Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Mechanical Seals 403
alignment, parallel misalignment, end play, and radial shaft runout (Figure
17-41). If wear has occurred between seal parts or between seal parts
and a shaft sleeve or shaft, the pump should be checked for the type of
condition illustrated and corrective measures should be taken. A pump
must operate within the pump manufacturer's tolerances.
Angular misalignment results when the mating ring is not square with
the shaft. As shaft rotation begins, the primary ring will try to follow the
surface of the mating ring. This often results in uneven wear of the softer
carbon primary ring, possible internal wear of seal parts, fretting of seal
hardware or shaft sleeve. Fretting on a shaft or shaft sleeve occurs when
the protective oxide coating is removed from the metal by rubbing motion
from a secondary seal in a pusher-type seal design. A hard coating
on the shaft or shaft sleeve can be used to eliminate this type of wear. A
full convolution elastomeric bellows is designed to protect the shaft or
shaft sleeve by taking all the movement in the bellows convolution instead
of along the shaft. Angular misalignment may also occur from a
stuffing box that has been distorted by piping strain developed at operating
temperatures. In this case, damage to the wear rings in the pump may
also be found if the stuffing box has been distorted.
Parallel misalignment results when the stuffing box is not properly
aligned with the rest of the pump. Generally, with small amounts of parallel
misalignment, the seal will operate properly. However, with a large
amount of parallel misalignment, the shaft may strike the inside diameter
of the mating ring. If damage has occurred, there will also be damage to
any bushing located at the bottom of the stuffing box. The point of damage
will be in the same location as that on the mating ring.
Excessive axial end play can damage the sealing surfaces and can also
cause fretting. If the seal is continually being loaded and unloaded, abrasives
can penetrate the seal faces and cause premature wear of the primary
and mating rings. Thermal damage in the form of heat checking on
the seal faces can result if excessive end play occurs and the seal is operated
below its working height.
Radial runout in excess of limits established by the pump manufacturer
will cause excessive vibration at the seal. This vibration, coupled with
small amounts of other types of motion which have been defined, can
shorten seal life. Radial runout from a bent shaft can damage the inside
diameter of a mating ring through 36X3°.
Seal installation begins with a review of the assembly instructions and
any seal layout drawings. The proper seal installation dimension or spacing
is required to ensure that the seal is at its proper working height (Figure
17-42). The installation reference can be determined by locating the
face of the box on the surface of the sleeve and then measuring along the
sleeve after it has been removed from the unit. It is not necessary to use