Page 389 - Centrifugal Pumps 2E

17

Mechanical

Seals

by James P. Netze!
John Crane Inc.

The most common way to seal a centrifugal pump is with a mechanical
seal. The basic components of a mechanical seal are the primary and
mating rings. When in contact they form the dynamic sealing surfaces
that are perpendicular to the shaft. The primary ring is flexibly mounted
in the seal head assembly, which usually rotates with the shaft. The mat-
ing ring with a static seal, forms another assembly that is usually fixed to
the pump gland plate (Figure 17-1). Each of the sealing planes on the
primary and mating rings is lapped flat to eliminate any visible leakage.
There are three points of sealing common to all mechanical seal installa-
tions:
• At the mating surfaces of the primary and mating rings.
• Between the rotating component and the shaft or sleeve.
• Between the stationary component and the gland plate.

When a seal is installed on a sleeve, there is an additional point of seal-
ing between the shaft and sleeve. Certain mating ring designs may also
require an additional seal between the gland plate and stuffing box.
Normally the mating surfaces of the seal are made of dissimilar materi-
als and held in contact with a spring. Preload from the spring pressure
holds the primary and mating rings together during shutdown or when
there is a lack of liquid pressure.
The secondary seal between the shaft or sleeve must be partially dy-
namic. As the seal faces wear, the primary ring must move slightly for-
ward. Because of vibration from the machinery, shaft runout, and ther-
mal expansion of the shaft to the pump casing, the secondary seal must
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