allowable stress during hydrostatic test 4

[Dennysurf]

design data:
- applicable design code ASME VIII Div 1
- hydrotest pressure acc to UG 99 and PED

The design calculations (allowable stresses according to ASME II part D) indicates a Maximum Allowable Pressure (new and cold) lower than the applicable hydrostatic test pressure.

Is it accapbable to calculate with higher stresses for the hydrostatic test? Do somebody know where I can find this instruction in the ASME code?

Thanx,
Denny

 

[Dennysurf]

Thank you for the clear explanation and your quick reply.

With 90% of the yield the hydrotest-calculation is suitable for this vessel. You wrote that it's common to use this 90%. Does this mean it isn't writen in the ASME code?

 

[TomBarsh]

ASME Section VIII Division 1 does not specify any acceptance criteria for the hydrotest, such as allowable stresses, etc, (other than that the inspector may reject the test if there is "visible permanent distortion" as per UG-99(d)).

However, ASME Section VIII Division 2 does limit the stresses during test. In this code the membrane stress is limited to 90% of yield (I don't know offhand if this still applies in the new 2007 Edition, which has been entirely rewritten); higher stresses are permitted for local membrane plus bending (at structural discontinuities, such as cone-cylinder junctions, nozzle openings, etc).

Some vessel owners or fabricators will limit the stress at test for Div 1 vessel to the same % of yield as permitted by Div 2. This protects their investment by removing the danger of that visible permanent distortion.

 

[gr2vessels]

Hi Denny,
Please note that the ASME I states in the PG-99.1 that "No part of the boiler shall be subjected to a general membrane stress greater than 90% of its yield strength (0.2% offset) at test temperature."
This concept has been extended to ASME VIII with the limits applied on Div 2, Clause 4.1.6.2 (note the primary stress limited to 95% of the yield, not 90%). As a conservative approach, it has not been prohibited in Div 1, hence a tacit approval of ASME VIII board has been assumed and the 90% of yield at ambient conditions is an acceptable approach, as long as no permanent distorsion of the tested vessel is experienced. However, even this, I read somewhere that an acceptable 0.1% distorsion is acceptable....can't remember where, for the moment. Other codes are even more lenient, accepting larger distorsion during hydrotest (up to 3%) under controlled conditions.
Cheers,
gr2vessels

 

[ggarciae]

Besides all what is previously said ,
Most european codes pre-PED (AD-Merkblatt, Codap, Racolta) usually stated Safety factors in test condition or excepcional conditions as S=1,1 (>< 10%)of yield strength or R0,02.
For austenitic steels, where R0,02 will apply, some parts of vessels (where a greater distorsion is acceptable without affecting other parts such us "pure" cylinders) R0,1 may be used.
The same concept is stated in one of the notes at the end of ASME II part D Table A, so for every grade of stainless steel two maximun allowable stresses are listed.
UNE-EN 13445-3-2006 (which is an harmonized standard) in table 6.1 even states safety factors down to 1,05 for some types of steels in testing or exceptional conditions.
Hope this helps


Regards from Barcelona
G. García