Page 30 - Valve Selection Handbook

Fundamentals 17

• Graphite can be used without misgivings in the food industry.

Common gasket constructions include:

• Plain graphite gaskets
• Graphite gaskets with steel sheet inserts
• Graphite gaskets with steel sheet inserts and inner or inner and outer
edge cladding
• Grooved metal gaskets with graphite facings
• Spiral wound gaskets

Because of the graphite structure, plain graphite gaskets are sensitive
to breakage and surface damage. For this reason, graphite gaskets with
steel inserts and spiral wound gaskets are commonly preferred. There
are, however, applications where the unrestrained flexibility of the plain
graphite gasket facilitates sealing.

Spiral Wound Gaskets

Spiral wound gaskets consist of a V-shaped metal strip that is spirally
wound on edge, and a soft filler inlay between the laminations. Several
turns of the metal strip at start and finish are spot welded to prevent the
gasket from unwinding. The metal strip provides a degree of resiliency to
the gasket, which compensates for minor flange movements; whereas,
the filler material is the sealing medium that flows into the imperfections
of the flange face.
Manufacturers specify the amount of compression for the installed
gasket to ensure that the gasket is correctly stressed and exhibits the
desired resiliency. The resultant gasket operating thickness must be con-
trolled by controlled bolt loading, or the depth of a recess for the gasket
in the flange, or by inner and/or outer compression rings. The inner com-
pression ring has the additional duty of protecting the gasket from ero-
sion by the fluid, while the outer compression ring locates the gasket
within the bolt diameter.
The load-carrying capacity of the gasket at the operating thickness is
controlled by the number of strip windings per unit width, referred to as
gasket density. Thus, spiral wound gaskets are tailor-made for the pres-
sure range for which they are intended.
The diametrical clearance for unconfined spiral wound gaskets
between pipe bore and inner gasket diameter, and between outer gasket