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allowable stress during hydrostatic test
4

allowable stress during hydrostatic test

allowable stress during hydrostatic test

(OP)
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

 

RE: allowable stress during hydrostatic test

2
The hydrostatic test is designed to ensure that the vessel is safe for use at a given maximum allowable pressure or design pressure. When you calculate the maximum allowable pressure you do so with allowable stress values that have been factored from the minimum yield and tensile strength values for that material to incorporate an acceptable factor of safety.

The hydrostatic test pressure is then calculated in accordance with UG-99 to increase the pressure to a level that ensures that the vessel is at least accurately designed and manufactured to meet that factor of safety.

It is common for the hydrostatic test pressure to produce stresses up to 90% of the minimum yield of the material but not to exceed that.

The pressure equipment directive requires in section 7.4 of annex 1 that you perform a hydrostatic pressure test to the maximum one of two values (see the directive for the formulae used). It does state in section 7 that this section is not mandatory as long as an equivalent level of safety is provided, however, it has become apparent through other codes like PD5500 that this is not interpreted to mean that you use the minimum hydrostatic test pressure calculated by the code.

To this end it is wise to calculate the minimum hydrostatic test pressure in accordance with UG-99 and 7.4 of Annex 1, use the higher value and, if the higher value comes from the PED then rerun your design calculations using an allowable stress of 90% of the minimum yield stress for the material at the testing temperature and using the hydrostatic test pressure as your design pressure.

***i sure hope that makes sense***

RE: allowable stress during hydrostatic test

(OP)
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?

RE: allowable stress during hydrostatic test

2
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.

RE: allowable stress during hydrostatic test

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

RE: allowable stress during hydrostatic test

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

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