Flange rotation is known to be important but there are
no simple ways to estimate it. Section Vlll of the ASME Code,
Table UA-49.2 Effective Gasket Width acknowledges rotation
by introducing an effective width for a gasket, which is equal
to or less than half the width of the gasket or joint-contactsurface
seating width. This allowance leakage assumes that
at least half of the gasket will have been unloaded by rotation.
But flanges often rotate more or less than this. Proper installation
and bolt tightening procedures greatly reduce the chance
for flange rotation.