PVElite allows users to check-on 90% of material yield strength during Hydrotest .

Extracted from HELP in PVELite :

Hydrotest allowable is 90 percent of the Ambient Yield Stress

But we only found ASME VIII-2 stated max. allowable stress during HT is 90% of Yield Strength. On VIII-1, UG-99 only specified in case of over hydrotest than desired value case, Authorized Inspector has to judge accept/reject based on visual check of any permanent distortion. No max. value or 90% of Yield Strength as above was stated.

Anybody can share opinion, where this 90% of Yield Strength for Div. 1 vessel came from?

S K Tan

Value of 90% Yield strength comes from Section II D - Mandatory appendix.

Thanks for the sharing, Mech Engg.

The relevant appendix is ASME's concept on derivation of higher material allowable stress. It is applicable on certain material and advisable to check suitabality of application. It deploys higher material capabality than greater of (TS/3.5; 2/3 of YS) but still limited to 90% of YS.

We already noted the same statement on note G5, Table 1A. We can see the good example on Austenitic Steel Grade 316L for example. Higher material stress shell and head using SA240 Gr 316L is acceptable but SA182 F 316L will not be suitable for flanges in order to prohibit leakage due to excessive distortion. Both of these materials have standard and higher material stress in Part IID.

May be as general Pressure Vessel practice, 90% of YS is adopted as limit in design inclusive overstress-check during Hydrotest. But apart from design aspect, UG99 still did not restrict un-intention overshoot of Hydrotest to comply < 90% YS on back-flush calculation but judgement solely based on significant permanent distortion. Actual yield stress of material usually is higher than value stipulated on ASME II-D. The over HT vessel may not deform yet even exceeding 90% of theoritical YS occurs.

If really overshoot 90% of YS during back-flush calculation but no significant permanent distortion occur, the VIII-1 vessel is still acceptable to ASME Authorized Inspector.

Other opinions are welcome.

I agree that the Div. 1 code does not specifically provide the allowable to use for the hydrotest case. It cautions against visible and permanent deformations.

TEMA std. also includes 90 % of yield as a suggestion.

So, 90% of yield is typically used in industry. Theoretically, the permanent deformation will start as you exceeded the yield point.

Because allowable stress of fracture sensitive steel materials are coincidentally governed by fraction of tensile (1/3, 1/3.5) rather than 2/3 yield, and yield is typically ca 55% of tensile, hydrotest primary stresses need not normally be near 90% of yield. The recommended safe temperature for resistance to brittle fracture, which goes with the lower expected hydrotest stress level, is 30°F above the MDMT for material curve and thickness. I would suggest considering whether this is adequate if going to 90%. When going to 90% of yield specifically with a fracture sensitive steel, you might be way out of bounds in fracture safety. If you do the fracture mechanics for plausible flaw sizes, you might find the material is incapable of the required toughness, unless very high temperatures relative to the curve are used. The 90% provision was first used with Div 1 vessels to allow large field fabricated low pressure pipe stills and fractionators to be tested, a once in a lifetime event. It was a big deal event with a lot of sctrutiny and great observation distance during the test, I assure you. I would be very cautious about pressure testing just any erected vessel maybe 25 to 50 years old at the same 90% stress level. Div 2 more specifically governs maximum test stress but has a whole range of supplimentary reinforcement and other calculations, and stress limitations for other than primary stress, that goes with 90% allowance. Just allowing 90% for primary stress is an abridged story. If you do it, consider doing it to Div 2 even if a div 1 vessel and consider defects like fatigue cracks and corrosion that may have occurred in an old existing vessel.