Page 106 - An Introduction To Predictive Maintenance

96 An Introduction to Predictive Maintenance

Figure 5–12 Rolls should be uniformly loaded.

Centrifugal
Centrifugal pumps can be divided into two basic types: end-suction and horizontal
split-case. These two major classifications can be further broken down into single-
stage and multistage. Each of these classifications has common monitoring parame-
ters, but each also has unique features that alter their forcing functions and the resultant
vibration profile. The common monitoring parameters for all centrifugal pumps
include axial thrusting, vane-pass, and running speed.

Axial Thrusting. End-suction and multistage pumps with inline impellers are prone
to excessive axial thrusting. In the end-suction pump, the centerline axial inlet con-
figuration is the primary source of thrust. Restrictions in the suction piping, or low
suction pressures, create a strong imbalance that forces the rotating element toward
the inlet.

Multistage pumps with inline impellers generate a strong axial force on the outboard
end of the pump. Most of these pumps have oversized thrust bearings (e.g.,
Kingsbury bearings) that restrict the amount of axial movement; however, bearing
wear caused by constant rotor thrusting is a dominant failure mode. The axial move-
ment of the shaft should be monitored when possible.