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Monday, November 16, 2009

Radial Thrust

In a single volute pump casing design, uniform or near uniform pressures act on the impeller when the pump operates at design capacity (which coincides with the best efficiency). At other capacities, the pressures around the impeller are not uniform and there is a resultant radial reaction. A detailed discussion of the radial thrust and of its magnitude is presented in the other article. The unbalanced radial thrust increases as capacity decreases from threat at the design flow.

For any percentage of capacity, this radial reaction is a function of total had and of the width and diameter of the impeller. Thus, a high head pump with a large impeller diameter will have a much greater radial reaction force at partial capacities than a low head pump with a small impeller diameter. will have a much greater radial reaction force at partial capacities than a low head pump with a small impeller diameter. A zero radial reaction is not often realized the minimum reaction occurs at design capacity.

Although the same tendency for unbalance exists in the diffuser type pump, the reaction is limited to a small arc repeated all around the impeller. As a result, the individual reactions cancel each other out as long as flow is constantly removed from around the periphery of the diffuser discharge. If flow is not removed uniformly around the periphery, a pressure imbalance may occur around the diffuser discharge that will be transmitted back through the diffuser to the impeller, resulting in a radial reaction on the shaft and bearing system.

In centrifugal pump design, shaft diameter as well as bearing size can be affected by the allowable deflection as determinated by the shaft span, impeller weight, radial reaction forces, and torque to be transmitted.

Because of the increasing application of pumps that must operate at reduced capacities, it has become desirable to design standard units to accommodate such conditions. One solution is to use heavier shafts and bearings. Except for low liquid pumps in which only a small additional load in involved, this solutions is not economical. The only practical answer is a casing design that developed a much smaller radial reaction force at partial capacities. One of these is the double volute casing design, also called the twin volute or dual volute design.

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