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Thread: Hydrotest pressure limits for flanges

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    Hydrotest pressure limits for flanges

    Good day all,

    Earlier today we were instructed to conduct hydrotest on a pipework package for with ASME B16.5 Class 300 rating. Specified test pressure was 7320 kPag (1061 psig). Design code is B31.3 and flange material was A105.

    Referencing ASME B16.5 Group 2.1.1, the 100 deg F pressure rating was 740 psig. Section 2.5 of the same standard states that the maximum test pressure is 1.5 times the 100 deg F pressure rating (740 psig x 1.5 = 1110 psig). To quote the standard, "Testing at any higher pressure is the responsibility of the user, subject to the requirements of the applicable code or regulation".

    My question is, although there is a slight margin (49 psig) for safety in the specified test pressure, do you normally design pipework to that close of the maximum? I normally take 90% of the pressure rating as the absolute maximum design pressure. If the system is required to operate at higher than that, I will specify a higher pressure rating (eg, uprate the flange from Class 300 to Class 600). That way, I will have a 10% safety margin for any temperature/pressure gain that occur during hydrotest.

    I believe that it is the responsibility of the designer to take into account any temperature/pressure gain during hydrotest. An outdoor system tested in situ (project in Malaysia, average daytime ambient 33 deg C) is bound to experience some temperature and pressure gain.

    Would welcome any advise for future reference.
    "The client is always right - except when he is wrong":)

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  3. #2

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    Code itself covers a lot of safety margin, so don't worry for your case.

    If you still worry then check and ensure that stresses induced in the flanges remains within 90% of the yield strength.

    Overdesign many times involve a lots of costs which can be eliminated by taking a good decision based on good supporting calculation.

    Thanks.

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