Semi-open (sometimes called ‘partially open’, or, ‘semi-closed’) centrifugal pump impellers are utilised within radial flow centrifugal pumps. Semi-open impellers have only one shroud, which is mounted either 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.
Does not get easily blocked/clogged.
Good compromise between the open and closed types of impeller.
Axial thrust created by the impeller can be quite high thus requiring thrust bearings to be installed.
Bernoulli’s principle states that if a steady flow is exposed to a change in area, then the velocity and pressure will change correspondingly.
If a pipe diameter increases, the pressure will increase but the velocity will decrease.
If a pipe diameter decreases, the pressure will decrease but the velocity will increase.
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 thus there is a velocity decrease and a pressure increase.
Notice on the below 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 maximise the pressure and reduce the velocity as much as possible.
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