Page 232 - Root Cause Failure Analysis

SEALS AND PACKING

All machines that handle liquids or gases, such as pumps and compressors, must be
sealed around their shafts to prevent fluid from leaking. To accomplish this, the
machine design must include seals located at various points to prevent leakage
between the shaft and housing. This chapter discusses sealing requirements and com-
mon seals.

CONFIGURATION
The two primary types of sealing devices used to seal around rotating shafts are
packed-stuffing boxes and simple mechanical seals.

Packed-Stuffing Boxes

A soft, pliable packing material placed in a box and compressed into rings encircling
the drive shaft commonly is used to prevent leakage. Packing rings between the pump
housing and the drive shaft, compressed by tightening the gland-stuffing follower,
forms an effective seal. Figure 18-1 shows a typical packed-stuffing box seal.

Simple Mechanical Seal
A mechanical seal is used on centrifugal pumps and other types of fluid-handling
equipment where shaft sealing is critical and no leakage can be tolerated, Toxic chem-
icals and other hazardous materials are primary examples of applications where
mechanical seals are used. These seals also are referred to asfriction drives, or single-
coil spring seals, and positive drives.

Figure 18-2 shows the components of a simple mechanical seal, which is made up of
a coil spring, O-ring shaft packing, and a seal ring. The seal ring fits over the shaft and

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