Page 202 - Know and Understand Centrifugal Pumps

Mechanical Seals

As the faces wear, the spring extends maintaining the rotary face in
contact with the stationary face. The shaft O-ring should be free to
move axially on the shaft within the operational tolerances of the
bearings. This is called axial play.
The liquid’s pressure in the seal chamber holds the faces together and
also provides a thin film of lubrication between the faces. This lubricant
is the pumped product. The faces, selected for their low frictional
characteristics, are the only parts of the seal in relative motion. Other
parts would be in relative motion if the equipment is misaligned or with
loose tolerance in the bearings.

The single, unbalanced, inside mounted mechanical seal

This type of seal mounts onto the shaft or sleeve inside the seal chamber
and pump. The pumped liquid comes into contact with all parts of the
seal and approaches the outside diameter of the internally mounted
faces keeping them lubricated. The environment outside the pump
approaches the ID of the seal faces.
The pressure inside the pump acts upon the faces to keep them
together and sealing up to about 200 psig. This is the most popular
type of mechanical seal in clean (no solid particles or crystals) liquid
service. A discharge bypass line connected to the seal chamber can
provide additional cooling. Some people prefer a suction bypass
connection with low vapor head (Figure 13-5).

Fiaure 13-5

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