Critical Considerations in the Hydraulic Design of a Lift Station with Un-assisted Siphon Recovery

A high capacity lift station has been used for flooding control and supplying cooling water uses for industrial/power plant water supply. The lift pump capacity varies, but it is not uncommon to have capacity of 500,000 gpm for each lift pump. For a lift station configuration with pumping over a levee or a barrier, a siphon recovery scheme is typically used to reduce motor size and satisfy economical lift station operation requirements. The resulting pumping head required simply equals the pool-to-pool lift head, plus the internal friction and minor losses of pump suction and discharge piping. In order to have a proper hydraulic design of a lift station with siphon recovery scheme there are several critical considerations to be made, including:

- Motor horsepower requirement to achieve initial self-priming

- Pump discharge nozzle minimum velocity to create siphon

- Evaluation of air coming out of solution during lift pump operation and its impact to the established siphon at pump discharge

- Sizing of vacuum breaker to prevent reverse flow in case of pump trip

This paper will address all the above and provide a case study on design evolutions of a large lift station for a power plant. For the case study, the lift station has four lift pumps, with capacity of about 200,000 gpm each. The paper will discuss various design considerations to satisfy these critical requirements. The paper will also provide a summary of good practices in lift station design including effective use of formed suction inlets in the lift pump suction as well as discharge arrangement to achieve a compact configuration.