Bolting Pattern in Circular Flange

Theoretically from the strength point of view, the hole pattern can be anything, provided you analyse it for stress/ strain and safety. But for standards point of view, it should be some symmetrical or otherwise with unsymmetrical, there will be literally infine combinations.

Normally the design of fastening is based on

a) equal load on each bolt

b) Equal contact pressure on the mating flange (pressure cone)

To have these normally the fasteners are kept equispaced.

Additional advantage is as told there is only one equispaced combination, but infinite no of non-equispaced combinations.

By keeping equisp one can ensure matching of flanges as well as relative rotation (re-orientation) whenever required.

Some times when re-orientation is not wanted, offset holes are provided but practically we ourself face problem (some times offsets are made mirror imaged). We now prefer a locating pin hole instead of an offset hole.

What do you mean by center hole?

The bolt can not pass through center of the flanges - it will obstruct flow path as well as bolting will be difficult- you will need elbows in each end.

Also considering the flange pressure distribution, bolting through center will result in less and unequal joint pressure. Even a minute perpendicualrity difference will result in a huge pressure difference.

Also, I don't know if you noted this in your statement because it a good statement, but if you change the hole flange pattern somewhere it will affect the orientation for any valves that are mounted inline anywhere.

Vaiku,

There are a number of reasons why the bolts are equispaced on a flange - many of which have already been identified here.

Every bolted joint contains a gasket to prevent leaks. These gaskets require a certain amount of force to seal them. This force is governed from the input torque on each bolt. If the bolts were not equispaced with one another then leak paths would appear in the gasket UNLESS the clamping force in each bolt varied from one to another.

It is therefore much easier for the bolting technician and designers to use flanges with uniformally distributed bolts.

There is an extensive calculation process involved in coming up with the required torque for each bolt, which would become much more difficult of the bolts were not equi-spaced on a PCD.

Also, when tightening the flange you do not go around the flange from one bolt to the next tightening them as this will deform the gasket too much and could possibly take it past its yield limit for the material. Instead, you go across the flange tightening the bolts. So for example, in a flange with 4 bolts you would tighten at positions 12, 6, 3, 9 (points on a clock face) and for a flange with 12 bolts you would typically tighten in a similar fashion across the flange and not around the flange. This would bring the flanges together more uniformally and prevent overstressing the gasket, and ultimately prevent leaks. This is the main reason for having the bolts equi-spaced.

If you requrie any more information on this subject I would suggest you contact a company involved in the bolting industry - check out my profile, my company will be able to help but you would need to contact Technical Services or a sale representative for your area.

The UKOOA Guidelines contain a lot of relevent information: http://www.ukooa.co.uk/

The Heat Exchanger Handbook may help too: http://books.google.com/books?id=G52EfFF4uQYC&pg=PA549&lpg=PA549&dq=bolt+spacing+on+a+flange&source=web&ots=ClmKbj4_sG&sig=JgyFNJY4zLO6iMvI78Lvj60Eggc&hl=en&sa=X&oi=book_result&resnum=3&ct=result#PPA2,M1

And here's some more info: http://www.pipingdesigners.com/Training%20-%20section%201d.htm

Kind Regards

Kev Brown