The pump model SSP is an open impeller pump. This means that the impeller does not have a front shroud or wear ring surface to reduce recirculation. Instead, the impeller vanes themselves are positioned very close to the wear plate surface. Standard impeller-wear plate clearances are between 10 and 15 thousandths of an inch (0.010" to 0.015").

This clearance significantly affects pump performance. The closer the vanes are to the wear plate, the better the pump performance. When the vanes are spaced further away from the wear plate, pump performance is reduced. The clearance between the impeller vanes and the wear plate is established by inserting spacer shims behind the impeller on the motor shaft (figure 1).

If the impeller clearance is set too close, the impeller can rub against the wear plate, causing damage to both the case and the impeller. This rubbing may not be evident during pump assembly, but may occur during operation. This is due to pump differential pressure causing the impeller to flex towards the wear plate. The closer the pump operates to shut-off, the more of a problem this can be. As a general rule of thumb, impeller/wear plate clearances should never be less than 7 thousandths of an inch (0.007").

The number of shims added to achieve a the proper impeller clearance is often different from pump to pump. This is due to shaft length tolerances from the motor manufacturer, as well as manufacturing tolerances in the pump parts. See the Installation Operation & Maintenance Manual for shimming procedures.

The performance curves on the following pages show performance characteristics for our standard line of open impeller pumps. The solid line head-capacity curve, labeled "B", represents the performance of a pump with standard shimming of 8 to 12 thousandths of an inch. Performance curves for different impeller/wearplate clearances are also shown, labled "A", "C", and "D." Caution is advised when selecting a pump with the "A" clearance as impeller rubbing may occur (see Impeller Shimming above).