Page 435 - Centrifugal Pumps 2E

Mechanical Seals 399

Figure 17-36. Package mechanical seal for CO 2 service (courtesy of John
Crane).

The inboard seal faces are a metal alloy running against tungsten car-
bide. The choice of this material combination was to avoid blistering in a
carbon material that would be exposed to the liquid carbon dioxide. The
seal faces at the outboard position are running at a higher level of PV and
here, carbon versus tungsten carbide is being used.
Double seals are also considered and used for harsh environments in-
volving high temperature applications. Figure 17-37 shows a double-op-
posed stationary seal design. This seal is operating at the following con-
ditions:

• Liquid sealed: residual bottoms
• Vapor pressure at pumping temperature: 1.25 psia
• Viscosity: 0.6 cp
« Temperature: 700°
• Buffer liquid: gas oil
• Temperature: 150°
• Stuffing box pressure: 200 psig
• Speed: 3550 rpm
« Seal size: 3.000 inches

Seal faces are carbon versus tungsten carbide. Double seals on applica-
tions such as this have a seal life that can be 2V2 times longer than a single
seal installation.
Figure 17-38 shows another multiple installation. This seal can be used
as a double or a tandem seal, depending on pressure between the seal