Page 159 - Water Loss Control

Modelling W ater Losses 137

• Typical background leakage for service connections if in good condition
(at some standard pressure can also be measured as above—see ICF
calculations in Sec. 10.6).
• Typical numbers of residents using toilets at 3 to 4 a.m. each morning (or other
relevant minimum night-flow period).
• Typical toilet flush volume (toilet use is one of the largest residential individual
uses and the most common use of water at night other than in areas where
irrigation is being undertaken).
• Typical toilet leakage.
• FAVAD N1 values for different types of leaks and pipe materials.
• FAVAD N3 values for pressure dependant and nondependant consumption.
• ICF values for estimating background leakage volumes and separating them
from reported and unreported breaks volumes.

A simple example showing the need for care when applying coefficients and defaults
values is shown below:
A night flow analysis model is used to estimate the amount of leakage present in a
zone. The zone consists of residential properties and no commerce or industry (infra-
structure and system data).
One of the key factors in this model is to identify estimated legitimate night con-
sumption and subtract it from the night flow. To do this the model makes some assump-
tions based on preprogrammed coefficients and default values. In our example, the
model was built in the United Kingdom and is being applied in the United States.
The model assumes that most of the use at night in a residential zone is from toilet
flushes. In our example, the toilet flush volume was 1.5 gal per flush (default value).
However in the zone in which the model is being applied the toilets have not been ret-
rofitted and the flush is really 4 gal (default value).
So the model will ask for the population in the zone and multiply this by the esti-
mated number of people active at night. Let's say 6% (coefficient) during our analysis
window of 3 to 4 a.m. in the morning.
If the population in our zone were 6000 (infrastructure and system data) then the
model would assume that 6% were active at some time during that period, which would
be 360 active flushes.
The model then identifies the flush volume from the default value and multiplies
this by the number of active flushes. In our example this would be 360 flushes multi-
plied by 1.5 gal per flush, which would equal 540 gal used between 3 and 4 a.m. in the
morning, which is 540 gal/hr or 9 gal/min.
However a closer estimation using the correct flush volume would be 360 flushes
multiplied by 4 gal per flush, which would equal 1440 gal, used between 3 and 4 a.m.
in the morning, which is 1440 gal/hr or 24 gal/min.
If the measured night flow was 50 gal/min (field data) the model would then sub-
tract the estimated legitimate usage and identify the rest as leakage. If the coefficients
and default values were incorrectly applied as shown above the model would identify
the example zone as having 41 gal/min of leakage, where as really it would only have
26 gal/min of leakage.
Then there are the allowances for leaking toilets; what percent of households have
leaking toilets, what is the typical leak flow rate. Leaking toilets are a significant com-
ponent on night consumption in North America and many other countries.