PED Allowable stresses 2

[andyecho]

Hi
I would like to here of other peoples' experience on the use of PED design stresses , ESR 7.1.2.
At present I use these stresses and the required joint coefficients only for shell internal pressure calculations, as these are the only general membrane stresses. For local load,external pressure, flat end, saddle and nozzle reinforcement (away from seams) etc. I use those design stresses from the particular design standard specified - mainly PD 5500.
Recently a colleague has told me that HSB insist on the use the ESR stress for all calculations but I can see no justification for this restriction.

This seems a fundamental area and any comments are greatly appreciated

 

[abbver98]

Take care, you should read the guidelines : if you apply a code or method you should apply it entirely.
For this case you don't find coefficient in PED (PED isn't a code) so you had to apply the harmonised standards or a recognised code.

 

[andyecho]

Thanks for your responses.
It seems that I have been conservative, using the lower of PED or code stress for internal pressure.
My colleague spoke to HSB and they now seem to be in agreement with the 'whole code' method but they insist that the test pressure must be at least that given in ESR

 

[abbver98]

Fawkes,
I received from our notified body a copy of the Annex Z to ASME VIII - 1 where they claim :
"the manufacturer must use the test pressures specified in §7.4 of Annex I for the hydrostatic test. (Note : the hydrostatic test pressure of 1.3 in UG-99 of the code does not meet the PED requirement".
I was obliged to review my calculation accordingly.
The other code requirement remains applicable.

 

[abbver98]

I agree with you Fawkes. I argued my choice as you say but I was told that this decision was taken by the NB forum, and I received the annex Z where they inform that the hydrotest factor of 1.3 doesn't met the requirements of the directive and so the PED factor must be adopted.

 

[abbver98]

Fawkes,

In the guidelines 8/6 and 9/5 there is nothing that support or contradict what was said. 8/6 concerns the achievement of an equivalent overall level of safety for exceptions to rules and sent back to 9/5 that speaks about alternatives to harmonized standard. Note in the last sentence "the use of a "recognised" code is not, in itself, sufficient to demonstrate an equivalent overall level of safety". The documentation had to be inspected.
Our NB sent us warning in this sens (U stamped tank is not sufficient, documentation had to be assessed regarding the directive). In the other hand they sent us a list of comparison between the annex I and the ASME VIII div.1 (Annex Z) where they show the considerations when applying this code. I can e-mail it to you if you send me your address.
I have U stamped equipment that I can't sell because the manufacturer doesn't want to send the documentation. With dollar low I am considering my position.
Our NB is AIB "0026".

 

[abbver98]

Hi Fawkes !
As promised, I send you an excerpt from Annex Z (Additional considerations for application of section VIII, division 1 vessels within the scope of PED 97/23/EC, Annex I). In July we have had an expected visit from our notified body. Again we had a sulphurous discussion concerning the hydrotest. Normally according to PED we have to consider the temperature in evaluating the test pressure. According to ASME Test pressure = 1.3 P Sr/St (P: Maximum allowable pressure or design pressure, Sr : allowable stress at test temperature, St : allowable stress at temperatue). According to PED, Test pressure = max (1.43 P, 1.25 P fa/ft) where fa : allowable stress at test temperature and ft : allowable stress at temperature (Note that : 1-allowable stress in EN standards are before safety factor while in ASME II are with safety factor. 2-There is a guideline specifying this test pressure). As we have lot of product and customers asked product with his own design pressure it's hard for us to specify test pressure for each case because the system will become very heavy to acheive jobs, I do a study to compare the both ASME test pressure and PED test pressure to 1.5 P (which is our standard) and it appears that this one is the greater. The problem is that I had discussion in the past with the NoBo concerning calculation in test conditions. I told him that normally it's not required by ASME VIII div.1 and practically the stress had not to exceed 90% the allowable stress. He reject the argument and obliged us to do calculation in test conditions (for flange Appendix 2 with pressure of 1.3PSr/St) because "the equipment had to be checked in all forssen conditions".
Now the problem he asks us to revise the calculation in test conditions with pressure of 1.5 P. He's right but many years ago I heard that when applying a code It had to be entirely. But here we apply ASME with PED formula and it gives a strange mix !
Best regards,