Introduction to Magnetic Flow Meters
A
magnetic flow meter (mag flow meter) is a volumetric flow meter which
does not have any moving parts and is ideal for wastewater applications or any
dirty liquid which is conductive or water based. Magnetic flow meters will
generally not work with hydrocarbons, distilled water and many non-aqueous
solutions). Magnetic flow meters are also ideal for applications where low
pressure drop and low maintenance are required.
Principle of Operation:
The
operation of a magnetic flow meter or mag meter is based upon Faraday's Law, which
states that the voltage induced across any conductor as it moves at right
angles through a magnetic field is proportional to the velocity of that
conductor.
E is proportional to V x B x D where:
E = The voltage generated in a conductor
V = The velocity of the conductor
B = The magnetic field strength
D = The length of the conductor
To
apply this principle to flow measurement with a magnetic flow meter, it is
necessary first to state that the fluid being measured must be electrically
conductive for the Faraday principle to apply. As applied to the design of
magnetic flow meters, Faraday's Law indicates that signal voltage (E) is
dependent on the average liquid velocity (V) the magnetic
field strength (B) and the length of the conductor (D) (which
in this instance is the distance between the electrodes).In the case of
wafer-style magnetic flow meters, a magnetic field is established throughout
the entire cross-section of the flow tube (Figure 1). If this magnetic field
is considered as the measuring element of the magnetic flow meter, it can be
seen that the measuring element is exposed to the hydraulic conditions
throughout the entire cross-section of the flow meter. With insertion-style flow
meters, the magnetic field radiates outward from the inserted probe .
ELECTROMAGNETIC FLOW METER
• AC type magnetic flow meters apply line
voltage to the magnetic coils. The signal generated is a low level AC signal in
the high microvolt to low mV range.
• In the pulsed DC type Magnetic flowmeter, the magnet coils are
periodically energized. DC excitation may be either on-off or plus-minus
excitation. The principle is to take a measurement of the induced voltage when
the coils are not energized and to take a second measurement when the coils are
energized and the magnetic field has stabilized.
•In all the pulsed DC approaches, the concept
is to take a measurement when the coils are excited and store that information,
then take a second measurement of the induced voltage when the coils are not
excited. The voltage induced is a combination of both noise and signal when the
coils are energized and it is only noise when the coils are de-energized.
Subtracting one from the other will yield only signal.
AC/DC Technology
Comparison DC AC
Accuracy High Low
Auto Zeroing Yes No
Power consume. Low High
Effect of stray
Noise Low High
Process noise
rejection Not good Good
•Design
Temperature Upto 120 °C with Teflon liners; Upto 180
°C with Ceramic liners.
•Type of flow
detected Volumetric flow of conductive liquids, including slurries of corrosive or abrasive materials.
•Flow Ranges 0.01
to 100000 GPM (0.04 to 378000 l/m)
•Size Ranges From
0.1 to 96” in diameter
•Accuracy ±1%
of full scale for AC excitation.
±1% of actual flow for
10:1 range and ±0.5% of actual flow for
2:1 or 5:1 range.'
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MAGMETER SELECTION:
The characteristics of the fluid to be metered, the liquid
flow parameters, and the environment of the eter are
the determining factors in the selection of a particular type of flowmeter.
Conductivity:
Electrical conductivity is simply a way of expressing the
ability of a liquid to conduct electricity. Just as copper wire is a better
conductor than tin, some liquids are better conductors than others. However of even greater importance is the fact
that some liquids have little or no electrical conductivity (such as hydrocarbons
and many nonaqueous solutions, which lack sufficient conductivity for use with magmeters).
Conversely, most aqueous solutions are well suited for use with a magmeter.
Depending on the individual flowmeter, the liquid conductivity must be above the
minimum requirements specified. The conductivity of the liquid can change
throughout process operations without adversely affecting meter performance, as
long as it is homogeneous and does not drop below the minimum conductivity threshold.
Several factors should be taken into consideration concerning liquids to be
metered using magnetic flow meters. Some
of these are:
1. All water does not have the same conductivity. Water varies greatly in
conductivity due to various ons present.
The conductivity of “tap water” in Maine might be very different from that of
“tap water” in Chicago.
2. Chemical and pharmaceutical companies
often use deionizer or distilled water, or other solutions which are not
conductive enough for use with magnetic flow meters. 3. Electrical conductivity
is a function of temperature. However, conductivity does not vary in any set
pattern for all liquids as temperature changes.
Therefore, the temperature of the liquid being considered should always be
known.
4. Electrical conductivity is a function
of concentration. Therefore, the concentration of the solution should always be
provided. However, avoid what normally is a logical assumption, such as: That electrical conductivity
increases as concentration increases. This is true up to a point in some solutions,
but then reverses. For example, the electrical conductivity of aqueous
solutions of acetic acid increases as concentration rises up to 20%, but then
shows a decrease with increased concentration to the extent that, at some concentration
above 99%, it falls below the minimum requirement.
