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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. >>The
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