Page 328 - Water Loss Control

Contr olling Real Losses—Speed and Quality of Leak Repair 297

easy to locate and are repaired quickly, actually only contribute a small volume to the
annual real loss figure. The reason for this is that, even though they usually have a high
leakage flow rate, the utility responds quickly to such an event and the pipe section of
the break is promptly shut down, therefore the leakage loss volume is relatively small.
Conversely, small leaks, especially on service lines, generally contribute the greatest
volume to the overall real loss volume due to their average long run time. Small leaks
can run for periods of weeks, months, and even years before being discovered and
repaired.

17.2.1 Reducing the Awareness Time
Some leaks are reported and others are unreported and whether a leak is reported or
unreported has an impact on the total leak run time, especially the awareness time.
The time it takes to become aware of an unreported or a reported leak is significantly
different. Reported leaks have a short awareness time since they either become visible
on the street or ground surface (sometimes in form of catastrophic failures) and are
reported to the utility, or they cause a drop in supply pressure and again are reported
quickly to the utility.
Unreported leaks, however, can run for very long periods of time (up to years)
before they become big enough to surface, cause a catastrophic failure, and the like, and
therefore become reported leaks.
There are two activities that help reduce the awareness time of unreported leaks.
1. Active Leak Detection: Conducting an active leak detection campaign covering
the entire distribution system once a year reduces the awareness time of an
unreported leak to on an average 6 months. Doubling the intensity of active
leakage control effort, that is, completely sounding the system every 6 months
instead of every year, would reduce the duration for which the unreported leak
runs to an average of about 95 days, reducing the leakage volume related to
unreported leaks by half. However, a reduction in activity to sounding the system
every 2 years would allow breaks to run for an average of 365 days before their
location and repair, doubling the losses resulting from sounding the complete
system annually. This illustrates why the detection of unreported breaks can be
of such importance to a water service provider. The various frequencies of leak
detection sounding carry differing levels of cost—personnel, equipment, and
materials—to implement, and these costs must be compared with the value of
the water that would be either saved by a greater frequency of active leakage
control activity or lost due to a lower level of activity (see Chap. 9).
2. District Metered Areas (DMA): Dividing the distribution system into small
hydraulically discrete zones where the total inflow is monitored continuously
allows the water utility to become aware of a new leak shortly after it emerges in
the DMA. With a DMA in place the utility can analyze the minimum nighttime
flows (MNF) on a daily or weekly basis to identify the emergence of new leakage
as indicated by an increase in MNF. The size of the leaks that can be identified by
DMA analysis depends on the size of the DMA—the smaller the DMA the smaller
the leakage events that can be discerned. A leakage management strategy
including both DMAs combined with active leak detection is usually a more
efficient approach than regular sounding alone, but a combined approach incurs
a higher capital cost to create and install the DMA.