CENTRIFUGAL PUMP

WHAT IS  CENTRIFUGAL  PUMP

            A centrifugal pump which employs centrifugal force to deliver liquid from one place to other is called centrifugal pump, here the kinetic energy of the leaving water from the impeller is converted in to potential energy which is utilize axial flow and radial flows

            Centrifugal pump works on the principle that when a certain mass of liquid is made to rotate along the impeller from the central axis of rotation, it impresses a centrifugal head. It causes the water to move radially outwards at higher velocity and causes the water to rise to a higher level. The motion of water is restricted by casing of pump,  it results into pressure build up. In addition, the change in angular momentum of liquid during its flow results  into increase in pressure head.

Impeller

 The rotating part of a centrifugal pump is called ‘impeller’. It consists of a series of backward curved vanes. The impeller is mounted on a shaft which is connected to the shaft of an electric motor.

Functions of impeller

            The functions of impeller is to increase the liquid velocity by its rotation this increasing the energy. Some energy being partially converted pressure energy by suitable design of the impeller vanes suction of the pump being created by the differences in pressure between the suction and discharge side of the impeller    

            When the pump rotates liquid in the casing is swirled by the rotating impeller the swirling action causes liquid to move towards the outside and away from the center the backward curving vanes and the rotation give the liquid a combined radial and circular motion

Volute casing

Volute casing is the type of casing that converts kinetic energy created by the             impeller into pressure. The impeller pushes water into the volute which converts     that energy into pressure and directs the flow toward the discharge point.

  PURPOSE OF VOLUTE CASING
It is provided to reduce the velocity of water after it leaves the impeller and convert part of kinetic energy to pressure energy

Diffuser

                                                         

Diffuser is the another method used in centrifugal pumps to convert energy into pressure. This is through the use of a diffuser. A diffuser functions similarly to a standard volute in that it contains vanes that begin close to the impeller and then gradually extend away from the impeller periphery
 A diffuser is fitted to high-pressure centrifugal pumps. This is a ring fixed to the casing, around the impeller, in which there are passages formed by vanes. The passages widen out in the direction of liquid flow and act to convert the kinetic energy of the liquid into pressure energy. Hydraulic balance arrangements are also usual. Some of the high-pressure discharge liquid is directed against a drum or piston arrangement to balance the discharge liquid pressure on the impeller and thus maintain it in an equilibrium position.

The centrifugal pumps are not self priming why?

A centrifugal pump cannot pump a gas therefore a differential pressure necessary for flow, the flow will not be created if the impeller is having air or vapour.

          A centrifugal pump is a rotodynamic pump. This means that the head developed (in meters of liquid that is pumped) depends on the velocities determined by diameter of the impeller and the impeller speed (rpm.). As the pressure developed is related to the head  by the equation  head = pr / sp. weight, the pressure available will be proportional to the specific weight of the liquid. This means that the pressure (or pressure difference) created with air will be only around 1/800 times that with water ( density of water = 1000 kg/ m3  and dry air at S.T.P has a density of 1.2 kg/m3). Therefore, if the pump is not primed, the suction pressure created will not be sufficient to lift water.

Why centrifugal pump started with discharge valve shut?

            It is a common practice to start large capacity centrifugal pump with the discharge valve closed. If the characteristic curve for a centrifugal pump are examined it will be seen that when the quantity of water discharged is zero the power required by the pump is zero or very small amount. by starting the pump with discharge valve closed the power demand made by the pump on the pump motor is kept to very minimum after the pump has started and the momentary high motor current demand has stabilized the discharge valve is opened.