Allowable stress during testing, which?

[carletes]

Hello all,

I have a question, perhaps too easy , but whose answer I can't find.
I am calculating a little pressure vessel under internal presssure, according to ASME Sec. VIII, Div.1. I calculate the required thickness according to the design pressure, using the maximum allowable stress in Sec. II, but my question is, do I have to check the vessel's strength during hydrostatic test? Some if my colleages teel me that I have to check it, using as design pressure the testing pressure but the allowable stress should be 90% of the yield strength of the material. Is it right? I can not find that figures in ASME SEc. VIII. Perhaps is it just a "good practice" rule but is not included in ASME Sec.VIII? I have found something about it in ASME Sec. I.

Thanks a lot for any help.

Best regards,


Carletes

 

[Tankman650]

It looks like you are talking about hydro per UG-99(c) which is a special case and not normally done. Most vessels are tested per per UG-99(b).

I have never seen a 90% of yeild rule. On first thought I can not see how the test stresses can come close to yeild if the test pressure is 1.3 times design pressure. To increase the factor above 1.3 requires AI/customer approval.

 

[carletes]

Thanks Tankman650 for your prompt response!!

I think I see what you say, but, if I am not wrong the allowable stress coulb be 2/3 of yield strength (in some materials), so, in that case if the testing pressure was, aprox., 1.51 times the design pressure (3/2 = 1.5), the yield strength would be reached. Then, If I test the tank at 1.5 times the design pressure (in my case it is so because of client specifications), the yield strength would be reached so I have been said to increase the thickness of the tank to guarantee that during hydrostatic test at 1.5 tmes design pressure yield strength is not reached. Is it logical? Is there something I am not taking into account?

Thank you again,


Carletes

 

[prex]

carletes
that rule is actually specified in ASME VIII Div.2: the limit of 0.9Y applies to general primary membrane stresses during hydrotest; a limit of 1.35Y for primary bending stresses is also provided (this would be relevant for flat covers).
The advice you received is correct: of course if the circumferential stress in the shell due to hydro pressure goes beyond yield, your vessel will become a waste.
Duration of hydrotest is not critical, as far as the stresses are concerned: if the stresses are higher than yield, a few seconds will suffice to brake the vessel. prex

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[PPVE]

It is only true to limit the membrane stress at test to yield for materials that have a high yield stress compared to their ultimate stress and a low value of elongation at failure, ie., brittle materials. (However, beware stress concentrations at nozzles, etc.)

If you have good knowledge of your material, and it is ductile, you can go beyond the miniumum quoted yield during the test (need strain gauges to monitor it), but if you don't have such knowledge, and no strain gauges, stick to the code rules.

PPVE