Solid and Split Casings

Solid casing implies a design in which the discharge water ways leading to the discharge nozzle are all contain in one casting or fabricated piece. The casing must have one side open so that the impeller can be introduced into it. Because the sidewall surrounding the impeller are actually part of the casing, a solid casing, strictly speaking cannot be used, and design normally called solid casing are really radially Splits.

A split casing is made of two or more parts fastened together. The term horizontally split had regularly been used to describe pumps with casing divided by a horizontal plane through the shaft centerline or axis. The term axially split is now preferred, because both the suction and discharge nozzle are usually in the same half of the casing, the other half may be removed for inspection of the interior without disturbing the bearing or the piping. Like its counterpart horizontally split, the term vertically split is poor terminology. It refers to a casing split in a plane perpendicular to the axis of rotation. The term radially split are now preferred.

More about pump part:

• Propeller
• Gland Packing
• Material Construction
• Radial Thrust

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.

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 360^o 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.