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National Environmental Services Agency (NESA). Tom addresses the issues that bug you the most. And Tom knows!! With 35 years experience in providing environmental support services to public and private sector clients on a wide range of environmental issues. Tom has also co-authored and presented training courses on wastewater treatment systems.
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A Consumers' Guide to Full-Bore
Magnetic Flowmeters
Guest article by David Spitzer PE and Walt Boyes
Magnetic flowmeters utilize
Faraday's Law of Electromagnetic Induction to determine the
velocity of a liquid flowing in a pipe. Faraday's Law forms the
basis for electrical generation systems where wires travel through
a magnetic field and produce a voltage.
In a typical physics class
experiment to illustrate the phenomenon, a wire (conductor)
connected across a galvanometer can be moved through the magnetic
field of a horseshoe magnet and cause the galvanometer pointer to
move. Moving the wire in the opposite direction will cause the
pointer to move in the opposite direction due to the changing
voltage polarity. Moving the wire faster will cause more voltage
to be generated and the movement to move higher.
In magnetic flowmeters, a magnetic
field is generated and channeled into the liquid flowing through
the pipe. To accomplish this, the electromagnetic coils can be
located outside of the pipe (flow tube), however the flow tube
must be non-magnetic to allow penetration of the magnetic field
into the liquid. Locating the coils internal to the flowmeter
(closer to the liquid) can reduce the electrical power necessary
to deliver the magnetic field, as well as reduce the size of the
flowmeter and fabrication costs.
Following Faraday's Law, flow of a
conductive liquid through the magnetic field will cause a voltage
signal to be generated. This signal is sensed with electrodes
located on the flow tube walls. When the coils are located
externally, a non-conductive liner is installed inside the flow
tube to electrically isolate the electrodes and prevent the signal
from being shorted. For similar reasons, non-conductive materials
are used to isolate the electrodes for internal coil designs.
The fluid itself is the conductor
that will move (flow) through the magnetic field and generate a
voltage signal at the electrodes. When the fluid moves faster,
more voltage is generated. Faraday's Law states that the voltage
generated is proportional to the movement of the flowing liquid.
The transmitter processes the voltage signal to determine liquid
flow.
The voltage signal will take the
same general form as its electromagnetic excitation. When a
magnetic flowmeter is excited by a sinusoidal magnetic field (AC
waveform), the signal generated at the electrodes is also
sinusoidal. In earlier designs, these signals were subject to a
number of influences that affected measurement quality, including
stray voltages in the process liquid, capacitive coupling between
the signal and power circuits, capacitive coupling between
interconnecting wiring, electrochemical voltage potential between
the electrode and the process fluid, and inductive coupling of the
magnets within the flowmeter. These flowmeters required a zero
adjustment to compensate for these influences and the effect of
electrode coating.
Turning the electromagnetic field
on and off (DC waveform) causes the signal to resemble a square
wave. When the electromagnetic field is on, the signal due to flow
plus noise is measured. When the electromagnetic field is off, the
signal due to only noise is measured. Subtracting these
measurements cancels the effects of noise and eliminates the zero
adjustment, reducing the abovementioned drift problems and
improving performance. Waveforms other than those described above
are also in use.
There is a perception within the
instrumentation, systems, and automation community around the
world, that magnetic flowmeters have become a commodity product.
It has been said that magnetic flowmeters are pretty much equal,
that their specifications are pretty much equal, and therefore
their performance is pretty much equal. This has made it difficult
for users and manufacturers alike to differentiate magnetic
flowmeters. In order to compete, manufacturers have reduced prices
and stifled new product development across the product niche. High
development costs cannot be justified to develop innovative
products in a market where the only differentiation is on price.
We discovered that manufacturer
claims actually tend to support the perception that a magnetic
flowmeter is a commodity item. These claims typically refer to
claimed performance under ideal conditions, and are often
simplifications intended to make things easy for the purchaser/specifier.
So incredibly "easy" have things become that even the
accuracy of the widely used analog output signal is often not
stated or known. Yet this is important, since the analog output is
the most commonly used to control the process.
How we put together our study
Recent information we developed in
the course of doing research for our newly released series of
reports "Copperhill & Pointer's Competitive Intelligence
Survey: Magnetic Flowmeters," challenges the perception that
magnetic flowmeters are, or should be considered, a commodity
product. Originally, we collected data on 43 companies worldwide
who sell magnetic flowmeters. We found that 26 companies
manufacture meters, with the remainder private-labeling them from
one or more manufacturers. Since the study was completed, our list
has expanded to over 50 companies worldwide, including companies
in Eastern Europe, China, and India.
We asked the companies to
participate in our research, and all but one agreed to provide
product specifications. Their raw information was tabulated on
over 120 data sheets that were developed specifically for this
purpose. The types of magnetic flowmeters were further organized
into categories (see sidebar). Within each category, each model
was compared on the basis of its published performance
specifications. If there appeared to be an omission or
inconsistency in a published specification, we sought further
clarification from the supplier .
