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138 Cha pte r T e n

10.6 Modeling Background Losses
Background losses are individual events (small leaks, weeping joints, and the like)
with flow rates too low to be detected by visual inspection or traditional acoustic
leak detection techniques. They will continue to
flow unless either detected by chance or until they
If you are using a model gradually worsen to a point where they can be
from another region or country detected. The level of background leakage tends to
always ensure that the con- increase with increasing age of the network and is
cepts and coefficients applied higher for systems operated at higher pressure.
in the model are applicable to The type of pipe materials and jointing techniques
are also factors contributing to the level of back-
your system.
ground losses. It is important when modeling
components of real loss to separate out back-
ground loss from other components as the tools used to reduce background losses
are limited. Managing and reducing pressure is an effective option for reducing
background losses in well-maintained systems. In most cases, it is also a lower cost
option than the alternative of infrastructure replacement, however, often the latter
is a good long-term investment.
Table 10.7 provides flow rates for unavoidable background leakage (UBL) at a stan-
dard pressure of 70 psi, or 50 m; UBL corresponds to an infrastructure condition factor
(ICF) of one.
Another common error in modeling background leakage is to assume that UBL var-
ies linearly with pressure; this misassumption arose because of the way the data were
presented at standard pressure, in a table in the original paper. In fact, the standard
2
modeling assumption, based on available reliable data from various sources, is that
UBL varies with pressure to the power 1.5 (FAVAD N1 = 1.5).
Once the UBL values in Table 10.7 have been corrected for pressure, using a FAVAD
N1 of 1.5, they must be multiplied by ICF. The ICF is an unknown factor to most utilities
and without carrying out tests, it is difficult to estimate the ICF. Field tests used to esti-
mate the ICF can only be undertaken in small zones temporarily or permanently estab-
lished for the purpose of measuring minimum night-time flows and pressures. Methods
available to estimate the ICF are

Infrastructure Background Leakage
Component at ICF = 1.0 Units
Mains 2.87 gallons per mile of mains per day per
psi of pressure
Service connection— 0.11 gallons per service connection per day
main to curb-stop per psi of pressure
Service connection— 4.78 gallons per mile of service connection
curb-stop to meter per day per psi of pressure

Source: Adapted from Water Loss Control Manual 1st ed.
TABLE 10.7 Unavoidable Background Leakage Rates