Gasket Seating
There are two major factors to be considered with regard to gasket seating.
The first is the gasket material itself. The ASME Unfired Pressure Vessel Code Section VIII, Division 1 defines minimum design seating stresses for variety of gasket materials. These design seating stresses range from zero psi for so-called self-sealing gasket types such as low durometer elastomers and O-rings to 26,000 psi to properly seat 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. Table 1 indicated the more popular types of gaskets covered by ASME Unfired Pressure Vessel Code.
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 gaskets such as Spiral Wound 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 effective seal.
Spiral Wound gaskets, which have become extremely popular in the last fifteen to twenty years, do require some surface roughness to prevent excessive radial slippage of the gasket under compression. The characteristics of the type of gasket being used dictate the proper flange surface finish that must be taken into consideration by the flange designer and there is no such thing as a single optimum gasket surface finish for all types of gaskets. The problem of the proper finish for gasket seating surface is further complicated by the type of the flange design. For example a totally enclosed facing such as tongue and groove will permit the use of a much smoother gasket seating surface than can be tolerated with a raised face. Table 3 includes recommendations for normal finishes for the various types of gaskets.
