Risk Management in Real Time—Transient Control Testing on a Large Conveyance System

Minneapolis Water Works (MWW) is a 160 MGD (million gallons per day) municipal water utility utilizing the Mississippi River as source water. The MWW has several large pumping stations and transmission mains, including Pump Station No. 4 (PS 4) which consists of six pumps, 5,200 ft. of 66 in. diameter suction pipeline and twin 17,000 ft. discharge pipelines with 50 and 54 in. diameters. The pump station was recently refurbished and improvements included dual 2,100 cu-ft surge tanks for surge protection. Transient modeling performed during the design of PS 4 refurbishment led to the recommendation to install a 4,200 cu.-ft surge tank on the discharge side of the pump station and a standpipe on the 66" diameter suction main. Additional analysis during detailed design found that dual tanks with a total volume of 4,200 cu-ft (in lieu of a single tank) would provide adequate protection in the event of power failure The additional analysis also supported the recommended installation of two combination air/vacuum relief valves (CARVs) in the last few thousand feet of the 50" and 54" transmission mains to control negative pressures. MWW chose not to construct a standpipe on the 66" diameter suction main or the recommended CARVs. This decision was based on the history of reliable service from the pipelines. In lieu of these additional control measures, MWW decided to perform field testing on the renewed transmission system in order to define the actual risk from transients. Functional testing was performed in August, 2011 with the objectives of comparing measured system performance to predicted performance and using field data to evaluate risk at specific locations, particularly with regard to predicted negative pressures. It was found that the model results were generally conservative when compared to the collected data, although data acquisition rates may have skewed the findings somewhat. Negative pressures near the pipeline terminus were found to be less severe than predicted, while the vacuum pressures occurred as predicted in the suction pipeline. The results of the testing are currently being used to develop an overall transient risk approach for the pipeline.