Acid/Caustics:
The chemical composition of the liquid
slurry to be metered will be a determining factor in selecting the flow meter
with the proper design and construction. Operating experience is the best guide
to selection of liner and electrode
materials, especially in industrial applications, because, in many cases, a
process liquid or slurry will be called by a generic name, even though it may
contain other substances which affect its corrosion characteristics. Commonly available
corrosion guides may also prove helpful in selecting the proper materials of
construction.
Velocity:
The maximum (full scale) liquid velocity
must be within the specified flow range of the meter for proper operation. The
velocity through the flowhead can be controlled by properly sizing the meter.
It isn’t
necessary that the flowhead be the
same line size, as long as such sizing does not conflict with other
system design parameters. Although
the meter will increase hydraulic head loss when sized smaller than the line size (because the meter
is both obstructionless and of short lay length), the amount of increase in
head loss is negligible in most applications. The amount of head loss increase
can be further limited by using concentric reducers and expanders at the pipe
size transitions. As a rule of thumb, meters should be sized no smaller than
one-half of the line size. Because of the wide rangeability of magnetic lowmeters, it is almost never necessary to
oversize a meter to handle future flow requirements. When future flow
requirements are known to be significantly higher than start-up flow rates, it
is imperative that the initial flows be sufficiently high and that the pipeline
remain full under normal flow conditions.
Abrasive Slurries:
Mildly abrasive slurries can be handled
by magnetic flowmeters without problems, provided consideration is given to the abrasiveness of
the solids and the concentration of the solids in the slurry. The abrasiveness of a slurry will affect the
selection of the construction materials and the use of protective orifices.
Abrasive slurries should be metered at 6 ft/sec or less in order to minimize flow
meter abrasion damage. Velocities should not be allowed to fall much below 4
ft/sec, since any solids will tend to settle out. An ideal slurry installation
would have the meter in a vertical position. This would assure uniform
distribution of the solids and avoid having solids settle in the flow tube
during no-flow periods. Consideration should also be given to use of a
protective orifice on the upstream end of a wafer-style
magnetic flow meter to prevent excessive
erosion of the liner. This is especially true since Tefzel liner
have excellent chemical
resistance, but poor resistance to abrasion. In lined or non-conductive piping
systems, the upstream protective orifice
can also serve as a grounding ring.
Sludges and Grease-Bearing Liquids:
Sludges and grease-bearing liquids
should be operated at higher velocities, about 6 ft/sec minimum, in order to
reduce the coating
Viscosity:
Viscosity does not directly affect
the operation of magnetic flowmeters, but, in highly viscous fluids, the size
should be kept as large as possible to avoid excessive pressure drop across the
meter.
Temperature:
The liquid’s temperature is
generally not a problem, providing it remains within the mechanism’s operating
limits. The only other temperature
considerations would be in the case of liquids with low conductivities (below
around 3 micromhos per centimeter) which are subject to wide temperature excursions.
Since most liquids exhibit a positive temperature coefficient of conductivity,
the liquid’s minimum conductivity must be determined at the lower temperature
extreme.
Advantages of the DC Pulse Style:
From the principles of operation, it
can be seen that a magnetic flowmeter relies on the voltage generated by the
flow of a conductive liquid through its magnetic field for a direct indication of
the velocity of the liquid or slurry being metered. The integrity of this low-level
voltage signal (typically measured in undress
of microvolt's) must be preserved so as to
maintain the high accuracy specification of magnetic flow meters in industrial environments. The superiority
of the dc pulse over the traditional ac magnetic meters in preserving signal
integrity can be demonstrated as follows:
Quadrature:
Some magnetic flowmeters employ alternating
current to excite the magnetic field coils which generate the magnetic field of
the flowmeter (ac magnetic flowmeters). As a result, the direction of the
magnetic
field alternates at line
frequency, i.e., 50 to 60 times per second. If a loop of conductive wire is located
in a magnetic field, a voltage will be generated in that loop of wire. From
physics, we can determine that this voltage is 90° out of phase with respect to
the primary magnetic field.
The magnitude
of this error signal is a function of the number of turns in the loop, and the
change in magnetic flux per unit time.
In a magnetic flowmeter, the electrode wires and the path through the conductive
liquid between the electrodes represent a single turn loop. The flow-dependent
voltage is in phase with the changing magnetic field; however, flow independent
voltage is also generated, which is 90°out of phase with the changing magnetic
field. The flow-independent voltage is therefore an error voltage which is 90°
out of phase with the desired signal. This error voltage is often referred to
as uadrature. In order to minimize the
amount of quadrature generated, the electrode wires must be arranged so that they are parallel with the
lines of flux of the magnetic field.
In ac field magmeters, because the magnetic field
alternates continuously at line frequency, quadrature is significant. It is necessary
to employ phase sensitive circuitry to detect and reject quadrature. It is this
circuitry which makes the ac magnetic meter highly sensitive to coating on the electrodes.
