BOLT FLANGE-div1

mona2 · Oct 22, 2010

Hi, I have 2 questions concerning div 1 flanges.

I don´t really understand that overbolting equation.
Why is the overbolting only considered in the gasket seating condition, I understand that the bolt is tightend during the flange assembly, but during normal operation the bolt is still there causing stress.

I have a 24" ring flange that has Asme B16.5 dimensions, but it's cut from a plate. The flange fails Appendix 2 because of overbolting. Would it be a bad idea to use 3/4" bolt on a 1 3/8" hole to avoid overbolting (with proper bushing and special washer)and safe some money.
Does the norm says anything against it, or is that idea just bad engineering practise?

Sorry for english mistakes.

Thanks

jamesl · Oct 22, 2010

Read Appendix S in Div.1 code for answers to the first question. The real bolt stress during operation is hard to predict. The safety factor is built into the bolt allowable stresses.

There is nothing in code prohibiting oversized bolt hole for Appendix 2 flanges. It is just bad engineering practice. Make sure your client accept it.

Duwe6 · Oct 22, 2010

Just use the proper-fitting 1-1/4" studs, with a UNC coarse thread pitch = 7 threads-per-inch, as opposed to the typical UN8 pitch [8 threads-per-inch, on all large studs]. Then note on the drawing that the bolt torque should be 100-200 foot-pounds [or some reasonable value you derive].

I would be very supprised if your customer would accept undersize bolts and bushings witn washers. That is unheard of, in my 32 years of experience.

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