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Liquid
Flow Measurement in the Water & Wastewater Industry
The
World Market for Magnetic Flowmeters
Articles
about Liquid Flows
Water
is one of our most important natural resources.
We drink it, use it for cooking and cleaning, and depend on it in
many aspects of our lives. It
should not be surprising, then, that we have a need to measure the amount
of water we use. This is where flowmeters come in. Whether it is measuring household or office building water
consumption, measuring water in open channels coming from natural
reservoirs, or measuring water used inside chemical or power plants, there
are many situations where water flow needs to be measured.
As
is the case with most jobs, there are a variety of tools available to
measure water flow. These
tools are in the form of flowmeters that use different measuring
principles, and have different advantages and disadvantages. Which flowmeter is selected for a given job depends on
accuracy and reliability requirements, whether the water is dirty or
clean, cost, available suppliers, and many other factors.
Figure 1 gives the percentage spent on different types of
flowmeters sold into the water and wastewater industry worldwide in 2003.
While
positive displacement and turbine meters have dominated utility
measurements, new-technology meters such as magnetic and ultrasonic are
beginning to make inroads into utility applications.
One factor driving this change is the development of standards for
these types of flowmeters by the American Water Works Association (AWWA)
for the drinking water industry.
Turbine
Flowmeters
The
word “turbine” is derived from a word that means “spinning thing.”
Turbine flowmeters have a rotating blade that spins in proportion
to flowrate. Unlike positive
displacement meters, which excel at measuring fluids at low flowrates,
turbine meters do especially well at measuring medium to high-speed flows.
Turbine metes are also more adaptable to use in large pipe sizes,
including pipes over 12 inches in diameter, than are positive displacement
meters. Each of the eight
different types of turbine meters is designed for particular applications.
Turbine
meters are widely used for custody transfer of commercial and industrial
water and other liquids. They are also used for custody transfer of hydrocarbon-based
liquids and natural gas. Both
the AWWA and the American Gas Association (AGA) have formulated standards
for the use of turbine meters for custody transfer purposes.
With well over 100 suppliers worldwide, there are many types of
turbine meters available for different applications.
Compound
meters represent an interesting type of meter.
Compound meters use one type of technology when flowrates are low,
and another type when they are high.
Low flowrates in compound meters are usually handled either by
positive displacement meters or by single jet or multi-jet turbine meters.
High flowrates are handled by some type of turbine meter, usually
either Woltman or axial. A
typical use of compound meters is in apartment or office buildings, where
flowrates peak during the day or evening, but are very low at night.
Magnetic
Flowmeters
Magnetic
flowmeters are widely used in the water and wastewater industry, where
they have many advantages. They can measure flow in very small pipes, and also in very
large size pipes. Some
magnetic flowmeters measure more than 100 inches in diameter.
Magmeters are both accurate and reliable. Both inline and insertion models are available.
Different liners make them applicable for sanitary applications,
and enable them to handle almost any type of liquid.
Unlike
many other types of meters, magmeters can be used to meter dirty liquids
and slurries. This makes them
especially useful in the wastewater and pulp and paper industries.
While magnetic flowmeters are highly versatile for measuring liquid
flow, they have two main limitations.
One limitation is that they cannot measure steam or gas flow, and
the other is that they cannot measure the flow of nonconductive liquids.
This limitation means that magmeters cannot be used for measuring
hydrocarbon-based fluids.
Magnetic
flowmeters have coils that are mounted onto or outside of a pipe.
As current is applied to the coils, a magnetic field is generated. As liquids pass through this magnetic field, a voltage is
generated that is proportional to flowrate. This voltage is detected by
electrodes that are mounted on either side of the pipe.
The flowmeter computes flowrate based on the amount of voltage
generated, along with other values.
While
some early magnetic flowmeters used AC current to generate a magnetic
field, more recent models use pulsed DC current.
The use of pulsed DC current facilitates calibrating the meter in
zero-flow condition. However,
some pulsed DC meters have a difficult time generating a pulse
sufficiently strong to measure the flow of dirty liquids.
As a result, magmeters with AC current are still used for some
difficult-to-measure dirty liquids.
