Field Pressure/Leak Testing of Shop-Hydro'd Piping Systems

[jxr]

There is some discussion going on in our organisation regarding the field pneumatic pressure/leak-tightness testing requirements post assembly of shop hydrotested piping spools. In this instance, the service is fuel-gas, on an offshore gas production platform.

I have read through ASME B31.3 (the applicable code in this instance) and others (ASME PCC-1 & PCC-2 etc).

I hold a belief, from first-principles origin, that the assembled piping system should be leak tested to design pressure. I hold this conviction for several reasons:

1. The gross structural integrity of the piping spools has already been proved during the shop hydrotest, hence risk of explosive failure is negligible or zero.

2. I think it would be quite possible for a flange joint assembly to be field-assembled sufficiently tight to withstand a low pressure leak test (say 35% design pressure, per PCC-2), but not tight enough to necessarily be leak-tight at full design pressure. At design pressure, internal pressure could load insufficiently preloaded flange bolts and unload the gasket, leading to leakage.

3. If design pressure is intended to be a credible pressure-temperature combination with the process fluid, then there should not be any fear of taking the system to this operating point with an inert test gas.

As mentioned, I have searched reasonably extensively, but cannot find any code or standard to support this view. Furthermore, the ASME codes seem to hold to the view that low-pressure sensitive leak tests are adequate proof of joint leak-tightness.

What is the general consensus and practical experience of field installation of prefabricated piping spools?

 

[BigInch]

Leak testing to design pressure is not a "leak test". They are conducted at well below design pressure. A lower pressure test will adequately expose any joint tightness problems that once repaired correctly, will suffice for full charges of the product.

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[atmengr]

We usually leak test field assembled piping in hydrocarbon service to 100% of the operating pressure. As BigInch says, this will reveal any gasket (joint) leaks, and after all, isn't that the only thing left to test? Obviously you need to witness the shop hydro or review write-ups of an approved witness to ensure the leak test is all that is necessary.

The leak test could be accomplished with water, with pressure monitoring and visual inspection to ensure integrity. Be careful about drying the piping out though, especially if it will carry high temp process. You can also perform a pneumatic test as you mentioned, and use snoop or soapy water for visual inspection at each joint.

 

[aironyx]

is there anybody that can tell me if a pnumatic test at 12 bar it's not feasable on site.
I have form the Line List that the Testing Pressure (air) is 12 bar for Instrument Air & Utility Air.
What to do?
Do I have to propose something else? Which kind of test (hydrotest or leak test? The plant it's under Offshore regulation.

Another question.
Is it true that the demineralized water for Stainless Steel testing if stored in a Carbon Steel tank can be "contamined" and not used?

Thanks for your help