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Flow measurement applications in the water industry are
quite varied. They can range from small dosing and treatment flows in pipes
just a few millimeters in diameter, to the flow of treated water or wastewater
in trunk mains and aqueducts that are two meters in diameter and larger. Often
in larger cities, vast interceptor sewers are used to collect huge volumes of
effluent and spent water to convey them to wastewater treatment plants.
Therefore, flowmeter usage is diverse and central to the entire water cycle
control within the industry. The metering process directly or indirectly
influences resource management, process control, new works planning,
distribution management, leakage detection, financial control and environmental
Managing Water Distribution Systems
Preventing water imbalances is becoming a key focus to
increase the reliability and quality of the supply. The complete supply system
consists of a number of elements. Each component needs to be effectively
managed and controlled if the overall supply cycle is to remain within tight
control. A typical system, shown in Figure 1, usually consists of the following
water piping system between abstraction and primary treatment plant.
and components within the water treatment works.
mains and supply storage reservoirs.
distribution supply mains.
pipes to the consumer's dwellings.
in the consumer's premises after the point of final metering.
Water is drawn from the source, through an abstraction meter
(SP1) and into the treatment plant. A second supply meter (SP2) may identify
losses in the supply main. From the treatment plant, water passes through the
outlet meters (OMs) into the transmission mains and finally into local
reservoirs. District meters measure the amount taken out from these reservoirs
into each district and finally area meters (AMs) measure flow in smaller supply
mains into each subdivision when the supply mains split after the DMAs. In each
area, there may be individual meters for shops, apartment complexes and
factories. It is vital that all the meters within this system are of known
performance and regularly maintained so the water balance within any section can
be documented and the uncertainty correctly estimated.
Losses or imbalances can occur in any or all parts of this
process. In the initial stage, large volumes of water are pushed through a few
key flowmeters. For example, a large Venturi meter might be in a 1,600mm pipe
measuring a flow of 250 mld. Over a period of time, the inside surface of the
pipes and meter will be altered due to deposits and generally the meter may
begin to under-read.
The intrinsic accuracy of an installed meter is fundamental
to managing the water balance. If the same type of meter was used throughout
the supply system, then they all could drift in the same way and at almost the
same rate. In this way, the meters may balance, but the quantitative assessment
of the flow is in error. This has been the case in many of the older networks.
Invariably these meters have not been properly installed, calibrated or
maintained. However, they are still used to measure the amount of water put
into supply. From my experience of installing flowmeters, this total is never
precisely known. Therefore, estimating leakage becomes extremely difficult.
Sources of Water Imbalance and Leakage
Leakage rates within small areas (or small towns) can be
estimated by the direct measurement of flows at times when demand is low. Water
companies make regular "soundings" during late night hours to try to
estimate these system flows. However, most "macro-level" water
balances are made directly from totalled values over longer periods by taking the
difference between a measured input volume and the accumulated volume of many
estimated or measured outputs over the same period. The uncertainty of metering
directly affects the balance result. Thus, accurate flow metering is
fundamental in water conservation projects.
The total water balance is made up of a number of
consumption, estimated from the population statistics.
losses or leaks.
or inferred losses.
Metered consumption is very clear and has an estimated
uncertainty. Unmetered consumption is less clear and has a much larger
uncertainty. This larger uncertainty is partly due to the fact that the exact
number of consumers is never precisely known and the rate of consumption may
vary within an area depending on the balance between domestic and/or industrial
users. Real losses consist of leaks and losses prior to the point of final
measurement and may be measured through the use of system balance meters, district
meters or zone flowmeters. More measurements may mean a better estimation of
the flow totals, but regular verification is essential to error analysis.
Apparent losses are largely due to the errors in the assumptions made on
consumer demand, population, etc.
