Practical Limits For Centrifugal Pumps

This reference, based on a combination of theoretical study and practical experience, covers the "normal limits" of the typical general-purpose centrifugal pump. The intent is to broadly establish parameters defining probable pump capabilities under various conditions.

Capacity
Initial efforts should be made to apply a pump to a system operating near the pump's highest efficiency point. When operation is continuous, it is doubly important that the point of operation be confined within 50% of the pump's best efficiency point (bep) of its impeller diameter. Operation in this region normally eliminates the need to specifically check limitations based on hydraulic, mechanical and thermodynamic conditions.

One exception might involve a system with a low available NPSH condition which could dictate an over-size pump selection. Operation at significantly reduced capacity will result in a high differential pressure in the casing volute acting against the side profile of the rotating impeller. The resulting force becomes a radial  load which, acting on the impeller shaft, can cause shaft and bearing failure, premature impeller and wear ring wear, and mechanical sealing problems where inadequate mountings are employed.

A second problem resulting from reduced capacity — noted particularly in high head (over 140 feet) pumps operating at less than 20% design — is thermodynamic buildup. At partial capacities involving low efficiencies, much of the wasted energy is converted to heat and transferred to the liquid being pumped. As this process continues, the pump casing absorbs some heat, with a portion being dissipated through convection and radiation. Maximum temperature rise for a specific application must be determined based on hydraulic and mechanical conditions.

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