Page 263 - Intro Predictive Maintenance

254 An Introduction to Predictive Maintenance

Process- and machine-induced shaft instability also create seal problems. Primary
causes for this failure mode include aerodynamic or hydraulic instability, critical
speeds, mechanical imbalance, process load changes, or radical speed changes. These
problems can cause the shaft to deviate from its true centerline enough to result in
seal damage.

Chemical Attack
Chemical attack (i.e., corrosion or chemical reaction with the liquid being sealed) is
another primary source of mechanical seal problems. Generally, two primary factors
cause chemical attack: misapplication or improper flushing of the seal.

Misapplication. Little attention is generally given to the selection of mechanical seals.
Most plants rely on the vendor to provide a seal that is compatible with the applica-
tion. Too often a serious breakdown in communications occurs between the end user
and the vendor on this subject. Either the procurement specification does not provide
the vendor with appropriate information or the vendor does not offer the option of
custom ordering the seals. Regardless of the reason, mechanical seals are often
improperly selected and used in inappropriate applications.

Seal Flushing. When installed in corrosive chemical applications, mechanical seals
must have a clear-water flush system to prevent chemical attack. The flushing system
must provide a positive flow of clean liquid to the seal and provide an enclosed drain
line that removes the flushing liquid. The flowrate and pressure of the flushing liquid
will vary depending on the specific type of seal but must be enough to ensure
complete, continuous flushing.

12.12.2 Packed Boxes
Packing is used to seal shafts in a variety of applications. In equipment where the shaft
is not continuously rotating (e.g., valves), packed boxes can be used successfully
without any leakage around the shaft. In rotating applications, such as pump shafts,
the application must be able to tolerate some leakage around the shaft.

Nonrotating Applications
In nonrotating applications, packing can be installed tight enough to prevent leakage
around the shaft. As long as the packing is properly installed and the stuffing-box
gland is properly tightened, seal failure is not likely to occur. This type of application
does require periodic maintenance to ensure that the stuffing-box gland is properly
tightened or that the packing is replaced when required.

Rotating Applications
In applications where a shaft continuously rotates, packing cannot be tight enough
to prevent leakage. In fact, some leakage is required to provide both flushing and