centrifugal pumps unlike the reciprocating displacement pump. the simple centrifugal pump, which does not have a positive suction action,consists of a rotating impeller within a stationary pump casing.The fully shrouded.single entry impeller is the type most widely used.
The impeller is mounted on a shaft driven by a prime mover.it consist of a number of vanes backward from the direction of the rotation between the shrouding.
Water enters the rotating impeller through the entry,known as 'eye',at the centre. Due to the circular motion given it is thrown hy certrifugal force to the open periphery of the impeller where it leaves tangentially and enters the space in the casing.and then directed to the outlet branch.
Centrifugal pumps are non-positive and fairly low pressure units. Here a closed discharge valve.will not cause excessive pressure to build up.
centrifugal pumps can be divided into volute, diffuser, mixed-flow, axial flow and turbine or regenerative types.
centrifugal pumps find their widest application for dealing with large volumes with medium-to-low heads and Viscositis.Centrifugal or radial flow pumps give regular delivery,and are quiet in operation.
Axial-flow and semi-axial-flow rotary pumps are frequenLly used for large volumes and low heads,as they are simple.occupy little space and are low in capital cost.
Vertical pumps are in general use.because floor space is usually more valuable than height in engine rooms.
Centrifugal units require to be primed if the liquid level is below the pump. Small integral reciprocating displacement priming pumps are sometimes provided,hut there is a wider use of water-ring pumps for priming purposes, because of their simplicity.
Various other means. such as internal recirculation,steam ejectors,etc. are also used in appropriate cases.
In general. reciprocating pumps are suitable for delivering small quantities at high pressure, rotary pumps are used for moderate quantities at moderate pressure,and centrifugal pumps are more suitahle for large quantities at low pressure.
Centrifugal pumps,however,can he designed with a number of impellers in series to attain a high final delivery pressure. They are popular because they are very simple.
in construction,and can produce smooth,constant,non-pulsating discharge.