BPVC Section VIII-Div 1 Appendix Y Flanges: Allowable Bolt Stress

[Gumpmaster]

I'm designing some flanges per Section VIII-Div 1 Appendix Y. I'm having some problems with my bolt stresses. I'm using B7 bolts and Section II Part D only gives you 23ksi allowable in Table 3 (20% of the minimum tensile strength). This thread has a similar question: thread292-251881 .

So, I understand that the flange design is based on limiting the stress in the bolts to prevent flange distortion and leakage. There are some bolt materials (such as A354 Grade BD which allow stresses up to 30ksi), so the flange design should be acceptable up to that limit. What is the purpose of limiting the bolt stress for B7 bolts so low? In my particular application I won't have any fatigue or thermal problems (I have a static load at 60deg F). AISC limits the stress to 56.5ksi, so it seems strange that ASME would be so much more conservative.

Are there any provisions for increasing the allowable bolt stress?

Thanks.

 

[TGS4]

In VIII-1, there are no provisions for increasing the allowable bolt stress. Your design will either need more bolts or higher strength bolts.

You can't compare AISC to VIII. They are completely different beasts with different design margins and different failure modes. Appendix Y is for flanges containing pressure where leakage is a concern - AISC doesn't deal with leakage in this manner, does it?

That said, design of flanges in VIII-1 is a paperwork exercise. Refer to Appendix S for commentary on actual bolting in service.

 

[Gumpmaster]

I understand that AISC & ASME are completely different beasts, but from the point of view of the flange & it leaking, does the flange or water care if I use B7 or A354 bolts? If they're the same size and spacing, they both have a similar modulus of elasticity, so why would one prevent leakage and the other not?

AISC only deals with the safety of the bolt (it doesn't care where the load comes from, it treats pressure & gravity loads the same), so I would think the bolt itself would be OK at higher stress levels.

 

[TGS4]

In VIII-1, there are no provisions for increasing the allowable bolt stress. Your design will either need more bolts or higher strength bolts. The design margin on yield and/or ultimate is not negotiable for the ASME Code for design of flanges.

Operating of flanges is different - see Appendix S and PCC-1.

Not that I necessarily disagree with your contention. Just that the ASME Code disagrees with you.

 

[Gumpmaster]

OK, good to know. I usually like to know the why of the code provisions as well as the what. Sometimes the why is lost to history.

I can use A354 BD bolts and make it work, so I'll just do that.