APPENDIX 41 - Flange Calculation - Bolts strength question 1

[MkOlivero]

Hello all,

I am currently designing a flanged heater, when doing the calculations of Appendix 41, I noticed that when I select bolts SA320-L7 I need more flange thickness than using SA320-L7M, although L7 has better strength.

My question is why the bending stress depends on bolts strength, should't it be better (from a mechanical point of view) to have more strength?

Thank you!

 

[Geoff13]

Give the installers a stronger bolt, and they might tighten it more ??

 

[SnTMan]

MkOlivero, See definition of "W", Eqn 2-5(e)5, function of bolt stress Sa. Higher Sa, higher W.

I'd say gasket seating governs your design.

Regards.

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[GD2]

Load, strength, stress for a bolted flange assembly will depend on the material strength of all the three components -flange, gasket and bolt.
Assembly analysis makes sure that we don’t crush one component at the cost of others.
Selection and matching of correct bolt material is always critical as it can easily be abused by field techs. Imagine a muscle bodybuilder tightening the flange joint as opposed to a normal tech.

GDD
Canada

 

[JD-JP]

I enjoy everyone's cool responses.
But you're right.
If you think about whether a flange or a bolt will cause a
fatal loss when it breaks, it's the flange.
At least four or more bolts can be attached to the flange,
but if one bolt is damaged by excessive tightening,
the flange can be fastened if the remaining three bolts do not exceed the yield point.
So, paradoxically, we're trying to design a flange that will last until the bolt breaks.

At some point, ASME decided to calculate the stiffness of the flange. This made it safer.
If the bolt is excessively tightened, the flange around the hub will deform.
This is sometimes called flange rotation. This causes leakage from the gasket contact surface.
Stiffness calculations were introduced to prevent this.

Therefore, the stronger the bolt, the thicker the flange.