Types of casing in centrifugal pump

 

             Casing is a Receptacle for enclosing the impeller both at the suction and delivery end so as to form a pressure vessel. Casings also provide supporting and bearing mediums for the shaft and impeller assembly.

Casings are of two types :-

1. Volute Casing
2. Diffuser casings.

 

Volute casing

             A volute is a curved funnel increasing in area to the discharge port. As the area of cross section increases, the volute reduces the speed of liquid and increases the pressure of the liquid.

Image by Intro to Pump via https://www.introtopumps.com/wp-content/uploads/2012/11/Volute-Pump-Assembly.png

           One the main purposes of the volute casing is to help balance the hydraulic pressure on the shaft of the pump. However, this occurs best at the manufacture's recommended capacity Running volute style pumps at lower capacity than the manufacturer recommends can out lateral stress on the shaft of the pump, increasing wear and tear on the seals and bearings, and on the shaft itself, Double volute casings are used when the radial thrusts become significant at reduced capacities.

 

Diffuser casing

              Diffuser casing have stationary diffusion vanes surrounding the impeller periphery that convert velocity energy to pressure energy. Conventionally , the diffusers are applied to multi-stage pumps.

            The casings can be designed either as solid casings or split casings. Solid casings implies a design in which the entire casing including the discharge nozzle is all contained in one casting or fabricated piece. A split casing implies two or more parts are fastened together. When the casing parts are divided by horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis , the casing is described as vertically split or radially split casing . Casing wear rings act as the seal between the casing the impeller.

Difference between Volute and Diffuser Casing :-

	VOLUTE CASING	DIFFUSER CASING
1	UNSYMMETRICAL LAYOUT	SYMMETRICAL LAYOUT
2	SIMPLE CONSTRUCTION IN SINGLE STAGE PUMP	COMPLICATED DUE TO STATIONARY ANES IN SINGLE STAGE PUMP
3	RADIAL THRUST AT PART LOADS AND OVER LOADS THRUST SHARPLY INCREASES AS HEAT INCREASES	NO APPRECIABLE RADIAL THRUST BUT HIGH AXIAL THRUSTS ARE THERE
4	QH CHARACTERISTIC IS MODERATE AND NON OVERLOADING TYPE STABLE OPERATION AT PART LOADS	DROOPING AND OVER LOADING TYPE QH UNSTABLE OPERATIONS AT PART LOADS
5	MOSTLY HORIZONTAL AND SINGLE	VERTICAL TYPE AND MULTISTAGE
	STAGE TYPE

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