A centrifugal pump is pump in which an impeller (propeller) rotates inside a closed casing draws in liquid at the centre and by virtue of centrifugal force, throws out through an opening to the delivery pipe.
The centrifugal pump is one of the most widely used pumps all over the world for different applications.
So let’s check out how the centrifugal pump works by understanding its construction and working.
Construction of a centrifugal pump
Following are the main parts of the centrifugal pump:
Foot valve with strainer
An impeller is the most important part of a centrifugal pump. The rotating part of a centrifugal pump is called the impeller.
It imparts kinetic energy to the fluid by rotating at a high r.p.m.
It is mounted on a shaft coupled to a prime mover like an electric motor or an engine.
The impeller consists of a number of backward curved vanes mounted on the circumference of the impeller. Casing-
The casing of a centrifugal pump is similar to the casing of a reaction turbine i.e. it is of volute shape.
It is an air-tight passage surrounding the impeller. The kinetic energy of liquid coming out of the impeller is converted into pressure energy by a casing. Suction pipe-
It is a pipe that is connected at its upper end to the inlet of the pump or to the centre of the impeller.
The lower end of the suction pipe dips into liquid in a suction tank or a sump from which the liquid is to be pumped or to be lifted up.
The diameter of a suction cup is kept larger than the delivery pipe to avoid cavitation. Delivery pipe-
It is a pipe that is connected at its lower end to the outlet of the pump and it delivers the liquid to the required height. Foot valve with strainer-
The lower end of the suction pipe is fitted with a foot valve and strainer. The strainer is provided to filter the debris such as leaves, wooden pieces away from the pump.
It then passes through the foot valve to enter the suction pipe. A foot valve is a non-return or one-way type of valve which opens only in an upward direction.
Thus the liquid can pass through the foot valve upwards only, but cannot return downwards.
When the electric motor or the prime mover is started, the impeller starts to rotate.
This creates a vacuum at the center of the impeller where the suction pipe is connected.
The water in the suction pipe is pulled up, it reaches the impeller which then rotates the water at high speed building up centrifugal force on the water.
This causes the water to be thrown away from the impeller. The water enters the casing which converts the kinetic energy of water into pressure energy.
Thus the water exits the pump from the delivery pipe at high pressure.
Note that it is very important that the suction pipe and the impeller must be underwater and there isn’t any air or else the pump won’t work.
This is because the air would keep rotating at the same place as it’s less dense than water.
The process of keeping the suction pipe and the impeller full of water is called as priming. Priming can be done manually or automatically.
What is a Multistage centrifugal pump?
When more power is needed or the pump is expected to deliver over long distances or at very high pressure a pump with a single impeller isn’t effective.
At this time two or more impellers are fitted on the same shaft to increase power.
A centrifugal pump containing two or more impellers mounted on a single shaft is called a multistage centrifugal pump.
The impellers may be mounted on the same shaft or on different shafts. At each stage, the fluid is directed to the center before making its way to the discharge on the outer diameter.
For higher pressures at the outlet, impellers can be connected in series. For higher flow output, impellers can be connected in parallel.
Multistage centrifugal pumps are commonly in boilers as feedwater pumps.
For example, a 350 MW unit would require two feed pumps in parallel. Each feed pump is a multistage centrifugal pump producing 150 l/s at 21 MPa. The submersible pump is a type of multistage centrifugal pump commonly used in bore wells, swimming pools, drainage systems, etc.
And unlike normal pumps, the submersible pump is placed vertically & fully submerged in the fluid, hence the name submersible pump.