Components of a centrifugal pump

The main components of a centrifugal pump are shown in Figure 9 and described below:

􀂃 Rotating components: an impeller coupled to a shaft
􀂃 Stationary components: casing, casing cover, and bearings.


Figure 9. Main Components of a Centrifugal Pump (Sahdev)

a) ImpellerAn impeller is a circular metallic disc with a built-in passage for the flow of fluid. Impellersare generally made of bronze, polycarbonate, cast iron or stainless steel, but other materialsare also used. As the performance of the pump depends on the type of impeller, it isimportant to select a suitable design and to maintain the impeller in good condition.

The number of impellers determines the number of stages of the pump. A single stage pumphas one impeller and is best suited for low head (= pressure) service. A two-stage pump hastwo impellers in series for medium head service. A multi-stage pump has three or moreimpellers in series for high head service.
Impellers can be classified on the basis of:

􀂃 Major direction of flow from the rotation axis: radial flow, axial flow, mixed flow
􀂃 Suction type: single suction and double suction
􀂃 Shape or mechanical construction:
− Closed impellers have vanes enclosed by shrouds (= covers) on both sides (Figure10). They are generally used for water pumps as the vanes totally enclose the water.This prevents the water from moving from the delivery side to the suction side, whichwould reduce the pump efficiency. In order to separate the discharge chamber fromthe suction chamber, a running joint is necessary between the impeller and pumpcasing. This joint is provided by wearing rings, which are mounted either overextended portion of impeller shroud or inside the cylindrical surface of pump casing.A disadvantage of closed impellers is the higher risk of blockage.
− Open and semi-open impellers (Figure 10) are less likely to clog. But to avoidclogging through internal re-circulation, the volute or back-plate of the pump must bemanually adjusted to get the proper impeller setting.
− Vortex pump impellers are suitable for solid and "stringy" materials but they are up to50% less efficient than conventional designs.
Electrical Energy Equipment: Pumps and Pumping Systems


Figure 10. Closed and Open Impeller Types (Sahdev)

b) Shaft
The shaft transfers the torque from the motor to the impeller during the startup and operationof the pump.
c) Casing
The main function of casing is to enclose the impeller at suction and delivery ends andthereby form a pressure vessel. The pressure at suction end may be as little as one-tenth ofatmospheric pressure and at delivery end may be twenty times the atmospheric pressure in asingle-stage pump. For multi-stage pumps the pressure difference is much higher. The casingis designed to withstand at least twice this pressure to ensure a large enough safety margin.
A second function of casing is to provide a supporting and bearing medium for the shaft andimpeller. Therefore the pump casing should be designed to
􀂃 Provide easy access to all parts of pump for inspection, maintenance and repair
􀂃 Make the casing leak-proof by providing stuffing boxes
􀂃 Connect the suction and delivery pipes directly to the flanges
􀂃 Be coupled easily to its prime mover (i.e. electric motor) without any power loss.

Electrical Energy Equipment: Pumps and Pumping Systems
There are two types of casings
􀂃 Volute casing (Figure 11) has impellers that are fitted inside the casings. One of the mainpurposes is to help balance the hydraulic pressure on the shaft of the pump. However,operating pumps with volute casings at a lower capacity than the manufacturer’srecommended capacity, can result in lateral stress on the shaft of the pump. This cancause increased wearing of the seals, bearings, and the shaft itself. Double-volute casingsare used when the radial force becomes significant at reduced capacities.

􀂃 Circular casing has stationary diffusion vanes surrounding the impeller periphery thatconvert speed into pressure energy. These casings are mostly used for multi-stage pumps.The casings can be designed as:

− Solid casing (Figure 12): the entire casing and the discharge nozzle are contained inone casting or fabricated piece.
− Split casing: two or more parts are joined together. When the casing parts are dividedby horizontal plane, the casing is called horizontally split or axially split casing.