Centrifugal pump impeller explained; Open, Semi-open and closed impellers

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Impeller is a rotating disc inside the housing or volute casing of the centrifugal pump. In this article we will explain the relationship between centrifugal pump impeller diameter to flow relation. Impeller of a centrifugal pump can be categorized in three main types, which are:

  1. Open impeller
  2. Semi-open impeller
  3. Closed impeller
Impellers type


Impellers is a rotating disc, having bore in the middle which has to couple with motor shaft. The source of driving force transfers through shaft and connection at the bore of impeller at the center.

Vanes are the fin type guides on impellers, which basically energize the fluid. Number of vanes on an impellers defines the design specification of the pump; high number of vanes for fluids having low suspended solids and vice versa. Clear water pumping, number of vanes are usually more than 5 upto 10, and for solution/fluid having more suspended solids number of vanes could be 5 or less than 5.

Spacing between the vanes called channels. Channels spacing are increasing from inner to outer side of the impeller; spacing will be smaller at the bore end and gradually increasing to outer direction at a specific angle.

Shroud is the plate with vanes in case of semi-open of closed impellers. No shroud present in open impeller type. Single shroud on semi-open impellers and double on closed impellers.

Basically, impellers job is to convert kinetic energy into pressure energy. The gradual change in vanes spacing is the reason that kinetic energy converts into pressure energy. Bernoulli equations states that high velocity is to low pressure and low velocity is to high pressure. At the eye of the impeller vanes, low spacing creates high velocity, by increase in area velocity drops and pressure increases. That how a centrifugal pump works.

Change in Area=Change in Velocity=Change in Pressure (Bernoulli Equation)

Increase in Area=Decrease in Velocity=Increase in Pressure

Decrease in Area=Increase in Velocity=Decrease in Pressure

Impellers design

Open Impeller

Open impellers are best suited for fluids having large amount of suspended solids. Its structurally weak due to lack of support and inefficient by performance.

Semi-Open impeller

Semi-open of semi-closed impellers have average mechanical strength and supports small amount of suspended solids. It has better efficiency than open impeller and lower efficient than closed impeller.

Closed Impeller

Closed impellers are the most efficient type impeller. It has high strength than the other two, because of having two shrouds on both sides of the vane. It can pump slurries and fluid having high suspended solids but the wear on impellers will be high.

Impeller are also calssified by suction type:

  1. Single suction type
  2. Double suction type

Classification of impeller in term of flow type:

  1. Radial flow impeller
  2. Axial flow impeller
  3. Mixed flow impeller

Centrifugal pump impeller diameter to flow relation is directly proportional. Increase and diamter results in increase in the flowrate by keeping the pump RPM constant. The number of vanes are also important to maintain the flowrate of the centrifugal pump.

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