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 manufacturer's technical manual.

The clearance between the impeller wearing ring and the casing-wearing ring (fig. 6-23) must be maintained, as shown in the manufacturer's 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 manufacturer's technical manual. (Wearing rings are located on the main feed pump, as shown in figure 6-24.)

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 permissible.

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 manufacturer's 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.