Centrifugal Pump Working

   

               

• The pump consists of rotating impeller within a stationary casing.

• The impeller construction has two discs joined at in between surface by a set of internal curved vanes.

• Impeller has an eye (opening) at the center and is mounted on shaft, which is driven by an electric motor, steam engine through crank mechanism or turbine, or other prime mover. 

• Opening in the sides of the impeller near the shaft, called eye, communicates with the suction branch as shown in figure

•   Assume there is a certain amount of fluid at the eye of the rotating impeller.

• The fluid will flow radially outwards (because of centrifugal action) along the curved vanes in the impeller, increasing its linear velocity

• The fluid leaves the impeller in a similar manner to sparks shooting from a Catherine wheel.

• The high velocity fluid is collected in specially shaped casing (volute casing), where some of the kinetic energy of the fluid is converted into pressure energy.

• Fluid under pressure now leaves the impeller producing a drop in pressure behind it at the eye of the impeller.

•  This causes fluid from the suction pipe to flow into pump under atmospheric pressure and subsequently that fluid also gets discharged like earlier one.  

• This way fluid in the pump acts like a piston moving outwards and causing drop in pressure behind it.

• However, if initially there is not liquid at the eye, there will be no pumping action as explained. *Centrifugal pump therefore is not a self-priming pump

• In such case, where normally at start of the pump the level of the liquid is below the eye of the centrifugal pump, we have following alternatives:-

• Prime the pump: Use an air pump initially to draw out air from the suction branch & thus make liquid rise to the eye under atmospheric pressure.

• Example: Emergency fire pump. Bilge and ballast pump,