Design Development and Final Evaluation of an Air Entrainment Reduction Alternative for a Deep Well Tunnel Dewatering Pump Station

The New York City Department of Environmental Protection (NYCDEP) owns and operates several raw water supply tunnels as part of their overall water supply infrastructure. Inspection, repair, and rehabilitation activities require continuous dewatering of the tunnel to maintain a safe work environment due to infiltration. During the design of a new dewatering pump station, NYCDEP utilized a physical model study of the station to ensure that no hydraulic conditions would exist that would adversely affect pump performance. The model study also revealed that vertical drop pipes immediately upstream of the station were delivering a significant amount of air to the pumps. While this source of air entrainment was identified during design of the station, the ability of the drop pipes to break the air up into miniature bubbles that saturated the pump sump water volume was not foreseen. Since the inclusion of air in pumped systems can lead to reduced pump performance, vibration, air binding, etc., elimination of this air saturated condition was necessary. The purpose of this paper is to review the initial physical model of the pump station and discuss the mechanisms that led to the air saturated condition within the pump sump. The various conceptual design alternatives identified as capable of eliminating the air saturated condition will then be discussed and the method for selecting the preferred alternative identified. Finally, the hydraulic design and performance of the preferred alternative in the physical model will be demonstrated. Overall, the information provided in this paper allows future designers the ability to identify a possible air saturated condition in similar designs and provides a basis for identifying potential design modifications required to eliminate it.