Flange bolt pattern

[csr17]

In my past experience using ASME flanges I was always using small flanges (generally 2" or smaller) and usually of pressure classes anywhere from 600# to 2500#. These flanges always had 4-bolt patterns and occasionally a slightly larger flange would have 8 bolts. I am now working with much larger flanges (but much smaller pressure ranges - 150#) and have noticed there are many more bolts - enough to fill the entire circumference.

Is the reason for this simply that a larger pipe OD = larger area = larger applied force by the internal pressure and therefore more bolts required to ensure proper flange tightness? Is there any more to it? Are these bolt patterns governed by the ASME code or have they just become industry "best practices" over the years?

Thanks

 

[MickMc]

csr17,

The number of bolt holes is governed by ASME B16.5 & ASME B16.47 for Class 150 to Class 2500 and the logic behind it is that larger flanges need more bolt holes to effectively seal the increase in area/cirumference.

 

[bellowsmfg]

MichMC hit the nail on the head. The standards he referenced - B16.5 covers flanges up for connecting to pipes up to 24 inch and B16.47 above 24" pipe up to 60". The details and dimensions are the ASME standard and suppliers in the US adhere to these.

Bellows Manufacturing and Research, Inc.

https://bellowsmfg.com