Explain the construction and working of a centrifugal pump

The hydraulic machines which convert the mechanical energy into pressure energy by means of centrifugal force are called Centrifugal Pumps.

The centrifugal pump works on the principle that forced vortex flow which means that when a certain mass of liquid is rotated by an external force, the rise in pressure head of the rotating liquid takes place. The rise in pressure head at the point of the rotating liquid is proportional to the square of tangential velocity of the liquid at that point;

 i.e. rise in pressure head = V² / 2g

Construction of centrifugal pump:

Main parts of the centrifugal pump are as follows :

1. Impeller

2. Casing

3. Suction pipe with a foot valve and a strainer

4. Delivery pipe

1. Impeller : It is the rotating part of centrifugal pump.

                                    It has series of backward curved wings.

                                    It is lifted on the shaft which is connected with the shaft of an electric motor.

2. Casing : It is an air-tight cover surrounding the impeller.

                  It is designed such that the K. E. of the water discharged at the outlet of the impeller is transformed into pressure energy before the water go from the casing and enters the delivery pipe.

3. Suction pipe with a foot valve and a strainer :

                    Suction pipe is a pipe whose one end is connected to the inlet of the pump and other end dips into the water in a sump.

                    A foot valve which is one way type of valve and a strainer is fitted at the lower end of the suction pipe. The foot valve opens only in one direction i.e. upward.

4. Delivery pipe : It is the pipe whose one side is connected to the outlet of the pump and other side delivers the water at a height which is required.

Working of centrifugal pump :

In case of centrifugal pump, work is done by the impeller on the water.

The expression for the work done by the impeller on the water is obtained by the drawing velocity triangles at inlet and outlet of the impeller, as for a turbine.

The water enters the impeller is arranged in such a way, at inlet for the best efficiency of the pump, which means the absolute velocity of water at inlet makes an angle of 90° with the direction of motion of the impeller at inlet.

Therefore, angle = 90° &. Vw = 0

   Let N = speed of the impeller in r.p.m.

         D1 = diameter of impeller at inlet

        u1 = tangential velocity of impeller at inlet, = ( πD1N) / 60

         D2 = diameter of impeller at outlet

         u2 = tangential velocity of impeller at outlet, = (πD2N) / 60

V1 = Absolute velocity of water at inlet

Vr1 = Relative velocity of water at inlet

As the water enters the impeller radially which means the absolute velocity of water at inlet is in the radial direction and hence angle = 90° and Vw1 = 0.

A centrifugal pump is the reverse of a radially inward flow reaction turbine. But in case of a radially inward flow reaction turbine, the work done by the water on the runner per second per unit weight of the water striking per second is given by the equation as ;

Work done = 1/g [ ( Vw1 × u1 ) – ( Vw2 × u2 ) ]

Therefore, work done by the impeller on the water per second per unit weight of water striking per second ;

                                                 = – { work done in case of turbine }

                                                 = – { 1/g [ ( Vw1 × u1 ) – ( Vw2 × u2 ) ] } = 1/g [ ( Vw2 × u2 ) – ( Vw1 × u1 ) ]

                             As Vw1 = 0 ,

                                               = 1/g ( Vw2 × u2 )

Work done by impeller on water per second = W/g . Vw2 . u2

where, W = weight of water