Since coatings cause a phase shift in the voltage
signal, phasesensitive circuitry leads to rejection of the true voltage flow
signal, thus leading to error.
Since dc pulse magmeters are
not sensitive to phase shift and require no phase sensitive circuitry, coatings
on the electrodes have a very limited effect on flowmeter performance.
Wiring:
In ac magnetic flowmeters, the
signal generated by flow through the meter is at line frequency. This makes these meters susceptible to noise pickup
from any ac lines. Therefore,
complicated wiring systems are typically
required to isolate the ac flowmeter signal
lines from both its own and from any
other nearby power lines, in order to preserve signal integrity. In comparison,
dc pulse magmeters have a pulse frequency much lower (typically 5 to 10% of ac
line frequency) than ac meters. This lower frequency eliminates noise pickup
from nearby ac lines, allowing power and signal lines to be run in the same
conduit and thus simplifying wiring. Wiring is further simplified by the use of
integral signal conditioners to provide voltage and current outputs. No separate
wiring to the signal conditioners is required.
Power:
By design, ac magnetic flowmeters typically
have high power requirements, owing to the fact that the magnetic field is constantly being powered. Because of the pulsed nature
of the dc pulse magmeter, power is
supplied intermittently to the magnetic field coil. This greatly reduces both power
requirements and heating of the electronic circuitry, extending the life of the
instrument.
Auto-Zero:
In traditional ac magnetic flowmeters,
it is necessary after installation of the meter to “null” or “zero” the nit.
This is accomplished by manual adjustment
which requires that the flowmeter be filled with process
liquid in a no-flow condition. Any
signal present under full pipe, no-flow conditions is considered to be an error
signal. The ac field magmeter is therefore “nulled” to eliminate the impact of
these error signals.
Many Magmeters feature automatic
zeroing circuitry to eliminate the need for manual zeroing. When the magnetic
field strength is zero between pulses, the voltage output from the electrodes
is measured. If any
voltage is measured during this period,
it is considered extraneous noise in the system and is subtracted from the
signal voltage generated when the
magnetic field is on. This feature insures high accuracy, even in electrically
noisy industrial environments.
Magmeter Selection
The
key questions which need to be answered before selecting a magnetic flowmeter
are:
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Is the fluid conductive or water
based?
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Is the fluid or slurry abrasive?
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Do you require an integral display or
remote display?
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Do you require an analog output?
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What is the minimum and maximum flow
rate for the flow meter?
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What is the minimum and maximum
process pressure?
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What is the minimum and maximum
process temperature?
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Is the fluid chemically compatible
with the flow meter wetted parts?
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What is the size of the pipe?
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Is the pipe always full?
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Installation Considerations:
Select
a location for the sensor where the flow profile is fully developed and not
affected by any disturbances. A minimum of 10 pipe diameters of straight run
upstream and 5 diameters downstream is recommended. Some situations may require
20 pipe diameters or more upstream to insure a fully developed turbulent flow
profile. The insertion magmeter is sensitive to air bubbles at the electrodes.
If there is any question that the pipe is absolutely full, mount the sensor at
a 45 to 135 angle.
Grounding requirements:
Magnetic
flow sensors are sensitive to electrical noise which is present in most piping
systems. In plastic piping systems, the fluid carries significant levels of
static electricity that must be grounded for best magmeter performance.
Instructions are included with the installation manual on how to best ground
the magnetic flow meter.
In Line Magmeters
The
in line type magnetic flow meters offer a higher accuracy. They can
be as accurate as 0.5% of the flow rate. The insertion styles offer a 0.5 to 1%
accuracy. Omega's FMG-600 series in line flange and wafer style meters offer
higher flow rates of 1 to 10 m/sec. These in line meters are offered in pipe
sizes up to 12".
Minimum conductivity:
5
micro Siemens/cm
Installation Considerations:
In
line flow meters do not require as much straight pipe as the insertion styles.
A minimum of 5 to 10 pipe diameters of straight run upstream and 1 to 2
diameters downstream is recommended. In vertical pipe runs, flow should always
run up and not down. These flowmeters are very sensitive to air bubbles. The
magmeter cannot distinguish entrained air from the process fluid; therefore,
air bubbles will cause the magmeter to read high.
Advantages
:
1) Obstruction
less and has no moving parts. No pressure drop across the meter.
2) Bi-directional
3) Viscosity
of liquid has no effect on metering.
4) Can
be used for slurry service.
5) Can
measure very low to very high volume flow rate with minimum size less than 1/8” inside diameter to size as large as 10
feet.
6) Suitable for most acids, bases, waters and
aqueous solutions.
Limitations
:
1) Can work only
with conductive fluids. Pure substances, hydrocarbons
and gases cannot be measured.
2) Electrical
installation care is essential.
3) To periodically check
zero on AC type Magnetic Flowmeters, block valves are
required in either side to bring the flow to zero and keep meter full. Cycled DC units do not have this requirement.
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