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District
Metering: A Means of Addressing NRW
by Steve Ditcham
Regional Manager - Asia Census Metering Systems
ABSTRACT
The use of bulk water meters at various points on a water distribution
network can help in identifying problem areas, i.e. those zones where
leakage or other problems are greatest. This then allows those areas
with the greatest apparent loss to be given highest priority.
The use of meters with higher accuracy, when installed at customer billing
points, will normally result in an increase in apparent flow totals,
due to their ability to measure lower flow rates.
The use of Remote meter reading systems can facilitate data collection
from sites, where access is difficult, such as locked buildings and
where meters are installed in pits, which are subject to flooding.
Automatic meter reading, while primarily installed for billing purposes,
allows an online check of meter condition and can rapidly identify meter
failure, vandalism and attempts at fraud.
DISTRICT METERING: A MEANS OF ADDRESSING NON-REVENUE WATER [NRW]
PROBLEMS
A water distribution network will, by definition, supply water to consumers,
both domestic and commercial/industrial. Depending on the country, varying
amounts of water are "lost" or "unaccounted for"
between the reservoir or storage facility and the consumer. Non-Revenue
Water (NRW) is expressed as the difference between the volume of water
entering the distribution system and the volume of water billed to customers.
Depending on the country, NRW figures can range from single figure percentages
to more than 60%.
NRW is made up of two components - physical losses and apparent tosses.
Physical losses comprise leaks in the pipes, valves and other components
of the network. They may be obvious, as in the case of a burst water
main or leaking valve stem, or less so if the leak is from a joint in
a buried pipe. They tend to be pressure-related, and an analysis of
NRW versus pressure can be useful in determining the magnitude of these
leaks. Illegal connections also contribute to the physical losses.
Apparent losses arise from the continued use of old consumer water meters.
Almost any type of water meter will under read with time: the magnitude
of this under-reading depends on the manufacturer, type and age of the
meter. Under-reading obviously has a negative impact on the revenue
accruing to the water utility and contributes to the overall NRW figure.
It should also be noted that certain types of meter may over-read with
time, depending on the condition of the water. The continued use of
these meters could mask a potential NRW problem, as the apparent sum
of the billed consumption may equal or exceed the amount of water entering
the network, whereas in fact an unknown amount of water may be lost
due to leakage or theft.
In order to localize the problem areas, it is recommended that the network
be divided into several District Meter Areas, (DMA), and that accurate
bulk water meters be installed at the entry point into these districts.
Ideally, the sum of the flow totals recorded by the district meters
should equal the quantity of water measured at the outlet of the water
source (reservoir or water tower), allowing for recognized meter uncertainties
as defined in international standards. If there are significant differences
between the source reading and the sum of the district meter readings,
the upstream network needs immediate investigation. However, as the
pipe sizes, flow rates and pressures are likely to be greater in this
part of the network, leaks are more likely to be obvious.
Once general agreement
is reached between the supply and DMA meter readings, attention can
then be turned to each individual DMA, addressing any difference between
the reading of the bulk meter and the sum of the readings of the consumer
meters in the area. Areas with the greatest losses can therefore be
addressed on a priority basis, allowing effective use of available manpower.
In some networks, district meters are already installed. In these cases,
a meter evaluation and, if necessary, replacement programme is an essential
part of the NRW reduction exercise. This should be implemented at an
early stage and should include meters installed at large commercial
and industrial customers which .may represent a significant source of
revenue for the utility. A detailed study needs to be conducted into
the age, size and type of meter installed. A meter change-out programme
has the combined benefit of (1) producing more reliable information
for the NRW evaluation team and (2) increasing the revenue for the utility,
as older under-reading meters are replaced with newer, more accurate
meters.
It should be obvious that the district meters should be of good quality
and performance. While most bulk water meters meet international standards
for initial accuracy, care should be taken to ensure that the meters
have good long-term accuracy. The utility's available maintenance personnel
would be better employed dealing with problems on the network than replacing
cheap but sub-standard and under-performing meters.
While absolute accuracy of water metering cannot be achieved-some degree
of uncertainty will always be present - the installation of properly
sized meters will allow optimal flow measurement. Data logging of existing
installed meters - assuming that the meters can provide the appropriate
outputs - can help determine whether the installed meter is sized correctly.
An under-sized meter will run too fast and may experience premature
wear or failure, while over-sized meters fail to register now flow rates,
leading to loss of revenue and adding to the overall NRW. When selecting
new meters, factors such as the consumption profile, flow range and
pressure loss must be taken into consideration. Care should be taken
that the meters have the capability to provide an output suitable for
connection to a data logger. Meter replacement should ideally be combined
with regular servicing.
One method of leakage control involves the continuous monitoring of
minimum night flows into defined areas (District Meter Areas - DMAs).
