Page 158 - Water Loss Control

136 Cha pte r T e n

• Number of service connections
• Location of customer meters relative to the curb stop
• Number of households, population, and consumption
• Number of nonhouseholds and consumption
• Average zone pressure (at night, and 24-hour average)
• Numbers or frequencies of different categories of leaks and breaks
• Average duration of each category of leaks and breaks (depending on utility
policies for leak detection and repair)

Although it might seem on face value that most of this data would be readily avail-
able many utilities do not have good pressure data available.
Since pressure has a significant impact on the leakage flow rates and subsequently on
the annual volume of real loses it is necessary to accurately assess the average system
wide pressure.
The average zone pressure (AZP) is a surrogate value for the average pressure that the
average leak within a distribution zone will experience. AZP can be used to determine
the average flow rate for a given type of leak within a distribution zone. AZP is there-
fore a key parameter in real loss component analysis. Unfortunately, some leakage prac-
titioners and researchers try to interpret leakage data without measuring or assessing
an AZP pressure, and use inlet pressures or critical point pressures instead; the model-
ing results then become unreliable to a greater or lesser extent.

Calculating AZP and Identifying an AZP Measurement Point
There are several ways to calculate AZP and identify an AZP measurement point. Where
network analysis models are available, this calculation can be based on node point data,
weighted by number of service connections. Alternatively, if hydrant pressures are
recorded, the average pressure can be estimated by taking an average of the hydrant
pressures recorded. Another method is to allocate numbers of service connections (or
properties, or hydrants) or mains lengths within contour bands, and obtain a weighed
average ground level for the selected type of infrastructure.
Once the weighted average ground level, or weighted average pressure has been cal-
culated, a hydrant that experiences that pressure in the center of the zone can be selected
as the AZP point for measurements, when field tests are being undertaken. It may be
necessary to consider seasonal variations in average pressure throughout the year, if there
are significant seasonal variations in demand causing seasonal pressure changes.

10.5.2 Coefficients and Default Values
Most statistical models use coefficients and default values developed from series of
field testing. It is important; however, that the operator understands the nature of the
coefficients and default values, how and why they were applied to the calculation so
that they make any necessary changes for local conditions.
Coefficients and default values often used may include

• Typical flow rates of each category of leaks and breaks at some standard pressure
(normally 70 psi or 50 m).
• Typical background leakage for mains if in good condition (per mi/hr, at some
standard pressure, this can be measured in an area where all locatable leaks
have just been repaired—see ICF calculations in Sec. 10.6).