Centrifugal pump motors use mechanical rotation to add velocity to a pumped liquid

Centrifugal pump motors use mechanical rotation to add velocity to a pumped liquid. This velocity increases a liquid’s pressure.

The fluid enters the pump axially at a smaller diameter known as an impeller eye and progresses radially outwards through a set of curved vanes and then into a discharge nozzle.

The Impeller

An impeller is the rotating pumping element that transmits mechanical energy from the motor to pumped medium. It is designed with backward curved vanes that are connected to a shaft and can be found in centrifugal pumps of all sizes.

The pump casing transforms the velocity generated by the impeller into a controlled and stable flow of water. It is also responsible for converting the kinetic energy discharged at the outlet to pressure energy.

This is accomplished when the impeller precesses - it moves away from the valve plate while reversing direction of rotation - until it reaches the top of the reciprocation cylinder or top-dead-center (TDC). This is when the connection between the trapped fluid chamber and the pump’s discharge port closes. The resulting pressure is transmitted through the pipework system to the application. The velocity of the pumped liquid can be adjusted by adjusting the casing diameter which can result in a different flow rate or head.

The Shaft

The shaft is a central element of the pump motor. It transfers power from the driver to the impeller. It must also withstand vibration from passing vane frequency, critical speeds and harmonics to prevent fatigue failure.

It must withstand radial hydraulic forces, especially at the keyway, fillet radius or snap ring grooves and it must be able to handle bending stresses at its weakest points such as the radius, shoulder and the key-seat. Shaft fractures from bending stress occur when the load exceeds the material’s elastic limit. The fracture type is dependent on the number of cycles, periodicity and distance (strain or amplitude) of the bending stress.

The shaft must be sealed from the pumped liquid using a mechanical seal or packed gland. The stationary part of the seal is held in place on the shaft by a set screw while the rotary portion is fitted to the casing or seal chamber bore. Mechanical seals require the pumped liquid to lubricate and cool the packing so they can perform their function correctly.

The Pump Head

The pump head is a circular, watertight casing that holds the impeller. It is the part of the centrifugal pump that imparts velocity to the liquid and generates head (pressure).

The difference between head and pressure is that given head is fluid independent while pressure is dependent on gravity. As a result, the same head will create different pressures for different fluids with different relative densities.

A mechanical seal or packed gland is used to envelop the shaft and casing providing a tight fit and sealing against pressure and frictional forces. Packed glands use a series of segmented rings that are sealed together with oil lubricating the packing which is cooled by the pumped fluid.

Centrifugal pumps are rated for a range of horsepower. Each pump has a performance curve that shows various performance metrics including head and flow at specific flow rates. The point where the pump curve and system curve intersect is the operating point.

The Pump Body

Centrifugal pumps are simple, cost effective and versatile. They are best suited to liquids that have low viscosity and pour like water or light oils and can be used in many applications.

They are typically made of cast iron, stamped steel or plastic. They can also be made of stainless steel, exotic alloys or bronze to handle corrosive or abrasive liquids. They can also be manufactured to meet sanitary requirements.

The pump casing (called a volute) is shaped similar to a snail shell and widens as it goes towards the discharge port. The casing helps decrease the speed/flow of the water and increases pressure.

The suction pipe is connected to the inlet of the casing and the water is dipped through a foot valve and strainer. This helps prevent the pump from flooding. The gland packing is a set of segmented rings that are wound around the shaft and require pumped liquid to lubricate and cool them.