Submergence requirements of high-specific-speed pumps that take suction from open sumps depend upon vortex formation in the sump and resulting air entrainment in the pumped liquid. Two interrelated requirements must be met in sump design. The sump must be of minimum size without affecting adversely the pump performance with respect to efficiency and head. Laboratory studies have been made on the effect of side-wall and bottom clearances, relative to the suction bell, on the pump efficiency, and also upon the submergence requirements to avoid vortex formation with air entrainment. Results show that side-wall and end-wall clearance of one quarter to one half the suction-bell diameter, and bottom clearances of one half the suction-bell diameter will not affect the normal pump performance adversely. Minimum stable depths of sump liquid also were obtained with these spacings. Model tests according to Froude’s modulus do not yield quantitative results since frictional effects apparently influence the results. However, qualitative studies may be made with models to determine optimum sump configurations to avoid vortex formation.