Overpressured pipe during Hydrotest.

[CalgarySS]

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)

 

[unclesyd]

I will defer to some of the more up to date code orientated members for the definitive answer, but you are OK as long as nothing deformed. We routinely tested a process piping system a 2X MAWP and never saw any harm.
This happens all the time on hydrotests.

 

[metengr]

I concur with unclesyd. Remember, the multiplier factor (1.3, 1.5) for hydrostatic testing in ASME Codes and Standards is a minimum requirement provided no permanent deformation or exceeding the yield strength (in B31.3 see paragraph 345.2.1) occurs during the hydrostatic test. Using the test pressure you stated, determine if the yield strength of the pipe was exceeded – I doubt that it was.

 

[zdas04]

I'm with the group that you're probably ok. I would calculate the hoop stress and see if you exceeded SMYS. If you did then you still might be ok as long as careful measurements and inspection don't show a serious deformation. Below the yield point steel is pretty elastic.

David

David Simpson, PE
MuleShoe Engineering

http://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

 

[Ashereng]

If MAWP is 1440 psi, and the PSV didn't go until 2300 psi, shouldn't that be a concern also? Shouldn't the pumps' PSV been set to 1900 psi or something?

Or, am I missing something?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[QC1971]

Typically the area of concern when overpressuring occurs is the components attached to the piping system such as the flanges. This is where I usually start inspecting for possible damage.

 

[stanweld]

If hydrotesting a boiler per ASME I PG-99.1, "No part of the boiler shall be subjected to general membrane stress greater than 90% of its Yield Strength at the test temperature"

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.

 

[JohnBreen]

I am also with the consensus.

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

 

[zdas04]

stanweld,
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

http://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

 

[timbones]

I would say one of the key questions is, what code is the piping designed to?

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.

 

[timbones]

Just noticed your name has "Calgary" in it, which I presume puts your plant in Alberta? If you really want a definative answer on your question, check with ABSA, as they will be responsible for certifying your piping and pressure vessels.

 

[stanweld]

zdas04,
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.

 

[zdas04]

Funny, in my college Materials text the first test listed under NDT is "Hydrostatic". If the intent of the hydro was other than non-destructive you'd do it before you spent a bunch of time and money welding it into fabrications and pipelines, wouldn't you?

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