Overpressured pipe during Hydrotest.
Overpressured pipe during Hydrotest.
(OP)
Just a quick question.
During a Hydrostatic test, A worker closed a valve and the piping briefly saw a pressure of 2300-2400 psi before the PSV on the pump released.The piping Mawp is 1440psi (Test pressure of 2160psi)
I cannot find the overpressurization maximum before pipe must be condemned. ( I assume that 10% is allowed but would like confirmation or a link to the appropriate code that covers overpressurization)
During a Hydrostatic test, A worker closed a valve and the piping briefly saw a pressure of 2300-2400 psi before the PSV on the pump released.The piping Mawp is 1440psi (Test pressure of 2160psi)
I cannot find the overpressurization maximum before pipe must be condemned. ( I assume that 10% is allowed but would like confirmation or a link to the appropriate code that covers overpressurization)





RE: Overpressured pipe during Hydrotest.
This happens all the time on hydrotests.
RE: Overpressured pipe during Hydrotest.
RE: Overpressured pipe during Hydrotest.
David
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
RE: Overpressured pipe during Hydrotest.
Or, am I missing something?
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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RE: Overpressured pipe during Hydrotest.
RE: Overpressured pipe during Hydrotest.
Hydrotesting of pipelines is often done at 90% of SMYS and up to 105% of the SMYS at lowest elevation.
Do the calculations but you can rest assured that your 10-11% over-pressurization did not impair your pipe.
RE: Overpressured pipe during Hydrotest.
Another way of looking at it is that if there was some plastic yielding during the hydrostatic testing and the pipe did not leak, deform or fail, the system will likely have a resulting residual stress in the opposite direction of the internal operating pressures stresses - so it will have additional fatigue life.
With very high pressure piping systems the methodology of "autofrettage" is commonly used and this creates enough radial stress to yield the inside diameter of the pipe. This produces a beneficial compressive residual tangential (hoop) stress at the inner bore of the tube and hence permits a significant enhancement of the fatigue resistance against pulsating internal pressure.
Regards, John
RE: Overpressured pipe during Hydrotest.
My understanding of the definition of SMYS is that it is the stress where the pipe can be expected to yield. I've never heard of a non-destructive test that was done anywhere close to the yeild point.
ASME B31.8 does not provide an explicit maximum hoop stress for a hydrostatic test, but language in the standard (see 841.322) indicates special proceedures are required if operating pressure results in hoop stresses greater than 72% of SMYS. I've never run into an engineer that was willing to test pipe to stresses above 75% of SMYS. I guess I've led an sheltered life.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
RE: Overpressured pipe during Hydrotest.
If it is B31.3, which seems likely, you are probably fine. I haven't got a copy of the code handy, but if I recall correctly, allowable stress for this code is about 50% of SMYS. With a hydrotest of 1.5x MAWP that gets you to 75% of SMYS. You have gone to about 1.6x MAWP which ends up around 80% of SMYS, which is probably plenty fine (but you should still check for deformation).
Contrast this with a code like B31.8, which under certain circumstances, like pipelines in remote areas, allows for very aggresive designs up to 72% of SMYS. Hydrotest requirements are lower for this code at 1.25x MAWP, but that gets you to 90% of SMYS on a hydrotest (again I am recalling from memory as I don't have the code with me).
Ultimately, it's best to check with your local pressure code certifying body, as they have rules on what is acceptable and what is not in your jurisdiction.
Incidentally, if you piping MAWP is 1440 psi, having your PSV set at 2300 psi is totally unacceptable. MAWP is "Maximum Allowable Working Pressure", not "allowable working pressure that can be exceeded sometimes during plant transients". Your PSV should be set near 1440 psi at the most.
RE: Overpressured pipe during Hydrotest.
RE: Overpressured pipe during Hydrotest.
The SMYS is the "Specified Minimum Yield Strength" of the material Specification. Most often the actual Y.S. is > than the SMYS.
The minimum hydrotest per 841.32 of B31.8 is not necessarily a non-destructibve test and must provide hoop stresses > 90% of the SMYS per 841.322 (a). Fatigue cracks at the toe of the longitudinal seam weld often occur during rail transport of SAW welded line pipe; "penetrator" type flaws/cracks often occur in ERW pipe. One of the purposes of the hydrotest, especially when operating near to or greater than 70% of the SMYS, is to expose these defects by leakage or rupture. During my tenure in the gas utility industry, nearly every hydrotest for large diameter welded API 5LX-60 or greater strength pipelines operating at 70% of the SMYS had at least one pipe fail during testing; better to fail during the test than in operation.
RE: Overpressured pipe during Hydrotest.
I've also seen pipe fail in test (not nearly as often as you, but I've seen it). It is definately better to have it fail in test than in service, and that is the reason we test. But a test to beyond the point where the manufacturer says a properly manufactured pipe might yield seems irresponsible to me--plastic deformation of the pipe would fail the test in a range where the manufacturer has told you that it might deform, there is no one to blame but yourself at that point.
David