Investigation of Severe Water Hammer in a Large Pump Station - Case Study

The Hoppers Crossing Pumping Station comprises two wells, each of which contains four pumps. Both wells may be considered as separate hydraulic entities as they are each supplied by separate suction lines from the penstock chamber. Within a well, each pump discharges into a rising main, which rises vertically for 40m. A pair of rising mains then discharges into a common discharge main that runs horizontally before discharging into the delivery chamber. Each pump can discharge up to 8m 3 /sec. An incident during commissioning tests demonstrated the potential for severe transients to occur in the rising mains. The particular incident occurred during rapid pump delivery valve closure on one pump. Investigation of the incident included detailed pressure and flow measurements under gentle transient conditions and the development of a numerical model. The model was verified against the detailed field measurements. The investigation showed that the cause of the incident was column separation in the rising main. No structural modification could be practically introduced to protect the system against column separation. Numerical simulations were then performed that lead to the development of valve operating protocols to avoid unacceptable transients. It was shown that unacceptable transients could be avoided, even during rapid total valve closure, if the valve was closed very rapidly over the first 72% and slowly over the final 28% of closure.