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

Mechanical Seals 405
Figure 17-42. Typical installation reference dimensions.
When mechanical seals are properly applied, there should be no static
leakage under normal conditions. Dynamic leakage can range from none
to several drops per minute, depending on the operating conditions for
the unit. For estimates of static and dynamic leakage, the user should
consult the seal manufacturer. When conditions call for a vacuum, a mechanical
seal is used to prevent air from leaking into the pump.
Seal leakage can occur in many ways. The cause of leakage may be the
result of an improper seal selection, installation, environment, or operation
of the pump. Leakage may occur past static and secondary seals or
past the seal faces. Leakage may occur while the pump shaft is rotating
and stationary or just when the pump shaft is rotating. Each observation
made about the pump and its operation, as well as the condition of the
failed seal parts, plays an important role in identifying the cause for seal
leakage. Analyzing the causes for seal leakage can best be done with pictures
of actual seal parts.
A leakage path past secondary and static seals are usually confined to
elastomeric seals. These types of seals are more vulnerable to nicks,
scratches, and cuts. Boor handling and installation practices are the most
common causes for surface damage to these types of seal parts. Care
should always be taken to remove all sharp burrs and edges from steps
and keyways as well as previous set screw identations from prior seal
installations. Extrusion failures of secondary elastomeric seals such as
O-rings are caused by excessive pressure and can be prevented by reducing
the pressure and/or temperature on the seal. This can be accom-