Centrifugal Pump
Centrifugal pumps make use of centrifugal force to work. The liquid is usually water. The water is led in through the center of the impeller. The impeller is rotated at a high speed so that the water gets thrown outwards with high velocity.
The fast moving water is then slowed down. For a volute centrifugal pump, expanding its volume slows the water down. The water is led through a volute shaped housing that gradually expands its volume. In the process, the highly energetic water is converted to pressure.
Another method of achieving the conversion to pressure is by changing the direction of the streams of fast moving water from the impeller. This can be done using diffuser blades. This type of centrifugal pump is called turbine centrifugal pump.
Whatever the method used, it is essential that the pump housing be completely filled with water. The water needs to flow in a solid stream. Any air in the housing will be compressed, and the conversion to pressure will be lost. Filling the pump housing completely is called priming. If there is water flowing though the pump housing, it can be achieved simply by releasing air at the top of the housing.
When the suction water level is higher than the pump, priming is not a problem in most cases. Submersible pumps are designed to be placed such that the impeller is always immersed. Self-priming pumps have a special pocket for ensuring that the impeller is always immersed. Sometimes special positive displacement suction devices are added to centrifugal pumps for priming purposes.
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