Bolt Load Reduction Results in Flange Thickness Reduction

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Last updated: Aug 29, 2024 by Mike Clark

Last Updated: 2024.4

Appendix 2 flanges consider two bolt areas: Ab is the actual bolt cross-sectional area; Am is the required total bolt cross-sectional area. The code requires Ab to be greater than or equal to Am, but it also penalizes excessive bolting.

The average of Ab and Am is used to determine the bolt load for gasket seating (W). As the actual bolt area increases - even if the required bolting area does not increase, the value of W increases; therefore, the calculated bending moment on the flange increases and drives up the flange thickness.

Look at the equations below that lead to the W value:


Wm1 = 0.785*G^2*P + 2*b*3.14*m*P

Wm2 = 3.14*b*G*y

Am1 = Wm1/Sb

Am2 = Wm2/Sa

Am = max (Am1 , Am2)

Ab must be satisfy Ab >= Am

W = 0.5*(Ab + Am)*Sa


Observations:

m and y are gasket values.  It is common to use Appendix 2 values that are very general and often conservative.  Check on the actual gasket data for the gasket being used.  The gasket manufacturer should provide m and y values and those values will be more accurate, and often less severe than the general values.  This will decrease Wm1 and Wm2.

Confirm that he allowable stress selected matches the bolting.  As bolting diameter goes up, the S values will typically decrease.  If you select the wrong size/thickness you could be penalizing your design even though the S value is artificially high.

Decreasing your bolt area, Ab, to as close to Am as is practical will help you get a thinner required flange thickness.