Semi-Open Impeller


Semi-open impellers (sometimes called 'partially open', or, 'semi-closed') are utilized within radial flow centrifugal pumps. Unlike open impellers and closed impellers, semi-open impellers have only one shroud, which is mounted on the front or back of the impeller. This type of impeller is very well suited to handling liquids with a moderate amount of suspended bodies.

Radial Flow Centrifugal Pump

Radial Flow Centrifugal Pump


Does not get easily blocked/clogged.

Good compromise between the open and closed type impellers.


Axial thrust created by the impeller can be quite high which may require thrust bearings to be installed.

How Impellers Work

The below video is an extract from our Introduction to Centrifugal Pumps Online Video Course.


Bernoulli's principle states that if a steady flow is exposed to a change in area, then the pressure and velocity will change correspondingly.

Example 1

If a pipe diameter increases, the pressure will increase, but the velocity will decrease.

Example 2

If a pipe diameter decreases, the pressure will decrease, but the velocity will increase.

Impellers are designed based upon Bernoulli's principle and the relationships between area, pressure and velocity.

Flow through the impeller vanes is radial. The impeller creates a negative pressure at the impeller eye (centre of the impeller) and this negative pressure draws liquid into the impeller. The liquid is thrown outwards radially due to the centrifugal force imparted onto it from the impeller. As the liquid flows through the vanes, the flow path area increases and there is a velocity decrease and a pressure increase.

Semi-Open Impeller

Semi-Open Impeller

Notice on the above image that the distance between the channels (area between vanes) gradually increases as the vanes stretch towards the outer periphery. This gradual increase in area gives a gradual decrease in velocity and increase in pressure. The purpose of a volute casing and diffuser is to continue this velocity to pressure change in order to maximize the pressure and reduce the velocity as much as possible.

Impeller Surrounded by Volute Casing

Impeller Surrounded by Volute Casing


Additional Resources

Encyclopedia - saVRee