The AWWA Manual "Water Audits and Leak
Detection,"1 lists six main sources of leakage that may occur in any
section of the system.
defects induced by poor design or insufficient planning at concept stage.
breaks caused by poor workmanship in construction phase--laying and support of
errors--over-pressure, water hammer valve operation, etc.
due to soil and/or water chemistry effects and groundwater effects (e.g.,
from any of the installed fittings (valves, saddles, bends, tees, hydrants,
or deliberate damage to hydrants and line air valves (including unauthorized
However, there also are hydraulic effects within the
pipelines that may bias the readings to a far greater extent than is realized.2
Leakage also can occur in each reservoir or storage tank through evaporation
(if open to the elements), by seepage (through defective concrete walls) and
because of simple overflows in times of heavy rainfall. Leakage is very time
dependant. Experience has shown that small leaks run undetected for long
periods and, in older systems these, are a common source of imbalance. Reported
studies generally indicate that losses in the water supply mains are less than in
transmission mains, which in turn are significantly less than in the water
mains. Thus, the majority of leaks are due to poor or non-existent metering in
the many smaller water lines in each area of a city. However, most leakage
policies concentrate first on the larger diameter lines, then each district and
finally each area or individual customer usage.
Flow Metering Techniques
British Standard 7405 (1991) shows many designs and methods
for measuring flow in closed conduits. Each design has strengths and
weaknesses, and no single technique fully meets the needs of the municipal
water engineer. With many ongoing developments, it takes a great deal more
skill and knowledge to select the optimum meter for a given application. The
operating principle forms a convenient way of classifying flowmeters and BS
7405 documents and names a number of basic groups (10 being used for closed
pipes, 1 for solids type metering and the last for open channel meters).
In water supply systems, the common methods used almost always
come from differential pressure types (BSI groups 1 and 2), displacement types
(group 3), inferential types (group 4), magnetic types (group 6), ultrasonic
types (group 7) and open channel types (not discussed in BS 7405). By far, the
domestic inferential meters for individual usage in houses, apartments,
factories, etc., are the most sold. Millions are used worldwide and have formed
the backbone of network supply management for the past three decades.
Bulk supply traditionally has been monitored with Venturi or
Dall tubes or large propeller meters. Recently, magnetic meters increasingly
are being used in most parts of the world for these applications. Table 1 is a
basic application table, largely based on experience and traditions, showing
what meters are used for each application. This table should not be considered
as the final means of selecting the best meter, but merely give an indication
of the current choices.
For the elements shown in Figure 1, the current practice is
to use ultrasonic or magnetic meters wherever possible, governed of course by
costs. As large meter costs can vary enormously, the balance has to be struck
between spending the right amount of money for an economic return. Normally
meters are chosen purely on cost grounds, but this may not necessarily be the
optimum choice. Life cycle costs are a much better means of judging instrument
Expected and Actual Installed Meter Performance
When purchasing any instrument, the supplier usually
includes a written specification for that device. While some manufacturers give
detailed performances backed by independent testing, a minority of
specifications barely enable the prospective user to determine what they are
purchasing. Flowmeters are a little different from other instruments. They are
tested in a flow laboratory under reference conditions. This means that
standard flow rates are used in long straight pipes under steady flow
conditions. Few manufacturers have comprehensive data on installation effects
and often are reluctant to part with this data. An installation effect is
defined as the variation from laboratory calibration to that obtained under
Examples of flowmeter installation effects are
in pipe characteristics (roughness, ovality, etc.),
of fittings (valves/bends) that are not present in reference testing,
in temperature (fluid and ambient),
of local RFI that is not present in reference testing, and
acquisition and processing errors of the local system.
The laboratory data is relevant for the meter in the
laboratory setup only. Once the meter is installed in the customer's pipe,
other changes may be introduced, such as
of the mating pipe is usually different to that of the meter,
flow rates in the transmission lines may be different from the lab data,
may be sediment, or calcification effects within the network, and
All these variables may introduce additional bias into the
meter readings from the initial day a meter is installed. This bias can be
estimated or quantified only from an in-situ verification. This verification
should be a part of regular network management activities. When purchasing
flowmeters, many users expect the manufacturer's specification to apply
immediately and be stable over time. This is a popular misconception. For
example, small mechanical meters always are tested at the manufacturer's
premises in accordance with local Weights and Measures regulations or
international metrology standards. This is simply a guarantee to the user that
when it leaves the factory it is within predetermined calibration limits. If it
is installed incorrectly, installed too close to valves or used in a water
supply with a high sediment content, its performance may shift. Usually, though
not always, it under-records. Over time, with component wear, this
under-registration may increase. There also may be a steady but noticeable fall
in accuracy of the meter. All flowmeters, regardless of the type or source,
show these time dependent effects to varying degrees. The key to successful
network management is to estimate the rate of degradation (if present) with
It is quite difficult to calibrate flowmeters to much better
than 0.2 percent total uncertainty. Therefore, this figure represents the
baseline when any meter leaves a manufacturing facility. When the user installs
the meter, this 0.2 percent lab figure almost certainly changes (increases).
