Simply stated, a pump is a machine used to move liquid through a piping system and to raise the pressure of the liquid. A pump can be further defined as a machine that uses several energy transformations to increase the pressure of a liquid. The centrifugal pump shown in Figure illustrates this definition. The energy input into the pump is typically the energy source used to power the driver. Most commonly, this is electricity used to power an electric motor. Alternative forms of energy used to power the driver include high-pressure steam to drive a steam turbine, fuel oil to power a diesel engine, high-pressure hydraulic fluid to power a hydraulic motor, and compressed air to drive an air motor. Regardless of the driver type for a centrifugal pump, the input energy is converted in the driver to a rotating mechanical energy, consisting of the driver output shaft, operating at a certain speed, and transmitting a certain torque. The power transmit ted from the driver to the pump is a function of the rotating speed times the torque.
The remaining energy transformations take place inside the pump itself. The rotating pump shaft is attached to the pump impeller . The rotating impeller causes the liquid that has entered the pump to increase in velocity. This is the second energy transformation in the pump, where the input power is used to raise the kinetic energy of the liquid. Kinetic energy is a function of mass and velocity. Raising a liquid’s velocity increases its kinetic energy.
After the liquid leaves the impeller, but before exiting the pump, the final transformation of energy occurs in a diffusion process. An expansion of the flow area causes the liquid’s velocity to decrease to more than when it entered the pump, but well below its maximum velocity at the impeller tip. This diffusion transforms some of the velocity energy to pressure energy.