Page 29 - Valve Selection Handbook

16 Valve Selection Handbook

irregularities of the flange faces are closed but also the capillaries in the
gasket. To close these capillaries, the gasket must be highly stressed.
The diffusion losses can be combated by making the compressed
asbestos gasket as thin as possible. The minimum thickness depends on
the surface finish of the flange face and the working stress required for
the gasket to conform to the surface irregularities while still retaining
sufficient resiliency. Because the properties of compressed asbestos vary
between makes and product grades, the manufacturer must be consulted
for design data.

Gaskets of Exfoliated Graphite 6

Exfoliated graphite is manufactured by the thermal exfoliation of
graphite intercalation compounds and then calendered into flexible foil
and laminated without an additional binder. The material thus produced
possesses extraordinary physical and chemical properties that render it
particularly suitable for gaskets. Some of these properties are:

• High impermeability to gasses and liquids, irrespective of temperature
and time.
• Resistance to extremes of temperature, ranging from -200°C (-330°F)
to 500°C (930°F) in oxidizing atmosphere and up to 3000°C (5430°F)
in reducing or inert atmosphere.
• High resistance to most reagents, for example, inorganic or organic
acids and bases, solvents, and hot oils and waxes. (Exceptions are
strongly oxidizing compounds such as concentrated nitric acid, highly
concentrated sulfuric acid, chromium (Vl)-permanganate solutions,
chloric acid, and molten alkaline and alkaline earth metals).
• Graphite gaskets with an initial density of 1.0 will conform readily to
irregularities of flange faces, even at relatively low surface pressures.
As the gasket is compressed further during assembly, the resilience
increases sharply, with the result that the seal behaves dynamically.
This behavior remains constant from the lowest temperature to more
than 3000°C (5430°F). Thus graphite gaskets absorb pressure and tem-
perature load changes, as well as vibrations occurring in the flange.
• The ability of graphite gaskets to conform relatively easily to surface
irregularities makes these gaskets particularly suitable for sensitive
flanges such as enamel, glass, and graphite flanges.
• Large gaskets and those of complicated shape can be constructed sim-
ply from combined segments that overlap. The lapped joints do not
constitute weak points.