Hi vinay,

"Gasket seating is the initial applied bolt load for assembling the joint at atmospheric temp and pressure. this is a function of gasket material and surface area occupied by gasket"

Please read ASME sec VII div-1,appendix-2 for detailed flange design. In para 2-7 you will get the formula for calculation of flange stresses.there are three basic formula available for:
1. longitudinal hub stress
2. Radial flange stress, and
3. Tangential flange stress

Please note that above formulas are common for calculation of stresses in operating case and gasket seating case. only the difference in these formulas are the value of " Total moment acting upon the flange, for the operating condition or gasket seating condition"

Allowable stresses for gasket seating and operating condition is the allowable stresses value of falnge material at atmospheric temp and design temp respectively which you can get it from B31.3 appendix A, table A-1.

In operating condition, the total flange moment is the sum of three individuAL moments in same direction mentioned above for stress.

For gasket seating, the total flange moment is based on the flange design bolt load, which is opposed only by gasket load.

Hope this will work for you.