In centrifugal pumps, it is common practice to provide front sealing means adapted to provide a seal between the front of an impeller and a front of a pump casing in a region bounding an inlet or suction opening and rear sealing means adapted to provide a seal between the rear of such impeller or its drive shaft and a rear of the pump casing for purposes of minimizing leakage of pumped fluid subject to high discharge pressure adjacent an outlet or discharge opening past the impeller towards the inlet opening and to atmosphere through the impeller shaft supporting bearings and stuffing box of the pump.
One widely used type of rear sealing means comprises a plurality of back, pump out or repeller blades, which are formed integrally with the rear surface of the impeller and arranged to cooperate with an annular surface of the casing rear wall to create a hydrodynamic seal. The facing or cooperating surfaces of the pump casing and the blades must be machined to relatively close tolerances and care must be exercised in pump design and construction to provide for a relatively small axial gap or spacing between such surfaces in order to obtain maximum efficiency of operation. It is well known that the efficiency or effectiveness of the seal is very sensitive to the size of the axial gap between cooperating surfaces of the casing and blades, and that even a small increase in the latter, due for instance to differential thermal expansion of the casing and the impeller or its drive shaft, will result in a marked reduction in seal efficiency.
The problem of maintenance of rear seal efficiency is particularly serious in a pump intended for use in handling fluids containing abrasive materials. In this respect, a pump of this type is typically provided with suitable means for adjustably moving its impeller axially towards its pump inlet opening, as required to compensate for the wearing away of casing and/or impeller surfaces defining its front sealing means. Thus, with each adjustment of the impeller the spacing between the cooperating surfaces of the repeller blades and the casing rear wall is increased with a corresponding steady reduction in the efficiency of the rear seal. As a result, the repeller blades cannot maintain a dry seal or provide for a steady predetermined reduction of the axial thrust to which the impeller shaft bearings are subjected.