Pump Type Follows:

Wednesday, November 4, 2009

Pump Casing and Diffusers

The Volute Casing Pump:
This pump derives its name from the spiral shaped casing surrounding the impeller. This section collects the liquid discharged by the impeller and converts velocity energy to pressure energy.

A centrifugal pump volute increases in area from its initial point until it encompasses the full 360o around the impeller and then flares out to the final discharge opening. The wall dividing the initial section and the discharge nozzel portion of the casing is called the tongue of the volute or the cutwater. The diffusion vanes and concentric casing of a diffuser pump fulfill the same function as the volute casing in energy convension.

A diffuser is seldom applied to a single stage, radial flow pump, except in special instances where volute passages become so small that machined or precission cast volute or diffuser like pieces are utilized for precise flow control. Conventional diffusers are often applied to multistage pump designs in conjunction with guide vanes to direct the flow efficiently from one impeller (stage) to another in a nimimum radial and axial space. Diffuser vanes are used as the primary construction method for vertical turbine and single stage, low head propeller pump.

Radial Thrust:
In a single volute pump casing design, uniform or near uniform pressures act on the impeller when the pump operate at design capacity (which coinsides with the best efficiency). At other capacity. the pressure arround the impeller are not uniform and there is a resultant radial reaction.

For any percentage of capacity, this radial reaction is a function of total head 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. A zero radial reaction is not often realized; the minimum reaction occurs at design capacity.

Although the same tendency for imbalance exist in the diffuser type pump, the reaction is limited to a small are repeated all around the impeller. As a result, the individual reactions cancel each out as long as flow is constantly removed from around the periphery of the diffuser discharge. If flow is not removed uniformly arround its 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 of pumping system.