Sacramento Lower Northwest Interceptor Transient Analysis
Sacramento Regional County Sanitation District
City of Sacramento, CA
The Sacramento Regional County Sanitation District (SRCSD) is constructing a 19-mile long pipeline from the rapidly growing northern and western suburbs of Sacramento to the existing Regional Wastewater Treatment Plant (WWTP) in the south. Five pipeline segments, three tunnels, and two pump stations comprise the $300-million project that is called the Lower Northwest Interceptor (LNWI). At maximum capacity the LNWI will transport up to 221 MGD of sewage to the WWTP in Elk Grove where it will be made suitable for discharge to the Sacramento River.
Early in the design process the program manager and pump station designer proposed installing multiple large pressurized surge tanks at each pump station to eliminate the possibility of vapor cavity formation that was predicted following loss of power to the pumps. The present value of the total capital and life cycle costs associated with this approach were estimated to be $24.9 million. Flow Science was retained to evaluate surge tank and other alternative surge control strategies (e.g., flywheels, sewage vacuum relief valves, open surge tank) for the LNWI. The results of this analysis showed that the installation of spring-loaded sewage vacuum relief valves, each with a controlled venting feature, at the pump stations and at several locations on the pipelines would (1) prevent leakage during initial low-pressure operation, and (2) eliminate the possibility of vapor cavity formation in the pipelines following power failure at the pump stations.
The program manager estimated that the present value of the total capital and life cycle costs for the spring-loaded sewage vacuum relief valve approach would be only $2.7 million. Compared to the costs associated with installing and operating pressurized surge tanks, the spring-loaded sewage vacuum relief valve approach represents a cost savings of approximately $22 million for the SRCSD and has been adopted for construction.