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.