Positive
Displacement
One
of the most popular type of positive displacement (PD) meters used today
as a water meter is the nutating disc meter.
Nutating disc meters were invented in 1830 by James and Edward
Dakwyne. The Dakwyne’s were
granted a patent for a hydraulic pump using this same principle.
In the early 1900s, these meters were improved, and the disc began
to be made of hard rubber. The
life of the meter was greatly extended by combining hard rubber on brass.
This rubber and brass design was widely used until the late 1950s,
when plastics and composites replaced the brass meter body and chamber.
Positive
displacement meters capture the fluid to be measured into a small
container of known size. They
have counters that increment each time the fluid is captured.
PD meters do very well in measuring the flow of viscous liquids,
like oil, honey, and syrup. They also excel in measuring fluids with low flowrates.
Besides being used for custody transfer of commercial and
industrial water, PD meters are also used for custody transfer of
hydrocarbon-based liquids to and from delivery trucks and airplanes.
Regulatory bodies have approved their use for this purpose.
In
addition to their use for commercial and industrial applications, PD
meters are widely used to measure water use in private homes and
apartments. Their ability to
measure low flowrates, along with their accuracy, makes them ideal for
this purpose. While
new-technology meters such as magnetic and ultrasonic are starting to make
inroads into the PD water flow measurement market, positive displacement
meters are still entrenched in water flow applications for residential,
commercial, and industrial applications.
Open Channel
Open
channel flowmeters are designed to measure the flow of water and other
liquids in rivers and streams, and in channels where flow occurs due to
gravity. The flow of liquids
in partially filled pipes that are not pressurized is also considered open
channel flow. Open channel
flowmeters are widely used in the water and wastewater industry.
They are used to measure flow in water treatment plants, and also
to measure water that is traveling from a natural resource such as a dam
or reservoir to a water treatment facility.
Some
open channel flowmeters use a weir or flume, a type of hydraulic structure
that water passes through. Flowrate
is calculated based on its level or depth as it passes through the weir or
flume. Other open channel
flowmeters use a velocity area technique.
Velocity area flowmeters use one method to compute the velocity of
the flowstream (e.g., electromagnetic), and another method to determine
the level or depth of the flowstream (e.g., radar).
Flow can then be calculated, provided the area of the flowstream is
known.
Ultrasonic
Ultrasonic
flowmeters come in two flavors: transit time and Doppler.
Transit time meters are mainly used with clean fluids, although
technology advances have widened their use to include fluids with some
impurities. Transit time
meters send one ultrasonic signal from a transducer across the pipe to
another transducer that receives the signal.
They also send a signal from the transducer on the other side of
the pipe back to the original transducer.
The signal moves faster when it travels with the flow than when it
travels against the flow. Flowrate is computed based on the difference between the two
transit times.
Doppler
flowmeters work somewhat like transit time meters, in that they send an
ultrasonic signal into the flowstream.
However, instead of sending a signal all the way across the pipe,
Doppler flowmeters bounce their signals off of particles in the
flowstream. As the signals
bounce off the particles, a frequency shift occurs that is proportional to
flowrate. A receiver detects
this frequency shift, and the flowmeter computes flow based on this
difference.
While
transit time meters get more attention than Doppler meters, Doppler meters
are of special importance in the water and wastewater industry.
Like magnetic flowmeters, Doppler meters can measure the flow of
dirty water and water containing particles, such as sand or gravel.
Measuring flow under these conditions is difficult for many other
types of flowmeters. Most
flowmeters perform best with clean fluids.
Even though Doppler flowmeters do not have the same accuracy as
transit time meters, they still play an important role in measuring flow
under difficult conditions. Doppler
meters are especially suited for flow measurement in the wastewater
industry, which often has a need to measure the flow of dirty water.
Other Flowmeters
Other
flowmeters used in the water and wastewater industry include differential
pressure, vortex, thermal, and Coriolis.
While these types are not as widely used as the ones described
above, they still have a place in the water and wastewater market.
The type of flowmeter selected for a given application depends on
the accuracy required, the condition of the fluid being measured, cost,the
experience of the end-user, and other factors.
No matter what type of measurement application in the water and
wastewater industry, there is likely to be a flowmeter that fills the
need.
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