A volute casing is a passage, which surrounds impeller or diffuser, if later is also used. Fluid enters into it tangentially from different impeller (or diffuser) passages and moves into volute casing. Volute casing is designed for constant velocity of flow. It changes direction of flow from successive passages so that fluid leaves the casing finally at the exit section of the pump. (It is also possible to take fluid from each diffuser channel directly to the place of need). These requirements are met by a spiral casing, whence the name spiral casing. Normally a volute casing is enough, since magnitude of velocity in pump is not high. The volute casing is also called the scroll casing.