An Experimental Study of Air-Entraining Vortices in Pump Sumps

Abstract

The factors affecting the formation of air-entraining vortices in sumps are enumerated, and their relative importance assessed from experiments with a number of different sumps. It is established that vortices are caused by rotation of the mass of water within the sump arising primarily from the entry conditions. Their severity, measured in terms of the minimum safe submergence to avoid air entrainment was also found to depend greatly on the velocity in the suction inlet; air entrainment rarely occurred below a velocity of 2 ft. per sec., and above 15 or 20 ft. per sec. the severity of the vortices did not increase substantially. The shape and disposition of the suction inlet only slightly affected the vortices, but sump boundaries were found to reduce appreciably the tendency for vortex formation when closer than 8 diameters to the suction pipe: with the walls very close to the pipe, vortices did not occur. Small sumps may thus be more satisfactory than spacious ones. Tests with scale models of existing or proposed pump installations are described, and for one sump, where vortices formed within the working range, it is demonstrated that suitably placed boundary walls would be considerably more effective in preventing vortices than modifications to the suction inlets. Measurements of the quantity of air drawn into the pump through a vortex showed that air entrainment of this type could seriously impair pump performance. Severe vortices could lead to depriming of pumps operating with suction lift.