Mechanical construction of a pump

Types of Impellers:
Closed impellers have vanes enclosed by shrouds;
Open and semi-open impellers;
Vortex pump impellers.

Purpose of Shaft :
The shaft transfers the torque from the motor to the impeller during the startup and operation of the pump. 

Function of Casing : 
Casings have two functions
The main function of casing is to enclose the impeller at suction and delivery ends and thereby form a pressure vessel.
A second function of casing is to provide a supporting and bearing medium for the shaft and impeller.

Types of Casings:
There are two types of casings
Volute casing  has impellers that are fitted inside the casings. One of the main purposes is to help balance the hydraulic pressure on the shaft of the pump.
Circular casing has stationary diffusion vanes surrounding the impeller periphery that convert speed into pressure energy. These casings are mostly used for multi-stage pumps. The casings can be designed as solid casing (one fabricated piece) or split casing (two or more parts together)

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