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The Global Market for Magnetic Flowmeters

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.

Flow Research, Inc.

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