Construction
Main components:
1. Impeller 2. Casing 3. Suction pipe 4. Delivery pipe
Function of main parts in detail as below:
1. Impeller
- It is rotor which is provided with a series of backward curves vanes or blades.
- It is mounted on a shaft which is coupled to an external source of energy (electric motor), which imparts required energy to the impeller.
- It gets mechanical energy and converts it to kinetic and pressure energy of the fluid.
- Liquid enters the impeller through an eye of the impeller, high energy liquid than enters the pump casing.
2. Casing
- It is an air tight passage surrounding the impeller, designed in such a way that kinetic energy of the water discharged at the outlet of the impeller is converted into pressure energy before the water leaves the casing and enters the delivery pipe.
- Material of the casing is generally cast iron or cast steel.
- The efficiency of the pump depends on the type of casing used. (Volute casing, Vortex casing and casing with guide blades.
Working
- “A centrifugal pump works on a principle that when the liquid is rotated by an external prime mover, it is thrown away from the axis of rotation and a centrifugal head is imparted which makes it possible to raise to the higher elevation.”
- Before starting a centrifugal pump, liquid is filled in the suction pipe, impeller, casing and a delivery pipe up to a delivery valve. This is known as priming. During priming delivery valve is kept close.
- After priming, prime mover (electric motor) is started; delivery valve is still kept closed.
- Energy given to the impeller by external source (i.e. prime mover) is transferred to working fluid which increases the kinetic energy and pressure energy of the fluid.
- The rotation of the impeller causes strong suction at the eye of the pump.
- After the impeller attains its normal speed, the delivery valve is opened and liquid is allowed to flow through the impeller vanes and it attains higher velocity at the outer periphery.
- Liquid enters into casing, due to special design of casing the velocity of liquid decreases and pressure energy hence increases.
- With high pressure energy and negligible kinetic energy liquid enters into delivery pipe and is lifted to the required height.
- At that instant partial vacuum is created at the eye of pump due to centrifugal action of impeller on liquid.
- This helps liquid to rush through the suction pipe towards the impeller eye, to take place of liquid which has left the impeller vanes.
- When the pump is to be stopped the delivery valve should be first closed to stop the back flow of liquid.
