Impellers are generally closed because of higher efficiencies and are less prone to wear in the front liner region. Open impellers are more common in smaller pumps or where particles blockage may be a problem or where the shear provided by an open impeller is a aid to pumping froth.
Another features of slurry pump impellers is the pump out or expelling vanes on the back and front shrouds. These perform the dual function of reducing pressure (thus inhibiting recirculating flow back to the impeller eye, and reducing stuffing box pressure) and keeping solids out of the gas between the casing and impeller by centrifugal action.
The impeller design is crucial as it influences flow patterns and ultimately, wear rates throughout the pump.
Some typical examples of the need for the non-standard impellers are:
a) Pumping coarse coal
Large particles may cause blockages with a standard 5 vane closed impeller. A special large-particle 4 vane impeller may be required.
b) Pumping fibrous material
Long fibers may get caught around the vane entrance of standard impellers. A special chokeless impeller can be used for these duties.
c) Reduced diameter impellers
In some special cases, reduced diameter impeller are required but are generally avoided as impeller wear is higher than with full diameter impeller.
d) Reduced eye impeller
In some extremely high wearing applications such as mill discharge, a special impeller with reduced eye can prolong impeller wear life.