The purport of my earlier response was that for ring type gasket, postulation of flange leakage is not very accurate in Caesar-II flange leakage calculation.

In critical services, either spiral wound gaskets or enough care in minimising insertion gap, flange misalignment need to be carefully prevented & criss-cross bolt-tightening sequence in stages (not to the maximum tightening stress at one go) to be followed. Conical spring washer in case of dissimilar bolt/flange material expansion or use of compatible bolt material of similar expansion coefficient as that of flange material is to be followed. In thermal shocks in start-up or shut-down, bolt may be at different temperature from flange. Spring washer will help in this case,too, in maintaining uniform gasket stress.

Maximum allowed preload stress to bolt should be adopted after checking flange design for that condition, as this can avoid the need for any risky hot bolt-tightening.

regards,

sam