Pressure testing of PE lined Carbon Steel Spools with Nitrogen Gas 1

[ram0608]

Dear Friends,

In a project, we have done Polyethylene Pipe Lining for carbon steel pipes and sucessfully tested for hydrotest at 74.10 Bar (1.5 x MAWP)with potable water. Now, During commissioning customer is asking to do the pressure test for the line with Nitrogen gas at 34.0 bar?

We would like to know can we test the above PE Lined carbon steel spools with nitrogen gas at 34.0 Bar? If so, what will be the duration of test? Is there will not be any effect of liner integrity? Any ref. Standard in this regard?
Any body can help me on this?

Thanks in advance

 

[patprimmer]

I can't really help other than to say, if it stands 74 bar hydraulic, what's the point of half that gas unless to test permeability which seems strange.

One caution is that a failure with gas pressure is a lot more violent than a failure with a liquid inside so take due care.

Regards
Pat
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[btrueblood]

Sorry not to read this sooner, it just seemed out of place in this forum. I'd suggest, past Pat's comments and mine below, that you further research pneumatic pressure testing. And maybe post in the Pipelines forum on this site, where people doing such testing more regularly hang out.

An issue for any polymer in pressurized gas testing is the possibility of explosive decompression damage. The idea is that polymers are permeable, and will dissolve/absorb a certain amount of gas under pressure. A sudden release of external pressure causes the absorbed gas to suddenly expand, causing damage to the polymer (this is more typically seen in elastomers).

A related concern for linings in pipe is the possibility for gas to permeate through the liner into a pocket (delamination?) between the liner and the pipe. Again, a sudden (or even just rapid, i.e. normal venting) release of pressure and no hole large enough for the pocket to vent, means the pocket expands and buckles the liner inwards.

Both damage mechanisms are caused by the test and operations methods, and are not necessarily a failure of the device being tested. Both damage mechanisms can be controlled and probably avoided by careful test procedures avoiding rapid pressure decay.

 

[patprimmer]

BT

I never considered the mode of failure you suggest, but to test for that, I would think the gas the pipe is being exposed would be a lot better test medium than nitrogen.

I have seen far to many in field failures because hydrolysis resistance tests where done with distilled water and the product was used in contact with tap water.

Every gas will have a different level of permeability to the PE as is demonstrated by the need for multiple barrier layers in food packaging.

eg PE layer for moisture barrier and nylon layer for oxygen barrier.

Regards
Pat
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