The centrifugal pump casing is the component of the pump that converts all of the velocity created by the rotating impeller into a controlled and stable flow and directs it out of the pump through the discharge point.

The most common type of casing is called a volute and it looks similar to a snail shell.

The impeller is placed within the volute. However, as you may have noticed in the image above, the impeller is not typically centered in the volute.

Instead, the impeller is positioned so that the outer diameter of the impeller is closest to the volute at the point just past the discharge. This point where the impeller is closest to the volute is called the cutwater.

Beginning from the cutwater, as we move around the impeller, the distance between the volute and impeller gradually increases until we reach the discharge point. This steady expansion of the area around the impeller means that pressure will build moving from the smallest clearance to the greatest and the increasing pressure will push the liquid out of the discharge point.

We’ve just touched briefly on the topic of pump casing design in this article. Additional information about the most common types of pump casings can be found in our article Fundamental Pump Components: Volutes, Casings, and Impellers.

Centrifugal Pumping Units

Centrifugal pumps aren’t much use on their own. They need to be combined with other equipment to be useful. The combination of equipment that renders a centrifugal pump operable is referred to as a pumping unit.

At a minimum, a centrifugal pumping unit includes at least two components: a pump and a driver.
In the majority of cases, the driver in a centrifugal pumping system is an electric motor. However, that isn’t always the case. Pumps can also be driven by other drivers such as natural gas engines or even steam turbines.

The most common pumping unit combines a centrifugal pump driven by an electric motor.

The image above is of a close-coupled end-suction pump. This is the simplest and most common type of centrifugal pumping unit. In this type of pumping unit the impeller actually mounts on the end of the motor shaft and the pump casing mounts right onto the face of the motor. With a close-coupled pumping unit, the motor bearings and shaft comprise the majority of the mechanical portion of the pump.

Another common pump design is the frame-mounted end-suction pump. With a frame-mounted end-suction pump, the pump does include a complete mechanical assembly and is coupled to a driver, both of which are mounted on a shared baseplate.