Night Flow Analysis & Step Testing
Night Flow Analysis can be used to determine the differences in consumer night use and water loss for a defined area. Differences in nighttime flow rates between 2:00 a.m. and 4:00 a.m.can assist in determining water leak quantities in a district-metered area (DMA)*. Nighttime flow analysis in conjunction with district-metered areas will provide the municipality with functional metering information to assess system water use and leakage flow rates. The utility can prioritize its active leak control program in areas that experience sudden or elevated nighttime flow rates within a DMA. Step Testing is a term used for isolating sections of a DMA by closing valves in the network of pipe. (The “Step” refers to the drop in flow on a graph if a leak is isolated from the flow rate being measured.)
*District Metered Areas
District-metered area (DMA) also referred to as Zone metering, allows utilities to evaluate water losses. DMAs can define various types of metered areas such as industrial, commercial, and residential but more often a DMA typically refers to a distinct pressure zone of the system (a pump system or pressure-reduced area). Zone metering allows a utility to analyze water use and determine water loss within a specific DMA of the distribution system. Water loss programs can be assessed from DMA analysis by comparing “Night Flow” rates among the zones to determine priorities for leak detection mitigations. DMAs can also be effective in capturing the benefits/results of a pressure management strategy and allowing utilities to conduct a water loss management audit.
The American Water Works Association (AWWA)
The AWWA M36 manual has an extensive section on NFA & Step Testing. In a simplistic explanation the procedure is to determine a minimum night flow at a source meter or reservoir. IKEN would typically measure reservoir depths to create volumetric calculations if a source meter is not available. With night flow information being acquired the step test procedure would be to isolate sections of the town. The diagram below depicts DMA meters. The step tests would take place if DMA meters were not available.
For example; with a steady drop at the reservoir due to leakage, if the section with excessive leakage is isolated, the drop at the reservoir will be less for that period of time.