Everything You Need to Know About Centrifugal Pumps

Centrifugal pumps convert the kinetic energy of the fluid to its pressure energy. The designers accomplish this phenomenon simply with the understanding of both Continuity and Bernoulli’s equations. The impeller imparts kinetic energy to the fluid by centrifugal force. The fluid then enters a casing that has a volute (spiral geometry), with a constantly increasing area. This helps in converting a high velocity into a high discharge pressure.

We know the simplest definition of the continuity equation; for an incompressible flow, the increasing area would mean decreasing speed. And Bernoulli’s equation (for incompressible fluid) suggests if all other things remain constant during a flow, fluid with decreasing velocity would conserve its energy by an increase in pressure.

Working Principle of Centrifugal Pumps

Centrifugal pumps work on the principle of centrifugal force. To understand it, let us consider an example. If you swing a bucket full of water, a force will exert on your arm. This is centrifugal force. If the speed of rotation is enough, water remains in the bucket. This is because the centrifugal force pushes it to the bottom of the bucket.

Now if you punch a small hole at the bottom of the bucket, a stream of water flows out. The distance that water travels is proportional to the amount of centrifugal force.

Centrifugal Pumps
Reference Figure: API-1071WB-Centrifugal Pumps

As the impeller rotates, a negative pressure (vacuum) develops at its eye. This draws in the liquid which is at a higher (atmospheric) pressure. The rotating impeller now exerts a centrifugal force on the liquid that forces it against the walls of the volute. As the liquid strikes the walls, its speed is increased. The casing or volute is wider at the outlet. This helps in converting the high kinetic energy (velocity) of the fluid to high discharge pressure. The higher discharge pressure then forces the liquid out to the pipelines.

As the impeller continues rotating, a continuous stream of liquid draws in and discharges out.

The main difference between a positive displacement pump and a centrifugal pump is: “The positive displacement pump gives a constant flow rate, irrespective of the pressure”. However, in the case of centrifugal pumps, the flow rate decreases with the increase in pressure. This is the reason why we don’t run a P.D pump with a closed discharge. So, understanding pump flow rate vs pressure is quite important.

Centrifugal Pumps Types

We can classify centrifugal pumps into different types depending upon the type of impeller and the number of impellers.

a. Type of impellers

There are three main types of impellers used in centrifugal pumps. Each type has a different use and design limitations.

  1. Open impeller which only contains the vanes. We generally use it for smaller pumps and abrasive liquids.
  2. Semi-open impeller which has a shroud covering on one side. We use it for slightly more viscous liquids.
  3. Closed impeller which has the shroud covering on both sides. Only the impeller eye remains open. We use it for more clear liquids.
Centrifugal pumps types

Reference Figure: API-1071WB-Centrifugal Pumps

b. Number of impellers

We can increase the number of impellers to meet the required pressure. Therefore, pumps can have two or more impellers. We call them multistage pumps. Following are some of the common types of multistage pumps.

  • Submersible Pumps: They have their motor and pump integrated into one unit. We use them to draw water or oil from the depth (wells). They are completely submerged into the source reservoir.
  • Can-Type Pumps: We use them to lift the liquid from storage tanks or sub-surface sources. In this case, the rotor is located above the surface of the reservoir. A shaft extends from it which is connected to the pump located some distance below into the liquid.
  • Horizontal Pumps: We use horizontal multistage pumps in the process industries where high pressure is required.

Centrifugal pumps can also be classified depending upon the number of suction inlets.

  • Single suction in which the fluid enters to the impeller aye at one side.
  • Double suction in which fluid enters at both sides.

The purpose of a double suction centrifugal pump is:

  1. either to reduce the inlet speed (because of the small cross-section) to avoid cavitation
  2. or when the flowrate required, is too large for the cross-section of single suction pump

Centrifugal Pumps Applications

Centrifugal pumps are ideal to be used for low viscosity applications. Almost 80% of the pumps used in each process industry are centrifugal because they can provide high flow rates. Centrifugal pumps are used:

  • for water supply to the residential areas
  • in Firewater system and also supplying water to the buildings
  • in oil industries
  • for irrigation
  • in chemical industries for the transportation of raw materials

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One Response

  1. Aw, this was an exceptionally nice post. Taking the time and actual effort to make a really good article… but what can I say… I hesitate a whole lot and don’t manage to get nearly anything done.

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