Pump Wearing Ring Location
In some designs, leakage is controlled by an axial clearance (see Figure 51). Generally, this design requires a means of adjusting the shaft position for proper clearance. Then, if uniform wear occurs over the two surfaces, the original clearance can be restored by adjusting the position of the impeller.A limit exists to the amount of wear that can be compensated, because the impeller must be nearly central in the casing waterways.
Leakage joints with axial clearance are not popular for double-suction pumps because a very close tolerance is required in machining the fit of the rings in reference to the centerline of the volute waterways. Joints with radial clearances, however, enable some shifting if the impeller for centering. The only adverse effect is a slight inequality in the lengths of the leakage paths on the two sides.
So far, this discussion has treated only those leakage joints located adjacent to the impeller eye or at the smallest outside shroud diameter. Designs have been made where the leakage joint is at the periphery of the impeller. In a vertical pump, this design is advantageous because the space between the joint and the suction waterways is open and so sand or grit cannot collect. Because of rubbing speed and because the impeller diameters used in the same casing vary over a wide range, the design is impractical in regular pump lines.