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Basics of Centrifugal Pump


Centrifugal pumps are the most commonly used turbo machinery devices. They are used to raise the pressure or induce flow in a control volume. Centrifugal pumps are radial flow devices. Various kinds of centrifugal pumps are available in the market with different construction details. But working principle behind all of them remain same



Working of Centrifugal Pumps


One of such pump (single suction, semi-open) is shown in figure below, with one part of its casing removed for ease of understanding.



Working of centrifugal pump is simple; as the impeller rotates it creates very low pressure at inlet of the impeller, called as eye of impeller. This low pressure helps in sucking fluid surrounding in. The fluid is pushed radially along the impeller to the casing. Casing collects the fluid , and it is pumped out through discharge nozzle.These processes are shown schematically in following figure. We will go through main components of a centrifugal pump in a detailed way.



Impeller

Impeller is the device which rotates, and transfer energy to fluid. It has got collection of vanes fitted to a hub plate. Shape and geometry of impeller blades are critical in pump performance.




Casing


Casing collects fluid from impeller in an efficient way. The casing has got a special shape, with area of cross section increases from inlet to outlet. As the impeller ejects fluid throughout casing, along length of casing mass flow rate increases. But, increasing area of casing helps in maintaining almost same velocity. Thus volute shaped casing helps in converting dynamic part of fluid energy to static part.


Construction Details of Casing



Casing is made on 2 volute curves, which are at offset. A three dimensional volute is made from this curves. A portion is removed from volute shape, in order to accommodate the impeller in it. A discharge nozzle is fit at exit portion of the casing, most of the time discharge nozzle is diverging in shape. The steps followed are shown in following figure.



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