Special consideration regarding the accuracy and installation
should be given to the outlet meters from a water treatment plant because they
are handling large volumes of treated water being put into the supply. Any
flowmeters of 250mm and above should be carefully selected and even more
carefully installed. Poor installation is the greatest source of error.
Standards give guidance on the effect of single fittings but little data exists
on the effect of multiple fittings close to flowmeters.4 Due to rapid
developments, even accepted standards currently in use may not be totally
correct.5 This is why site verification is vital to ensure the meter readings
All flowmeters are affected by a lack of attention to
details at the installation stage such as protruding gaskets, pipe bore
alignment, the proximity of valves and presence of pipe branches.6 In addition
to these hydraulic considerations, close attention must be given to
environmental aspects on secondary equipment to avoid excessive vibration,
flooding, ambient temperature swings and other effects. These factors can
greatly influence the actual measurement uncertainty that can be achieved. D.A.
Phair's "Errors Occur Often in Industrial Flowmetering" discusses
Taking into account these many installation factors, it
often is difficult to demonstrate total installed errors of much better than 2
percent. In order to achieve this long-term figure, it is necessary to specify
instruments with an intrinsic uncertainty of around 0.5 percent or better. This
then allows more than 1.5 percent for all the other unquantified effects.
More attention should be given to the nature of the meter
chosen and the application needs. If water companies would standardize on this
2 percent value, leakage figures would become much more understandable. It is
essential that all meters are site verified to ensure that this 2 percent value
is maintained, or at the very least any deviation is estimated.
It should be remembered that if each meter has a total
installation within the network of 2 percent uncertainty or better, summing all
these meter readings may still give total metered system imbalances of 10
percent or more. The performance critically depends on the number of meters,
the type of meter, the design of each installation and the degree to which
maintenance and field verifications are performed. There also are the
uncertainties due to unmetered consumption, losses due to undetected leaks and
any assumptions on inferred losses. Combined, these losses can give system
imbalance uncertainties of greater than 25 percent.
Installation and Cost Implications
Cost often dictates the meter choice as well as the design
of the complete installation. Unfortunately it usually is only the purchase
price of the equipment that is the driving factor. The full cost of flow measurement
is not simply buying the instrument. Recent independent studies have shown that
the purchase price is only about 40 percent of the total cost of owning and
operating the meter in the first year. The actual price depends heavily on the
meter specification such as pressure ratings, materials of constructions,
transmitter functionality, etc. Installing the meter is the single greatest
expense in the first year. This covers both mechanical and electrical
installation as well as the purchase of piping and ancillaries. As a result,
many water supply authorities have focused on reducing installation costs.
Previously, meters had been installed in chambers to allow access for
maintenance and calibration. However, modern flowmeters require little maintenance,
and recent advances in electronic signal processing and fault diagnostics have
allowed field devices to be developed that do not require direct access to the
meter. These devices use electronic fingerprinting to look for changes in both
the meter and transmitter characteristics.
As a result, the direct burial of flowmeters, pioneered in
the UK and South Africa, has gathered pace. This practice now has more than 15
years of practical experience from several large water companies in many parts
of the world. Several thousand meters of all sizes now are direct buried. The
three main benefits are
lower installations costs--building of concrete chambers is more than the meter
maintenance costs--magnetic meters offer a mean time before failure in excess
of 80 years.
reliability of the installation--tampering is eliminated.
