However, I have been in your operational situation before and what I have done is similar – although not exactly the same as you.

As I understand it, you are trying to leak-test a gas line that you want to place into service. This gas line normally operates at 9 bargs and you only have a utility supply of 7.5 barg nitrogen to do the leak test with. ITB dictates that the line should be leak-tested at 9.9 barg, but you can’t achieve that high a pressure with your nitrogen supply.

You haven’t stated what gas or fluid the line normally contains when it is operating. I’m going to assume it is a fuel gas or a fluid that is hazardous, should it leak. That’s why you are leak-testing the line prior to putting it into service. If that is the case, then I would obtain a conventional 7 m3, high pressure (145 barg) cylinder of nitrogen with a regulator and use it to boost the 7.5 barg utility pressure applied to the line and raise the leak test to the desired 9.9 barg pressure. This should not be a difficult or complicated procedure to apply. I recommend you do this even if you have never previously tested your lines for leaks with a pressure greater than your utility pressure (7.5 barg) because:

• You really haven’t complied with a true objective of a leak test unless you achieve at least the same operating pressure level plus a contingency for normal excursions;
• The operation of leak testing at 10% over the operating level is a relatively benign and safe procedure; a prior hydrostatic test on the same line would have involved probably a 150% increase in pressure;
• You have very little to worry about due to pressure testing at the levels you are working at. I don’t know the size of your test piping, but an operational pressure of 9 barg is rather low and falls well below the rating for nominal schedule 40 pipe in the usual sizes of 1” through possibly 30”, so I don’t think you have to worry about pipe failure during the leak test. However, I would still make sure that I knew exactly the maximum pressure rating of the test pipe and its physical condition – both internally and externally – before proceeding with a pressurization. This, I presume, you will routinely do.
• As I stated above, I presume that you are going to the trouble of a leak test because you cannot accept the risk of a future leak. Therefore, applying the 110% leak test rule is common sense in the case and the trouble of obtaining a cylinder of HP nitrogen and a regulator, in my opinion, is well justified.

I hope this experience helps you out.

[omnibus]

Thanks a million! Art, for your helpfull and thorough explaination.

The situation is that one of the clients has asked for a waiver of the condition menmtioned in their Invitation To Bid ( Project Execution Instructions). The condition says that the system or lines to be tested at 110% of the operating pressue prior to putting it in service. The gas lines( Gas is N2, which falls under categoryD in ASMEB31.3) which have been pneumatically tested( at 110% of the design pressure, in lieu of Hydrotest at 150%) for structural integrity,in section, are to be leak tested after reinstatement.

The waiver request states that they be allowed to leak test the lines at utility pressure as their Project Execution Instructions also state that' in case the operating pressure is above the available utility pressure, the test presssure will be determined based on the criticality of the service". Also they say that arranging for the bottles and further pressurization will affect the project schedule adversely.

Here is the horn of dilemma. There is no code which deals with this 'operational tightness test'

Since N2 is non reactive and non toxic gas and can cause harm only in enclosed places as a result of the leak by reducing O2 content in the air, I have specified that they can carry out the test at utility pressure in the areas where the line is in the open and to do the test in the sections which pass through enclosed areas at 9.9 kg/cm2g.

I mope you will share with me some more insights on this issue.

Thanks again are in advance.

[gvdlans]

Since N2 is non reactive and non toxic gas and can cause harm only in enclosed places as a result of the leak by reducing O2 content in the air,[...]

I do not fully agree with you, Nitrogen can also be dangerous just outside enclosed spaces, see http://www.csb.gov/s...gen-6-11-03.pdf and http://www.csb.gov/i...amp;page=index# , Valero Refinery accident.

For more information on leak testing, following document may be of interest: http://www.cpchem.co...;lib=CPC-Portal This document is intended for leak testing of polyethylene pipes, but procedures are quite similar for other pipes. Here they recommend to do the initial service leak testing at operating pressure. I can follow Art's recommendation to add 10% to the operating pressure for 'normal' pressure fluctuations.

[Satyajit]

Hi,
I would like to add following in this issue.
Although leak test at recommended pressure level is important ; what is more important in such pneumatic test is Pressre detention test: Hold the pressure for 4 hours and see that pressure drop does not exceed 0,5 bar in 4 hours. This is very important for Hydrocarbon services, chemicals and toxic ,flammable gas services.
Waiving of ITB regulations is a matter of decision for cost point of view ( as pressurising the line by mobile compressor or N2 bottle is always possible), but accepting a leaky system is very unsafe and may be devastating.

Bye.
Satyajit