This type of centrifugal pump is the most commonly used. The impellers may be open, closed or semienclosed, single suction, double suction, or nonclogging. They discharge into casings that are progressively expanding spiral designs of one or more stages (multistage). The casings housing the rotating elements may be vertically or horizontally split, and a few designs have casings divided on an angle from the horizontal. Pumps in this class usually have a specific speed below 4,000 rpm with single-suction impellers and a specific speed of 5,000 rpm with double-suction impellers.