Figure 6-23.Impeller, impeller wearing ring, and casing
wearing ring for a centrifugal pump.
bearings-laminated phenolic material grade FBM
(fabric-based Bakelite or Micarta), high lead content
bronze, graphite bronze, and lignum vitae.
The condition of all types of internal water-
lubricated bearings should be checked frequently to
guard against excessive wear which can result in
misalignment and shaft failure.
As for oil-lubricated sleeve or shell-type bearings,
the bearing clearances should be measured following
procedures described for the pump, and clearances
should be maintained within the limits specified in the
manufacturers technical manual.
The clearance between the impeller wearing ring
and the casing-wearing ring (fig. 6-23) must be
maintained, as shown in the manufacturers plans.
When clearances exceed the specified figures, the
wearing rings must be replaced. This replacement
requires the complete disassembly of the pump.
Information on disassembly of the unit, dimensions of
the wearing rings, and reassembly of the pump is in the
manufacturers technical manual. (Wearing rings are
located on the main feed pump, as shown in figure
When deciding whether the wearing rings need
renewing, you must consider the capacity of the pump
and the discharge pressure of the pump. On low-
pressure pumps, the wearing ring diametrical
clearance may be 0.015 to 0.030 inch more than the
designed amount without any appreciable effect on the
capacity of the pump. For pumps with a discharge
pressure up to 75 psi, a wear of 0.030 to 0.050 inch is
The percentage of capacity loss with a 0.030-inch
wearing ring clearance in excess of standard may be
large with a small pump but comparatively small with a
large pump. For high-pressure boiler feed pumps, the
effect of increased wearing ring clearance is readily
noticeable in the efficiency and maximum capacity of
the pump. For high-pressure pumps, the wearing rings
should be renewed when the clearance shown on the
manufacturers plans is exceeded by 100 percent. It is
usually not necessary to renew wearing rings uniess
the wear is at least 0.015 inch. If a pump has to be
disassembled because of some internal trouble, you
should check the wearing rings for clearance. Measure
Figure 6-24.Two-stage centrifugal pump.