This is especially relevant in a residential area where one would expect
night flows to be essentially zero. Minimum night flow is used as a
measure and it comprises distribution losses (leaking glands, joints
and minor bursts) and night flow delivered (consumer's use and losses
within the consumer's supply pipe).
It is quite common to find Class B bulk meters installed at the billing
points of larger consumers. Depending on the flow profile of the consumer,
it may be beneficial to install a meter of higher accuracy or higher
turndown ratio. These meters may be either Class C or compound meters,
depending on the consumption profile and network pressure. The higher
cost of these meters is usually offset by the increase in revenue resulting
from the measurement of lower flow rates. These meters are not recommended
for use as district meters, as they are usually only available in smaller
sizes, have a greater head loss and the flow rates in the distribution
mains tend not to vary in the same way as they do at the billing points,
so the ability to provide greater low flow accuracy is not important.
REMOTE READING
Data collection is essential in the analysis of the water supply network.
It serves no purpose to install a meter and then be unable to collect
the data because the meter is behind a locked fence, inside a locked
maintenance area or in a flooded meter pit However, the number of meters
with such access problems in the meter reader's route may be few: the
remaining meters may be read manually by meter readers equipped with
hand-held terminals. A fully automatic meter reading system might not
therefore be justified.
Remote meter reading can be performed using a radio walk-by or a touch-pad
system. Obviously, in a touch-pad system, the interface has to be located
at an accessible point (outside wall, kerbstone or post) and wired to
the meter, whereas the radio transponder can probably be located adjacent
to the meter, depending on the transmission power and. distance between
the meter and the street.
Meters must have
an electrical output. They can be equipped with pulse emitters or with
absolute reading encoders. While reed-type pulse emitters are the most
common, they respond to a pulse whether the flow is in the forward or
reverse direction. If the flow is zero, pulses could still be generated
due to slight movements of the meter impellor resulting from pressure
or temperature changes in the line, or even from vibrations. Therefore,
a discrepancy between the meter and the remote reading device could
arise. Electrical interference, e.g. from pumps, can also generate spurious
pulses. If the connecting cable is broken, data will be lost during
the time the break is present, and reconciliation should be made once
the connection is remade. In addition, power - usually from a better
- must be supplied to the reed switch in order to recognize the contact
closure.
The preferred solution is to use an absolute reading encoder, which
requires no power: the circuitry is energized by hand-held terminal
only when the meter is read. A permanent connection is made between
the encoder register on the meter and an interface device, e.g. inductive
touch-pad, which can be located at an accessible point. A reading cable,
attached to the hand-held terminal, is touched onto the interface device
and thus the meter is read. If the meter cannot be read, there is usually
a connection problem. When this is corrected, and the meter read, the
meter index and the collected reading will be identical. There would
be no need to make reconciliation between the external reading and the
meter index reading. Readings from encoder registers are not affected
by external interference and any reverse flow would be taken into account.
AUTOMATIC METER READING
The utility may benefit from billing large consumers on a more frequent
basis, perhaps weekly: this would improve the utility's cash flow. In
these cases, the use of an automatic meter reading system may be worthwhile.
However, an additional benefit, and maybe a justification in its own
right, is that the meters can be read online, allowing daily monitoring
of these meters. This in turn allows fast identification of meter failures
or attempts at fraud or vandalism, which might otherwise remain undetected
until the next visit by the meter reader (whenever that might be). Whereas
this may not be significant in the case of a domestic meter, it can
be of great importance where a bulk meter is concerned.
Automatic meter reading can be performed using radio fixed network or
telephone-based systems where the consumers are widespread, or using
a hard-wired Bus system where the consumers are more concentrated, e.g.
in an apartment block. Whereas walk-by radio systems require the use
of the existing meter readers, fixed network radio and telephone based
systems require no site visit.
The economics of automatic meter reading systems vary greatly from country
to country and depend on social considerations, labour costs, water
costs, prevalence of fraud and vandalism and the frequency of meter
reading. A detailed examination of the justification is required. It
is often difficult to justify a full AMR system based on reading cots
alone, but it can be attractive once the intangible benefits, such as
improved read-to-bill times, are taken into account. One utility in
Asia increased their water revenue from AMR-linked consumers by over
25% due to early detection of meter failures, and incidence of fraud
and vandalism were virtually eliminated.
The use of an AMR system can also help improve the accuracy of the NRW
calculation of NRW, since data can be collected from numerous sites,
almost instantaneously.
CONCLUSION
Accurate and reliable bulk meters installed on the network greatly assist
in localizing problem areas, resulting in reduction of losses and improving
the overall management of the water network.
More sophisticated methods of data collection allow a clearer picture
of the real NRW situation and often lead to increases in the Utility's
revenue.q
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