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Centrifugal pump: parts and working

Short answer

A centrifugal pump uses a rotating impeller with backward-curved vanes to impart kinetic energy to liquid; the volute casing converts it to pressure. Priming is necessary to remove air, as air binding prevents suction. Casings are classified as volute, vortex, and diffuser types.

Explain the construction and working of a centrifugal pump with a neat sketch. Why is priming necessary, and how are the casing types classified?

A centrifugal pump raises the pressure of a liquid by the centrifugal action of a rotating impeller: the impeller adds kinetic energy and pressure energy to the liquid, and the casing converts a part of the kinetic energy into pressure.

   Delivery pipe
        ^
        |      Casing (volute)
   +----+-------+
   |   .---.    |
   |  / ( ) \ <-+-- impeller with vanes
   | |  eye  |  |
   +-+---+---+--+
       |   |
 Suction pipe  <- foot valve with strainer
       |
   ~~~~~~~~~~~~~ sump

Main parts

  1. Impeller: a wheel with backward-curved vanes (shrouded, semi-open or open) fixed on the shaft; it imparts energy to the liquid.
  2. Casing: an airtight passage enclosing the impeller. Types: volute (spiral, with increasing area), vortex (a circular chamber between impeller and volute) and diffuser (guide vanes around the impeller, as in turbine pumps).
  3. Suction pipe with foot valve and strainer: the foot valve is a non-return valve which keeps the pump primed; the strainer prevents debris entry.
  4. Delivery pipe with delivery valve: the valve controls discharge, and is closed at starting to reduce the starting power.
  5. Shaft, bearings, stuffing box/gland to seal the shaft.

Working

When the impeller rotates in a casing full of liquid, the liquid in the vanes is thrown outward by centrifugal force. A low pressure forms at the eye, so liquid from the sump (acted on by atmospheric pressure) flows continuously into the eye. As the liquid leaves the impeller at high velocity, the volute casing expands the flow area and the kinetic energy turns into pressure energy before the liquid enters the delivery pipe.

Priming

If air is inside the casing, the pump produces only a small pressure rise because the density of air is very small (air binding), so it cannot lift liquid. Priming is filling the suction pipe and casing with the liquid before starting, so the pump can create enough suction. It is done by the foot valve, a filling funnel, a vacuum pump or by an ejector.

The delivery valve is kept closed at starting (for the radial pump the power is minimum at zero discharge) and opened slowly after the pump has reached full speed.

✓ Answer checked by Claude Sonnet 5.5

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