Flange Rotation / Lift-off
In all cases, the goal should be to achieve a construction as rigid as possible to overcome movements (lateral and/or axial) caused by varying pressure and/or temperature changes and external loads.
Flanges, bolts and seal can all be considered “spring elements” within the system of which the seal is often a highly non-linear element in its load recovery behaviour.
Therefore, an assembly where metal-to-metal-contact between flanges occurs after bolting is the most rigid and stable one.
After the seal has been compressed in its groove, further tightening of the bolts against the system pressure does not have any negative effect on the seal.
Bolted Seal Assembly
The initial bolt load will generate the initial load (cfr seating stress) on the seal. Because of the system pressure the hydrostatic load tends to “unload” the seal, resulting in flange lift-off.
The degree of residual stress (energy) left, will determine the final leak rate.