Civil Engineering Reference
In-Depth Information
In addition to implementing conservation at the tap, water purveyors should pursue
efforts to identify losses in the distribution system on an ongoing basis. A survey of
California water utilities showed that unaccounted-for water ranged from 2 to 30 per-
cent of all water use.
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Unaccounted-for water often includes losses from malfunc-
tioning or inaccurate water meters, leaks from the distribution system, overflows from
storage tanks, releases from pressure relief valves, and various unmetered uses.
Experience indicates that utilities should try to restrict their unaccounted-for water
to no more than 5 percent, although many utilities set a target of 10 percent. In service
areas where the infrastructure is relatively new and water is becoming more precious,
prudent management is needed to maintain the integrity of water distribution systems
and strive for low system water losses.
A leak-detection program can identify distribution system losses. A water utility
may need to invest in additional meters to monitor where the major demands or losses
are occurring in the system. Leaky pipes and fittings can cause water losses of 10 to
50 percent within a distribution system.
A utility should track water consumption regularly and flag any suspected higher-
than-normal demands in order to identify potential problems. Spillage from tanks or
a float control valve stuck in the open position on a reservoir can go unnoticed and
generate a considerable demand. Localized leaks from buried pipes often show up as
wet or swampy areas in unexpected locations. A faulty check valve on a well can
result in a backflow from the system into the ground.
Example
Consider an example involving a 10-mgd (38-ML / d) plant where water conservation
yields a 10 percent (1-mgd, or 3.8-ML / d) reduction in demand. Assume the discharge
pressure is 60 psi, the wire-to-water efficiency is 70 percent, and energy costs are
$0.07 / kWh. The horsepower required for this 1-mgd demand can be calculated as
follows:
QH
horsepower
3,960
efficiency
694 gpm
2.31 ft head
1 mgd
60 psi
mgd
psi
35 hp
3,960
0.70
where:
Q
flow, gpm
H
pump differential pressure, ft
By converting horsepower to kilowatt-hours, the resulting annual savings can be
determined:
0.746 kW
24 hours
annual savings
35 hp
hp
day
365 days
$0.07
$16,000 / year
year
kWh