It is important to understand the local soil chemistry to
eliminate external corrosion effects from either acidic or alkaline soil or
because of the ingress of groundwater that may contain pollutants or chemicals.
Usually the meter is specially coated and surrounded with sand to act as a
partial filter. Such techniques have been successfully applied in line sizes up
to 2,200mm. Special attention also needs to be given to those cases where
cathodic protection is used on the line. The cathodic current may bias readings
unless it is correctly insulated. Finally, earthing of the line is essential in
those areas where lightning strikes may be common. Despite these
considerations, experience has shown that very significant savings can be made
through such practices.
Water supply demand can vary greatly during the working day
as well as show variations through the working week. Figure 2 shows the flow
variations of a modern area meter (AM) installed in one street of a major U.S.
Peak demand in this case occurred on Sunday
morning--presumably when people were watering gardens and washing their cars.
Also notice the variations of peak demand between the average daytime velocity
and the average night-time velocity. At this location, reversed flow was
actually indicated. For this suburban location, flow velocities within the
supply line always are low, putting severe demands on the performance of the
flowmeter. Low line velocity is common in many supply systems, but what often
is not appreciated is the pipeline dynamic variations between the day and night
demands. This is a good example where modern metering has identified
operational and system design problems.
Line pressure can affect leakage significantly. The operator
has to give sufficient supply pressure while reducing the driving force that
may cause water to leak out of a defective pipeline. As demand reduces at
night, there is a tendency for line pressure to rise due to the supply pumps
running to cope with peak demand. If pressure was reduced as the flow rate
reduces, then the variations on stress at the pipe walls will not see a normal
cyclic variation. It is this cyclic variation that partly causes leakage at
connections, valves, etc. Several cities now implement pressure reduction
control during the night hours. Examples of this have been documented in
chapter 9 of Lambert's "Managing Water Leakage."8
The financial implications of incorrect metering often are
not appreciated. When it comes to project costing and particularly operational
costs, the monetary costs due to inaccuracy rarely are considered. Table 2 is
part of a study made in a French city four years ago. For the price of water
given, the financial uncertainty has been calculated for a range of flowrates
occurring in a number of different line sizes at various levels of inaccuracy.
For a 200mm flowmeter running at 0.7m/s with a total
installed uncertainty of 2 percent (a very real case), the uncertainty in
billing the volume passed is 25,000 Euros/year (approximately, U.S. $27,000).
This is almost twice the price of installing a traditional high quality water
meter. For a smaller 25mm area meter, the corresponding figure is 400 Euros.
Similar calculations for a major transmission line meter of 1,600mm show an
uncertainty figure of 1M Euros.
Future Role of Metering
It is clear that accurate metering is fundamental to the
future conservation of water resources and to the successful financial and
operational management of existing water supply networks. Without metering, the
true loss of water cannot be assessed, leading to financial under-recovery and
possible errors in capital investment within the industry. An economic
reduction in leakage rates can render the investment in new water sources,
dams, treatment works or reservoirs unnecessary, or at the least enable a
better economic justification to be made. Studies have shown that in major city
water supply networks, financial uncertainties in excess of several millions of
U.S. dollars per day are easily possible. When set against the cost of good
instrumentation and maintenance practices, such leakage costs are very
significant. This is why so many cities around the world are investing in
long-term projects for water loss reduction.
Figure 3 shows data from Calgary, Alberta, Canada.
Investment over a 20-year period has resulted in the saving of more than 100
million liters per day (mld), every year for the past decade. It also means
better financial management and more prudent capital investment that has been
based on accurate flow metering.
In the near future, as digital electronics and
communications protocols become more accepted, flow meters are expected to
evolve into small metering systems (pressure, flow temperature and alarm
capability at each point). Even the small domestic mechanical meters are
undergoing radical development and evaluation. They are evolving into solid
state devices, linked to telephone systems, satellite dishes, local cable TV or
other lines into properties, so that flowrate, consumption and other data can
be automatically transmitted back to central centers for collation and billing.
This article is partly based on a article appearing in the
Winter-Spring 2002 ITA Analyzer. The ITA can be reached at 702-568-1445 or