Page 31 - Valve Selection Handbook

18 Valve Selection Handbook

diameter and diameter of the raised flange face, should be at least 6mm
C/4 in). If the gasket is wrongly installed and protrudes into the pipe bore
or over the raised flange face, the sealing action of the gasket is severely
impaired. The diametrical clearance recommended for confined gaskets
is 1.5mm (He in).
The metal windings are commonly made of stainless steel or nickel-
based alloys, which are the inventory materials of most manufacturers.
The windings may be made also of special materials such as mild steel,
copper, or even gold or platinum. In selecting materials for corrosive flu-
ids or high temperatures, the resistance of the material to stress corrosion
or intergranular corrosion must be considered. Manufacturers might be
able to advise on the selection of the material.
The gasket filler material must be selected for fluid compatibility and
temperature resistance. Typical filler materials are asbestos paper or
compressed asbestos of various types, PTFE (polytetra fluorethylene),
pure graphite, mica with rubber or graphite binder, and ceramic fiber
paper. Manufacturers will advise on the field of application of each filler
material.
The filler material also affects the scalability of the gasket. Gaskets
with asbestos and ceramic paper filler materials require higher seating
stresses than gaskets with softer and more impervious filler materials to
achieve comparable fluid tightness. They also need more care in the
selection of the flange surface finish.
In most practical applications, the user must be content with flange
face finishes that are commercially available. For otherwise identical
geometry of the flange-sealing surface, however, the surface roughness
may vary widely, typically between 3.2 and 12.5 jam Ra (125 and 500
jiin. Ra). Optimum sealing has been achieved with a finish described in
ANSI B16.5, with the resultant surface finish limited to the 3.2 to 6.3 Jim
Ra (125 to 250 jiin. Ra) range. Surface roughness higher than 6.3 Jim Ra
(250 (Jin. Ra) may require unusually high seating stresses to produce the
desired flange seal. On the other hand, surface finishes significantly
smoother than 3.2 |U,m Ra (125 [lin. Ra) may result in poor sealing perfor-
mance, probably because of insufficient friction between gasket and
flange faces to prevent lateral displacement of the gasket.
A manufacturer's publication dealing with design criteria of spiral
wound gaskets may be found in Reference 7.