The simplest type of centrifugal pump is the single stage machine that consists fundamentally of a rotating element, called an impeller, and a casing. Liquid is led to the eye or center of the impeller and is set into rotation by the impeller vanes. By virtue of centrifugal force the liquid is thrown from the rim or periphery of the impeller with a considerable velocity and pressure. The casing, which closely surrounds the impeller, has a volute shaped passage of increasing area, which collects the liquid leaving the impeller, and converts a portion of its velocity energy into additional pressure energy. This casing passage leads to the discharge nozzle of the pump where it is forced into the discharge piping.