Working of a centrifugal pump

A centrifugal pump is a rotodynamic pump that raises the pressure of a liquid by the centrifugal action of a rotating impeller: it converts mechanical energy into pressure energy through forced-vortex motion.

Main parts

Impeller (with backward-curved vanes), volute (spiral) casing, suction pipe with foot valve and strainer, delivery pipe with delivery valve, and the shaft driven by a motor.

              delivery pipe
                   ||
            _______||______
          /   volute casing \
         |     _______       |
         |    / \ | / \      |
  suction|===|-- (eye) --|   |
   pipe  |    \ / | \ /      |
         |     impeller      |
          \_________________/
   |
  foot valve + strainer (in sump)

Working

1. Priming: the pump casing and suction pipe are first completely filled with liquid (priming) to remove air; an impeller rotating in air cannot create enough suction.
2. Starting: the motor rotates the impeller with the delivery valve closed (minimum power at zero flow).
3. Forced vortex and suction: as the impeller rotates, the liquid in it rotates as a forced vortex. Centrifugal force throws the liquid radially outward from the eye towards the tip, creating a low pressure (partial vacuum) at the eye. Atmospheric pressure on the sump surface pushes liquid up the suction pipe into the eye.
4. Energy transfer: the vanes give the liquid high velocity and pressure. The rise in pressure head due to the forced vortex is $\dfrac{u_2^2 - u_1^2}{2g}$, where $u$ is the peripheral speed.
5. Conversion in casing: the liquid leaves the impeller with high kinetic energy and enters the volute casing, whose area increases gradually; the velocity is reduced and kinetic energy is converted into pressure energy.
6. Delivery: the delivery valve is opened and the high-pressure liquid flows up the delivery pipe to the required height.
7. Stopping: the delivery valve is closed before stopping the motor to avoid backflow and water hammer.

A centrifugal pump works only after priming and is used for large discharge at moderate heads (water supply, irrigation, dewatering in powerhouses). Reversed, a pump-turbine works as a Francis turbine in pumped-storage plants.