Background Leakage and Backlog Removal

Background leakage is generally defined as the leakage below the level of detection (with current technology). The level of background leakage can be assessed using a number of methodologies.7 However, the level of background leakage is a function of the extent and method of leakage detection employed, which itself will have different operating costs associated with different levels of leakage. Therefore, a matrix of leakage detection costs versus level of background leakage can be derived, from which a view can be taken on the appropriate economic method of detection, and the associated level of background leakage.

Background levels of leakage have been related to system characteristics.9,10 Such as pressure, length of mains, and number of connections. From these, unit background losses at standardized pressure have been estimated. These can then be related to asset type, material, age, and condition. From this work it is possible to provide an estimate of the background level of leakage that might be expected in an area. By comparing this to the actual minimum achieved on that area, a view can be taken as to whether background levels have been achieved or whether it is likely that there are leaks on the area that would be possible to find.

These leaks will have gradually accumulated on the system over a number of years, and are essentially hidden in accepted minimum historic night flows. The leaks may be on parts of the network that are not normally checked for leaks, for example, large industrial complexes, mains which are believed to have been abandoned, private supply pipes, complex road junctions. The number of backlog leaks and hence the associated repair bill can be substantial, but they are one-off costs and the cost benefit can be readily assessed. However, it may be appropriate to take other action, for example, pressure reduction (described later) to reduce the frequency at which the system is breaking in order to allow for this backlog to be reduced over a period of time within the current repair budget. Alternatively, it is possible that these could be considered as a one-off capital cost depending on local accountancy rules.

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