Centrifugal pump

 

Definition :   Centrifugal pump is a machine which raises water (or any liquid) from a lower level to higher level and  from one place to another by the action of centrifugal force.

Centrifugal Force :

             It is that force which acts upon a body moving in a circular path, tending to     force it farther away from the centre of the circle in which it is moving.

  1. WORKING PRINCIPLE:

            When the rotating member of the pump (Impeller) gives fast rotary motion to a mass of the liquid contained in the casing surrounding it, the centrifugal force forces it out of the casing through the discharge outlet. The vacuum thus created makes atmospheric pressure to push in more water into the casing through the center, called the “eye” of the impeller.

 

CONSTRUCTION :

The pump has two main components :

  • Impeller
  • Casing

Impeller : 

  • It is the only moving part in the pump. 
  • The vanes of the impeller extend from the centre of rotation to the periphery. 
  • These vanes are closed by means of two discs one on each side. 
  • These discs are called shrouds. The vanes are curved slightly forward fin the same direction as that of rotation). 
  • It has a round opening in the centre which is called ‘eye’ of the impeller.
  • The main function of the impeller is to provide high velocity to the mass of water entering through its eye .

The Casing 

  • The casing is the enclosure in which impeller rotates. 
  • This is so designed as to reduce the turbulence created by the water as it is discharged in the volute which steadily increases in diameter till it reaches the delivery outlet.
  • The main function of the casing is to convert velocity energy into pressure energy.
  • The high pressure liquid in the volute recalculates behind the shroud of the impellor causing an end thrust tending to force the impellor off the shaft. 
  • A thrust ball bearing is installed to counteract this force.

Motor :

  • The pump is connected to a motor (Electric or diesel engine) by means of a shaft with the help of a coupling joint.
  • To prevent leakage of water a mechanical seal is provided between the casing and the shaft It is also done by providing a packing of fire resistant material such as asbestos called “gland packing”. 
  • A small leakage is retained in this case to prevent its damage by the factional heat.

VITAL PARAMETERS :

  1. Capacity :   The quantity of fluid discharged per unit lime. E.g. Liters/Minute or   Gallons/Minute.
  2. Head :      The increase in pressure e.g. kg/cm2.
  3. Power:      The energy consumed by the machine e.g. KWH
  4. Efficiency : The ratio of the energy supplied to the fluid by the machine and the energy supplied to the machine

CHARACTERISTICS :

  • At any given speed, when there is no flow the discharge pressure is at the maximum.
  • The pressure decreases progressively with an increase in the discharge rate.
  • Both pressure and flow increase with the increase in speed and vice versa.
  • The power absorbed by the machine increases as the flow increases and vice versa.
  • Keeping the revolutions per minute constant the discharge flow rate fails as the suction lift increases.

CAVITATION

 

  • It is the formation of vapor bubbles in the pump casing and is accompanied by a distinctive rattling noise.
  • This is the indication of water being drawn more than normal. 
  • This is the remedy is to reduce the discharge rate.
  • Water boils at 100° C at atmospheric pressure (760 mm Hg).
  • If the pressure is reduced it will boil at lesser temperature. 
  • If the pressure is reduced sufficiently, water can start boiling at atmospheric temperature.
  • When more and more of water is drawn from the pump, it keeps on reducing pressure at the suction till water starts boiling at the room temperature. 
  • This creates water vapors and bubbles which enter the impeller creating “Cavitation”.
  • This can also happen if there is obstruction in the suction line   e.g. blockage in the foot value or suction strainer.