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How does a centrifugal pump work?

- Oct 26, 2017 -

The purpose of this device is to convert prime mover’s energy into kinetic energy, then into pressure energy that helps in pumping the fluid.

These energy changes take place courtesy of two main centrifugal pump components – the volute or diffuser and the impeller.

The diffuser is the stationary part which converts kinetic into pressure energy while the impeller is the component that rotates and transforms driver force into kinetic energy.

There are two main components in a centrifugal pump:

Rotating system made up of a shaft and an impeller

A stationary part made up of a casing, its cover, and bearings

All these components are instrumental in ensuring that the pump can operate as required.

With the combination of these elements, the centrifugal pump can move fluid by accelerating the liquid radially outward.

These kinds of pumps consist of one or more impellers in a casing that guides the fluid in and out of the impeller, or from one impeller to another in multistage pumps.

The impellers can be single or double suction.

The rotating impeller conveys kinetic energy and pressure to the fluid. A collection zone in the casing transforms a significant portion of the kinetic energy into pressure energy before allowing the fluid to leave the pump.

Through the impeller, a free passage exists between the inlet and discharge side of the pump.

To prevent the draining or back-flow of fluid from the centrifugal pump, there is a need to ascertain that the impeller is rotating.

As a result, only a few centrifugal pumps are self-priming. A good number of pumps must be primed or filled with liquid before they can start.

Other components of the centrifugal pumps are:

Rings or wearing surfaces which create a running joint between the casing and the impeller to reduce fluid backflow from discharge to suction

The shaft that drives and supports the impeller, and

The seal or stuffing box that prevents leakages from occurring between the shaft and casing

The bearing housing encloses the bearings that are mounted on the shaft.

These bearings keep the rotor in alignment with the stationary components under the actions of transverse or radial loads.

The bearing system also incorporates a constant level oiler, an oil reservoir for the lubrication, and a jacket for cooling through cold water circulation.

Every centrifugal pump has a characteristic curve that illustrates the relation between the rate of flow or capacity and head or pressure against which the device will pump.

At a pressure difference of zero, maximum capacity is achieved in the absence of useful work. As the external flow resistance increases, the capacity decreases until flow ceases at a high pressure.

This is referred to as a shut-off head, and at this point, there is no work done. At constant rpm, the head and capacity differ in a fixed relationship.

When the needed head surpasses the practical for a single-stage pump, many phases are employed.

The multistage pump begins with a two-stage pump to pump with various stages of twenty or thirty for high lifts.

Centrifugal pumps almost never rely solely on the apparent centrifugal force* that is caused by the tangential acceleration of the fluid. If it were to be so, the fins would have to be radial. What you see instead in practice is that the fins are sloped tangentially at a high angle, so that they impart both tangential and radial acceleration. 

That acceleration is what causes the radial flow and causes the buildup 

of pressure against the volute. The volute acts as a diffuser and exchanges velocity with pressure. Part of the diffusion may occur inside the impeller, depending on its geometry.

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