Page 294 - Root Cause Failure Analysis

SEALS A 3D PACKING

Failure modes that affect shaft seals normally are limited to excessive leakage and
premature failure of the mechanical seal or packing. Table 30-1 lists the common fail-
ure modes for both mechanical seals and packed boxes. As the table indicates, most of
these failure modes can be attributed directly to misapplication, improper installation,
or poor maintenance practices.

MECHANICAL SEALS

By design, mechanical seals are the weakest link in a machine train. If there is any
misalignment or eccentric shaft rotation, the probability of a mechanical seal failure is
extremely high. Most seal tolerances are limited to no more than 0.002 in. of total
shaft deflection or misalignment. Any deviation outside this limited range will cause
catastrophic seal failure.

Misalignment
Physical misalignment of a shaft will either cause seal damage and permit some leak-
age through the seal or it will result in total seal failure. Therefore, it is imperative that
good alignment practices be followed for all shafts that have an installed mechanical
seal.

Process- and machine-induced shaft instability also creates 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 can cause the shaft to deviate from its true centerline enough to result in seal
damage.

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