Page 415 - Centrifugal Pumps 2E

380 Centrifugal Pumps: Design and Application

liquid. Then only the seal faces (primary and mating rings) and second-
ary seal come in contact with the pumpage and can be made of corrosion
resistant materials such as Teflon, ceramic, and carbon. Some of the
more common seal materials are shown in Table 17-3.
Temperature is another consideration in seal material selection. Both
secondary and static seals must remain flexible throughout the entire life
of the seal. If seal flexibility is lost by hardening of the elastomer from
high temperature, the seal will lose the degree of freedom necessary to
follow the mating ring. The temperature limits for common secondary
and static seal materials is given in Table 17-4. Temperature is not an
issue for a metal bellows seal. This type of secondary seal is specifically
designed for high temperature and cryogenic applications beyond the
limits of common elastomeric materials. However, this type of seal is
limited in terms of corrosion resistance and most high pressure applica-
tions due to the thin construction of the bellows.

Table 17-3
Most Common Materials of Construction
for Mechanical Seals

COMPONENT MATERIALS OF CONSTRUCTION

SECONDARY SEALS
0-RING NITRILE, ETHYLENE PROPYLENE .
PLUOROELASTOMER , PERFLUOROELASTOMER .
CHLOROPRENE
BELLOWS NITRILE. ETHYLENE PROPYLENE,
FLUOROELASTQMER, CHLOROPRENE
WEDGE FLUOROCARBON RESIN, GRAPHITE FOIL

METAL BELLOWS STAINLESS STEEL, NICKEL BASE ALLOY
PRIMARY RING CARBON. METAL FILLED CARSON,
TUNGSTEN CARBIDE. SILICON CARBIDE,
SILICQNIZED CARBON

HARDWARE I8-8 STAINLESS STEEL,
(RETAINER. DISC. 3!6 STAINLESS STEEL,
SNAP RING, SET NICKEL BASE ALLOYS, TITANIUM
SCREWS. SPRINGS)

MATING RING CERAMIC. CAST IRON, TUNGSTEN CARBIDE,
SILICON CARBIDE