Gasket Seating

There are two major factors to be considered with regard to gasket seating. The first is the gasket material itself. The ASME Unfires Pressure Vessel Section VIII, Division 1 defines minimum design seating stresses for a variety of gasket materials. These design seating stresses range from zero psi for so-called self-sealing gasket types such a low durometer elastomers and O-rings to 26,000 psi to properly seal solid flat metal gaskets. Between these two extremes there are a multitude of materials available to the designer enabling him to make a selection based upon the specific operating conditions under investigation.

The second major factor to take into consideration must be the surface finish of the gasket seating surface. As a general rule, it is necessary to have a relatively rough gasket seating surface for elastomeric and PTFE gaskets on the order of magnitude of 500 microinches. Solid metal gaskets normally require a surface finish not rougher than 63 microinches. Semi-metallic ets such as spiralwound fall between these two general types. The reason for the difference is that with non-metallic gaskets such as rubber, there must be sufficient roughness on the gasket seating surfaces to bite into the gasket thereby preventing excessive extrusion and increasing resistance to gasket blowout. In the case of solid metal gaskets, extremely high unit loads are required to flow the gasket into imperfections on the gasket seating surfaces. This requires that the gasket seating surfaces be as smooth as possible to ensure an efficient seal.