Page 246 - Root Cause Failure Analysis

234 Root Cause Failure Analysis

OPERATING METHODS
This section discusses operating methods for packed-stuffing boxes and simple
mechanical seals.

Packed-Stuffing Boxes

Packed-stuffing boxes commonly are used on slow- to moderate-speed machinery
where a slight amount of leakage is permissible. If the packing is allowed to operate
against the shaft without adequate lubrication and cooling, frictional heat eventually
will build up to the point of total packing destruction and damage to the drive shaft.
Therefore, all packed boxes must have a means of lubrication and cooling.
Lubrication and cooling can be accomplished by allowing a small amount of leakage
of fluid from the machine or by providing an external source of fluid. When leakage
from the machine is used, leaking fluid is captured in collection basins built into the
machine housing or baseplate. Note that periodic maintenance to recompress the
packing must be carried out when leakage becomes excessive.

Packed boxes must be protected against ingress of dirt and air, which can result in loss
of resilience and lubricity. When this occurs, packing will act like a grinding stone,
effectively destroying the shaft’s sacrificial sleeve and causing the gland to leak
excessively. When the sacrificial sleeve on the drive shaft becomes ridged and worn, it
should be replaced as soon as possible. In effect, this is a continuing maintenance pro-
gram that readily can be measured in terms of dollars and time.

Uneven pressure can be exerted on the drive shaft due to irregularities in the packing
rings, resulting in irregular contact with the shaft. This causes uneven distribution of
lubrication at certain locations, producing acute wear and packed-box leakage. The
only effective solution to this problem is to replace the shaft sleeve or drive shaft at
the earliest opportunity.

Simple Mechanical Seal
As with compressed packing glands, lubrication must be provided in mechanical
seals. The sealing-area surfaces should be lubricated and cooled with pumped fluid (if
it is clean enough) or another source of clean fluid. However, much less lubrication is
required with this type of seal because the frictional surface area is smaller than that
of a compressed-packing gland and the contact pressure is equally distributed
throughout the interface. As a result, a smaller amount of lubrication passes between
the seal faces to exit as leakage.

Most packing glands have a measurable flow of lubrication fluid between the packing
rings and the shaft. With mechanical seals, the faces ride on a microscopic film of
fluid that migrates between them and results in leakage. However, leakage is so slight