Open centrifugal pump impellers are utilised within radial flow centrifugal pumps. This type of impeller is very well suited to handling slurries and other liquids with a high amount of suspended bodies e.g. sewage.
The largest advantage with this impeller is that it does not block easily as there are no areas with small clearances.
Compared to the semi-open and closed impellers, this impeller is very inefficient as the flow is not guided between the impeller vanes.
The impeller vanes must be quite thick in order that they do not break or bend during operation, this leads to a reduced flow path and thus a reduction in efficiency.
Open impellers do not have any shrouds and are thus structurally quite weak.
Adequate space between the impeller and casing must be maintained in order to avoid damaging the impeller or casing. The gap between the impeller and casing represents a leakage path from the discharge to suction side and leads to a drop in efficiency.
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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