Figure 4 illustrates some of the more common typical gasket characteristics. On first
loading, as the bolts are tightened up, the gasket usually follows a non-linear and nonrecoverable
path. During this initial phase (O-A) the gasket is forced to conform to the
flange faces, absorbing any lack of flatness and filling the irregularities and asperities as are
present on any surface. Seal quality will depend on how effective the gasket is at fulfilling
these tasks since they minimise available leak paths for the contained fluid. The point at
which the gasket provides the minimum effective seal is known as the gasket seating stress
(y) and is given for various gasket types in Table 3.8.3(2) in PD 5500. The seating stresses
as given have been obtained from a combination of industrial experience and tests on typical
gaskets. For many years these values have been used successfully in the design of joints.