ASME VIII flange design bolt strength (with Appendix S)

[cornemo]

Dear all,

a general question.

When designing a flange according ASME VIII the bolt forces are calculated depending on pressure and gasket factors. This way a bending moment in the flange is calculated which is used to design the flange.
This force can also be used to size the bolts used as the force for each bolt can be calculated. The allowable bolt stress according ASME II part D is always very low.
According to Appendix S I may use higher values. Are these higher values also allowed when doing the design of the flange and bolts?

With regards, Corné

 

[gr2vessels]

Corne,
The flange design require understanding and engineering judgement, to select the applicable design conditions as specified in the particular application. You have to make distinction between a low pressure/low temperature application and the opposite case, when you cannot foresee every fluctuation of operating conditions which may give rise to permanent deformation of the flanged joint. In any case, the principle is that the light operating conditions of a large flanged joint might allow the use of higher allowable design stresses, but should not be used in applications where the leaking joint could be dangerous to people, equipment, etc. You wouln't use higher allowable for the design of exchanger's girth flanges, but in some cases you might use the higher allowables for the design of a manway flange, to save a buck. If not sure how to use it, you should forget the Appx S, otherwise it could prove dangerous and you'll be responsible for the consequences of the flange failure. I hope to have noticed that the Appx S is worded very cautiously, to prevent any novice engineer using it indiscriminately.
However, don't be dicouraged, try both methods of calculation and see for yourself, what are the benefits of cost saving and what could be the costs to you personally, if the joint fails.
Cheers,
gr2vessels

 

[cornemo]

Thanks for your reply.
I'm aware of the consequences. What I'm actually looking for is a statement in the code that this higher value is really allowable. The only statement I can find is in Appendix S but that is indeed very catious.
I'm only willing to allow higher allowables when somewhere else in the code these higher allowables are stated. If someone knows such a statement, please let me know.

 

[SnTMan]

cornemo, as Appendix S is non-mandatory, I don't believe you can use bolt stress values derived from it for flange design, if higher than basic values per Part D. You are of course free to use lower values if you wish.

What Appendix S discusses is that actual, applied bolt stress is generally higher than the the allowables used for design, and the effect this may have on flange yielding, gasket crush, retention of bolt load in service and so forth.

The equation given estimates applied bolt stress given by manual wrenching techniques. It can be seen that, for a given material, applied stresses will be higher than allowables up to some diameter, and will fall below for larger diameters, indicating manual wrenching may not do the job.

Regards,

Mike