The two ways of assessing real losses explained in the previous sections can be generalized as desktop analysis. However, an MNF analysis uses field test data to quantify the volume of real losses within the distribution network. The results can be directly compared with the volume of real losses obtained from the top-down water balance. A DMA is required in order to conduct MNF measurements. A DMA is a hydraulically discrete part of the distribution network that is isolated from the rest of the distribution system. It is normally supplied through a single metered line so that the total inflow to the area is measured (Fig. 5.2).
General DMA setup
General DMA setup
The MNF in urban situations, usually occurring between 2 a.m. and 4 a.m., is the most meaningful piece of data as far as leakage levels are concerned. During this period, authorized consumption is at a minimum and, therefore, leakage is at its maximum percentage of the total flow. The estimation of the leakage component at minimum night flow is carried out by subtracting an assessed amount of legitimate night-time consumption for each of the customers connected to the mains in the zone being studied. Typically, in European and North American urban situations, about 6% of the population will be active during the minimum night-time flow period. This activity is typically to use a toilet and the water use is almost totally related to the flushing of WC cisterns, although it can include substantial amounts of irrigation at certain times of the year. Analysis of minimum night-time flows therefore also requires the use of sophisticated techniques to determine legitimate night use. If it is known that there is significant or non-normal night use, otherwise known also as exceptional night use, within the zone, then this must also be estimated or measured by for example carrying out meter reading tests during the minimum night period.
The result obtained from subtracting the assessed night use and exceptional night use from the minimum night-time flow is known as the net night-time flow (NNF) and it consists predominantly of physical losses from the distribution network.
After completing these three initial components of a water loss control program it is now necessary to transfer volumes into values in order to determine the economic optimum volume of leakage.
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