Rotary pumps may be used for in-plant process and pipeline transfer of slurries when metered flow, medium-to-high discharge pressures are required. Since volumetric and mechanical efficiency are normally dependant on the clearances between the pumping elements of a rotary pump, take care in the selection and application of the pump in slurry service. Slurries containing hard particles can cause abrasive wear in rotary pumps.
The concentration of solids generally has the greatest effect on fluid characteristics of the mixture. A range of 5 to 15 percent by weight of paper stock slurry goes through three different stages of fluidity or shear characteristic.
The size and general shape of the solids in a slurry normally govern whether a particular rotary pump configuration and/or clearances can be used. Usually the clearances must be greater than the particle size. A clearance provision of greater than the maximum particle size for handling hard or fibrous particles is necessary in many rotary pump configurations to prevent binding, wedging and severe wear. This clearance, if greater than normal design clearances, could permit greater slip within the pump and thus should be evaluated for the range of slurry handled.
The hardness of solids greatly influences the wear rate of the rotary pump, since there is close relative motion of surfaces at rotor tip or flank velocity. The range of hardness is from soft solids (such as polymer chains of lubricating quality) to fibrous products (which, though generally soft, may carry hard particles) to very hard irregular particles (such as sand and carbon).
Regardless of the type of rotary pump used for abrasive slurries, pump speeds should always be reduced well below those for nonabrasive fluid applications. Speed directly affects product shear and the relative velocity of solids to the pump housing and rotor.
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