Page 235 - Root Cause Failure Analysis

Seals and Packing 223

bon ring and the stationary insert. Note that the surface areas of both rings must be
machined-lapped perfectly flat, measured in angstroms with tolerances of one-mil-
lionth of an inch.

PERFORMANCE
Performance of a packed-stuffing box seal depends primarily on the presence of a
small quantity of fluid through the box. This flow is needed to provide both lubrica-
tion and cooling of the packing.

A mechanical seal’s performance depends on the operating condition of the equip-
ment on which it is installed. Its efficiency depends on the condition of the sealing-
area surfaces, which are friction-bearing surfaces that remain in contact with each
other for the effective working life of the seal.

This type of seal is more reliable than compressed packing seals. Because the spring
in a mechanical seal exerts constant pressure on the seal ring, it automatically adjusts
for wear at the faces. Therefore, the need for manual adjustment is eliminated. Addi-
tionally, because the bearing surface is between the rotating and stationary compo-
nents of the seal, the shaft or shaft sleeve does not become worn. Although the seal
eventually will wear out and need replacing, the shaft will not experience wear.

INSTALLATION
This section describes the installation procedures for packed-stuffing boxes and
mechanical seals.

Packe&Suffing Box
The following sections provide detailed instructions on how to repack centrifugal
pump packed-stuffing boxes or glands. The methodology described here applies to
other gland-sealed units, such as valves and reciprocating machinery.

Tool List

The following list specifies the tools needed to repack a centrifugal pump gland:
Approved packing for specific equipment,
Mandrel sized to shaft diameter,
Packing ring extractor tool,
Packing board,
Sharp knife,
Approved cleaning solvent,
Lint-free cleaning rags.