Centrifugal pump- Construction, working and uses | The Mechanical post

Centrifugal pump- Construction, working and uses | The Mechanical post
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Welcome back to The Mechanical post! Today let's read about Centrifugal pump, its Construction, working and uses. Let's start by defining the centrifugal pump:

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.

So let's check out how the centrifugal pump works by understanding its construction and working.  

Main components of a centrifugal pump

Following are the parts of the centrifugal pump:
  • Impeller
  • Casing
  • Suction pipe
  • Delivery pipe
  • Foot valve with strainer
  • Prime mover
  1. Impeller-  The rotating part of a centrifugal pump is called as the impeller. 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.                                                                                                                                                                   
  2. 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 impeller is converted into pressure energy by a casing.                                                                                                                                                                       
  3. Suction pipe- It is a pipe which 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.                                                                                                                                                          
  4. Delivery pipe- It is a pipe which is connected at its lower end to the outlet of the pump and it delivers the liquid to the required height.                                                                                                                                                         
  5. 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 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.

Fantagu, Public domain, via Wikimedia Commons

Working of the centrifugal pump:

When the electric motor or the prime mover is started, the impeller starts to rotate. This creates a vacuum at the centre 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, as the air would keep rotating at the same place as its 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.

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 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 centre 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. A common application of the multistage centrifugal pump is the boiler feedwater pump. 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.

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Advantages of a centrifugal pump:

  1. As the impeller is the single rotating part, losses due to friction are reduced to a minimum.
  2. It is having no valves, pistons so on reciprocating parts, therefore maintenance is much less.
  3. Discharge is constant.
  4. No shock, vibration, so heavy foundation is not required.
  5. It is simple to design.
  6. Because of its high speed, the discharge capacity is high.

Applications of centrifugal pumps.

  1. Drinking water and drainage systems.
  2. Agriculture and irrigation processes.
  3. Thermal power plants.
  4. Hydraulic control system.
  5. Textile- bleaching of fabrics and silk.
  6. Food- sugar refining bleaching etc.
  7. Electronics- acid waste transfer.
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