ASME VIII _ HYDROTEST PRESSUREthread794-135893

[compo]

Just to clarify my understanding, when ASME updated the code on hydrotest pressure ratio to specify the minimum requirement of 1.3 what was the logic behind selecting 1.3 as the ratio, i recognise this is the minimum requirement and also that no upper limit for hydro is advised but what precludes or not if the case may be, one from simply maintaining 1.5 as a ratio, do we have to prove or complete additional calculations to use a ratio of 1.5?

 

[vesselfab]

You should do the calcs to prove that you are not exceeding the yield limits.

1.3 is used because we are increasing the allowable stress
used in calculations, effectively increasing stress in vessel.

 

[jte]

Plagiarizing myself from thread292-109371

The 1.5x hydro was changed to 1.3x with the increased allowable stresses introduced with the 1999 Addenda. Simply put, the stress during the hydrotest remains basically the same: The allowable stresses (those which were limited by ulitmate strength) increased from Sut/4 to Sut/3.5 and correspondingly the hydrotest was changed by the same ratio. 1.5*[3.5/4]=1.3. For example, a vessel built of SA516-70 (at 500°F or lower) material would have its circ stress brought up to about 1.5*17.5 = 26.25 ksi during the hydrotest. A vessel built to the 1999 or later addenda would have its circ stress brought up to about 1.3*20 = 26 ksi.

The post-construction hydrotesting requirement is up to the AI who may request a lower than VIII-1 hydro. In theory though, a 1.3xMAWP hydro would not meet VIII-1 for a vessel built to the 1998 or earlier addenda. It would not result in the stress in the vessel which it would have seen during the shop hydro. In this case, for the example in the above paragraph, the hydrotest circ stress would be only 1.3*17.5 = 22.75 ksi.

jt