Types of Volutes

Volutes: The word volute actually describes a specific type of pump casing that converts energy created by the impeller into pressure. The impeller pushes water into the volute which converts that energy into pressure and directs the flow toward the discharge point. In the picture on the right notice that the impeller is not located in the center of the volute. This is intentional. The portion of the volute that extends closest to the impeller is called the “cutwater”. It is the point where the flow is forced to exit through the discharge point rather than continuing to swirl around the impeller. The gradually increasing distance between the volute and casing and the direction of rotation of the impeller (noted by the arrow above the volute) combine to force the water around the volute in a counter-clockwise direction in the pump section shown, and once the flow reaches the cutwater it is forced to exit the volute.
Diffusers: There is a second way that is often used in centrifugal pumps to convert energy into pressure. This is through the use of a diffuser. A diffuser functions similarly to a standard volute in that it contains vanes that begin close to the impeller and then gradually extend away from the impeller periphery. However, unlike the volute casing a diffuser casing may contain many vanes. In the case of the assembly drawing shown the diffuser contains 10 vanes as compared the volute casing which only has one.