How we analyzed the data
Tabulated and graphical performance
data revealed significant differences between models and
manufacturers of magnetic flowmeters . Some magnetic flowmeter
performance was as much as 2-3 times worse than that of other
flowmeters in the same category (see chart).
The calculations also illustrate
much of the reason why magnetic flowmeters might be perceived as a
commodity product. Magnetic flowmeter performance specifications
are often intricate, and suppliers often simplify them to reflect
performance under ideal conditions. So incredibly
"simple" have things become that some suppliers cannot
quantify the accuracy of the analog output signal. Yet this is
important, since the analog output is the signal most commonly
used to control the process.
What the data means
End-users and consulting engineers
who know that several suppliers offer identical equipment (except
for nameplate) will be able to better control whose equipment they
purchase, and at what price. To simplify these relationships, the
flowmeter categories were tabulated by supplier along with country
of origin and/or source of manufacture.
To help select the best equipment
for an application, they would also like to know which models
perform better in a given category of magnetic flowmeters. To this
end, within each category, each model was ranked in order of its
calculated performance.
The results of the study
The report concludes, "Except
for specialized applications, the operating conditions within
which these flowmeters operate are similar within each design
category. Similarly, while there are differences in the electronic
features associated with different transmitters, flowmeter
performance at reference conditions was found to vary widely.
Differences were especially significant at low flow conditions
that are commonly encountered in actual flowmeter operation."
This series of reports is unique in
providing this comparison data. We consider the results to be
significant and expect that some buying patterns and marketing
strategies may be altered as an outcome of our research. Because
of the dynamic changes in the flowmeter marketplace due to
acquisitions, product additions and deletions, we intend to update
this report as events warrant , and make it available as a
consumers' guide utility on a continuing basis.
Magnetic Flowmeter Categories
- Ceramic-lined - Ceramic
magnetic flowmeters have abrasion-resistant liners typically
made of alumina ceramic instead of the typical elastomer
linings usually found in magnetic flowmeters. They often
permit higher temperature operation, and because their
electrodes are typically part of the ceramic substrate, they
tend not to offer a leak path between the electrode and liner.
- Electrodeless - So-called
"electrodeless" magnetic flowmeters employ
electrodes that are not in direct contact with the fluid.
These electrodes are either embedded in the liner or located
behind the liner, and are usually capacitatively-coupled to
the flowing liquid.
- Low flow (under 12
mm/0.5inch) - Low flow magnetic flowmeters include sizes below
12 mm (0.5 inch) in diameter. Many have ceramic linings with
embedded electrodes.
- Medium flow (12 mm/0.5
inch to 300-450 mm/12-18 inch) Medium flow magnetic flowmeters
include flanged and wafer-style meters that are between 0.5
inch (12 mm) and 300-450 mm (12-18 inch) in diameter. A large
number of magnetic flowmeter models fit into this category.
- High flow (over 300-450
mm/12-18 inch) - These magnetic flowmeters are larger in size,
ranging to over 2 meters in diameter.
- High-noise - Many
liquids, including slurries, produce signals that contain
large amounts of noise. These magnetic flowmeters are designed
to produce usable flow measurements even in high-noise
environments.
- Low-conductivity -
Traditionally, magnetic flowmeters could not be used for
liquids having an electrical conductivity of less than about
5-20 uS. Several designs permit measurement of fluids with
conductivities far less than the traditional level.
- Partially-full - Many
conduits, especially in wastewater and storm water runoff
applications, are only full part of the time. Partially-full
magnetic flowmeters are designed to measure flow using both
liquid velocity and liquid level to determine flow rate when
the conduit is completely not full of liquid.
- Fast response - While
many applications find the response time of traditional
magnetic flowmeters suitable to the service, other
applications require measurement where flow changes rapidly,
or where the duration of the flow may be on the order of only
a few seconds. Fast response magnetic flowmeters are designed
to quickly respond during these short time periods.
- Sanitary - Sanitary
magnetic flowmeters are designed and fabricated with materials
and finishes that allow application in the food and
pharmaceuticals industries where they may be cleaned in place
(CIP) or steamed in place (SIP) to reduce or remove bacterial
contamination.
- Two-wire - Traditionally,
to generate a sufficient magnetic field, magnetic flowmeters
required separate wiring to a source of power in addition to
analog signal wiring. Two-wire, or loop-powered magnetic
flowmeters are designed to operate on the power available from
the loop power supply. Most two-wire designs run on available
4-20 mADC loop power, but some designs require higher power
levels to operate over two wires.
About our author
David Spitzer has written a number
of books on flow measurement and flow measuring devices, some of
which are available through Amazon.com. These include:
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