How can the bathtub vortex at the bottom of a pump suction tank be eliminated as the liquid level is drawn down?
A. Since a dry-pit pump is usually located some distance downstream from its piping inlet in the tank, the effect of these flow disturbances on the pump is not as severe as with wet-pit pumps. For example, local flow separation, swirl or velocity non-uniformities, although creating greater head losses at the inlet, may be dissipated in the approach piping to the pump. The main problem is usually entrainment of air (or other tank gases) due to free-surface vortices, as this air may collect in the piping (causing air binding) or cause degradation of pump performance. Preventing the formation of free-surface vortices at tank inlets to pumps allows the tank to be drawn to lower levels than would otherwise be possible. Additional information on this subject can be found in HI Standard ANSI/HI 9.8, Pump Intake Design. HI is the largest association of pump manufacturers in North America.
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