Page 345 - Water Loss Control

314 Cha pte r Ei g h tee n

Using the internationally accepted average N1 of 1.15, our sample calculation of
reduced leakage changes too:
L = L (P /P ) 1.15 = New leakage = 500(50/80) 1.15 = 500 − 291 = 209 gpm
1 0 1 0
It can be seen that we have an additional saving of 104 gpm. This additional saving
is a function of the changing area of the leak(s) in the second example. We must there-
fore conclude that other than for systems with 100% fixed area leakage (which is very
hard to find), the traditional method of calculating potential savings from reduction of
pressure, is to say the least very conservative and misleading.

18.4.3 Background Leakage
While many utilities are undertaking very efficient leak assessment, detection, and
repair, there still remains an element of leakage, which is undetectable. This is often also
referred to as background leakage. This leakage is made up of many small pinhole
leaks, joint leaks, drips, and the like, which cannot be detected by traditional means.
The only efficient way of reducing the impact of background leakage, (other than infra-
structure management interventions such as mains and service replacement program,
as discussed in Chap. 19), is to efficiently control pressure.
High background leakage will often be found in systems with high service density,
high hydrant density, or systems where maintenance is difficult because of a highly
urbanized situation.
18.4.4 Reduction of New Leak Frequency
Pressure management helps to reduce not only the volume of leakage and background
leakage, but also reduces the frequency of new leaks occurring. It should be noted that
pressure is not the only influencing factor in the frequency of new leakage, however, it
is often a significant one. Other factors may include ground conditions, traffic condi-
tions, pipe material and condition, stray currents, temperature, and backfill. A method
for estimating the reduction in break frequency due to reduction in pressure is shown
in Chap. 10.

18.5 Overflow Control
When discussing pressure management and its impact on water loss it is important to
also discuss level management in reservoirs, tanks, and storage.
Water loss from overflows in storage facilities is
too often overlooked, as it is deemed not to be sig-
Pressure management nificant and often tanks are in out of the way places,
includes the management so overflow is not always evident.
of reservoir and tank levels, Overflows usually occur at night (when pres-
sure conditions are often at their highest due to lack
which can often be the source
of demand and head loss on the system) and are
of considerable annual losses.
caused by either lack of level controls or malfunc-
tioning controls. Level control can be performed
manually by pump control, by SCADA, which involves automatic control by computer-
linked software, or by simple hydraulic control, using either altitude valves or ball
valves. Sometimes a utility will have a sophisticated series of automatic controls, how-
ever, external forces such as lightening may affect them. A simple hydraulic backup is