Number of hydrotest a component can withstand (ASME) 2

[coolniks37]

I have a pipe, which after manufacturing, got hydrotested at 1.5 times design pressure as per ASME
Then it was erected, with the total piping and got hydrotested at site at 1.5 times again.
Now due to some modification, this pipe was modified. Again it is shop and site hydrotested?

My question is what happens to the metallurgy and stres,when u do multiple hydrotest.
Is there any limit on maximum times, a component can be hydrotested?
is it safe to do repeated hydrotests?

 

[gr2vessels]

The hydrotest is just an 'extension' to the normal service conditions of this pipe. As long as you don't get near the yield limit, you may test as many times you need, because no metalurgy change or excessive stress should happen to the pipe. When significant pipe modifications occur, you should re-hydrotest the pipe if feasible (most of the time is not).
Cheers,
gr2vessels

 

[coolniks37]

Hi..
how can you say that "hydrotest" is an extension to the normal service conditions of pipe.
How we will get near yield limit, when we do repeated hydrotest.
Could you pls elaborate the theory?

 

[moltenmetal]

A properly designed hydrotest puts no component into yield, so there's your reason! The safe allowable stresses and hydrotest ratio are selected based on the specified minimum yield of the material in question to avoid putting components into yield. There is a check required when using high stress ratios for pipe that is in service at high temperature, and the test pressure is de-rated to avoid putting any component into yield.

 

[TGS4]

I agree with moltenmetal's sentiment, but will slightly modify it.

A properly designed hydrotest puts no component into through-thickness yielding - however these is a small amount of yielding that will occur at local features. This yielding is beneficial as it can lead to a residual compressive stress on the inside surface. Any subsequent hydrotest would need to exceed the original hydrotest pressure to generate further yielding. If no yielding occurs in subsequent hydrotests, then the metal behaves linear-elastically throughout, and there is no cumulative damage.

There is a low-cycle fatigue limit, but you would have to hydrotest almost 1000 times before it might become an issue. You're not planning on doing that, right?

 

[coolniks37]

thank you molten metal and tgs4..
i am not much familiar into material loading..
wer can i read about the effect of hydrotest..? any reference or code cases?

moltenmetal, could you please explain the selection of safe allowable stress and hydrotest ratio, if possible through an example
TGS4, pls explain more about the low cycle fatigue situation.. and you are right, i am not going for 1000.. LOL.. That was superb

 

[gr2vessels]

Yep, you are right. I tried to use a language familiar to anyone else, I'm sorry. Again, you can hydrotest to the calculated hydrotest pressure every single day of your life, without damage to the equipment. Simple enough?
Cheers,
gr2vessels

 

[TGS4]

For fatigue, please see ASME Section VIII, Division 2, Part 5, Section 5.5. The curves are in Part 3 of the same book.

 

[moltenmetal]

SAS is selected to be no more than 2/3 of yield, and test pressure is 1.3x MAWP. Clear enough?

 

[jte]

Yeah, but the OP is referring to "piping" and mentioning a 1.5x multiplier so we may be looking at a general membrane stress of 2/3 yield x 1.5 or... yield. Or beyond, if adjusting for temperature. Depends on the CA. VIII Div. 2 keeps you to no more than 95% SMYS; in other cases a 90% yield criteria is imposed. Of course, as TGS4 mentioned above, certain areas of real geometries will easily exceed yield on a good hydro.

coolnicks37- Sometimes you just need a mentor or pay for a consultant. Eng-Tips is a great forum, but it is not a substitute for an engineering education, mentoring, and experience. This seems to be one of those times.

 

[coolniks37]

Thank you moltenmetal and jte..
Thank you for your time and answers.. Really appreciate all people and their answers..


Some more info which i got from internet is attached..

http://pipelineandgasjournal.com/pipeline-hydro-test-pressure-determination

 

[LittleInch]

That was going to be one of my questions to you, i.e. "got hydrotested at 1.5 times design pressure as per ASME" ASME what?? 31.3, 31.4 and 31.8, not to mentions all the other B 31's all refer to "pipe" or "pipeline" and trust me I know the difference, but you need to be specific as they are all different. As this is in the boiler forum, I assumed you meant B 31.3, but now you quote a pipeline reference. 1.5 times for a pipeline built to a 0.72 design factor results in a stress of 1.125 SMYS... Now that would start to strain your weakest link a bit more...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[weldstan]

coolniks37,
As LittleInch stated, pressure testing of cross country pipelines is quite different than that imposed by ASME B31.1 or B31.3. An 80% design criteria per B31.8 requires a pressure to impose a load in hoop at 100% of the SMYS, minimum; at 72% design, 90% of the SMYS. Portions of the line will experience stress greater tha 100% of the SMYS based on elevation differences. Be specific, is the pipe in question designed and constructed to B31.3, B31.1, B31.4, B31.8 or other?

 

[DSB123]

Pressure testing is basically a monotomic overload which is basically a form of stress relief for the component. Any high residual stresses due to welding or forming are re-distributed by the pressure overload such as to result in compressive stresses at any residual stress areas which are around yield. Residual welding stresses, where stress relief has not been applied, can approach yield stress levels. The monotomic overload provided during pressure testing is beneficial is stress re-distribution.