Magnetic Flowmeters Flow Reference Section

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Magnetic Flowmeters Flow Reference Section Powered By Docstoc
Flow Reference Section
Magnetic flowmeters are low
pressure drop, volumetric, liquid
flow measuring devices. The low
maintenance design–with no
moving parts, high accuracy, linear
analog outputs, insensitivity to
specific gravity, viscosity, pressure
and temperature, and the ability to
measure a wide range of difficult-to-
meter fluids (such as corrosives,
slurries and sludges)–differentiates
this type of metering system from
other flowmeters. Two basic styles
of magnetic flowmeter are currently
available from OMEGA Engineering:
1) Wafer-style, where highest
accuracy (up to +0.5% of reading)
measurements are required; and 2)
Insertion-style, for greater economy
and particularly for larger pipe sizes.
All OMEGA® magnetic flowmeters
employ the state-of-the-art dc
pulsed magnetic field system. The         Figure 1: In-line magnetic flowmeter operating principle
following discussion details the
principle of operation, as well as the
advantages, of dc pulsed type
magnetic flowmeters.

Faraday’s Law
The operation of a magnetic
flowmeter 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.
Faraday’s Formula:
E is proportional to V x B x D
E = The voltage generated
    in a conductor
V = The velocity of the conductor
B = The magnetic field strength
D = The length of the conductor           Figure 2: Insertion-type flowmeter operating principle
To apply this principle to flow
measurement with a magnetic               the magnetic field strength (B) and
flowmeter, it is necessary first to                                               considered as the measuring
                                          the length of the conductor (D)         element of the magnetic flowmeter,
state that the fluid being measured       (which in this instance is the
must be electrically conductive for                                               it can be seen that the measuring
                                          distance between the electrodes).       element is exposed to the hydraulic
the Faraday principle to apply.           In the case of wafer-style magnetic     conditions throughout the entire
As applied to the design of magnetic      flowmeters, a magnetic field is         cross-section of the flowmeter. With
flowmeters, Faraday’s Law indicates       established throughout the entire       insertion-style flowmeters, the
that signal voltage (E) is dependent      cross-section of the flow tube          magnetic field radiates outward
on the average liquid velocity (V)        (Figure 1). If this magnetic field is   from the inserted probe (Figure 2).

                                         4. Electrical conductivity is a
MAGMETER SELECTION                          function of concentration.            and expanders at the pipe size
                                            Therefore, the concentration of       transitions. As a rule of thumb,
The characteristics of the fluid to be                                            meters should be sized no smaller
metered, the liquid flow parameters,        the solution should always be
                                            provided. However, avoid what         than one-half of the line size.
and the environment of the meter
are the determining factors in the          normally is a logical assumption,     Because of the wide rangeability of
selection of a particular type of           such as: That electrical              magnetic flowmeters, it is almost
flowmeter.                                  conductivity increases as             never necessary to oversize a meter
                                            concentration increases. This         to handle future flow requirements.
Conductivity                                is true up to a point in some         When future flow requirements are
Electrical conductivity is simply a         solutions, but then reverses.         known to be significantly higher than
way of expressing the ability of a          For example, the electrical           start-up flow rates, it is imperative
liquid to conduct electricity. Just as      conductivity of aqueous solutions     that the initial flows be sufficiently
copper wire is a better conductor           of acetic acid increases as           high and that the pipeline remain full
than tin, some liquids are better           concentration rises up to 20%,
                                            but then shows a decrease with        under normal flow conditions.
conductors than others. However,
of even greater importance is the           increased concentration to the
fact that some liquids have little or       extent that, at some                  Abrasive Slurries
no electrical conductivity (such as         concentration above 99%, it falls     Mildly abrasive slurries can be
hydrocarbons and many non-                  below the minimum requirement.        handled by magnetic flowmeters
aqueous solutions, which lack                                                     without problems, provided
sufficient conductivity for use with     Acid/Caustics                            consideration is given to the
magmeters). Conversely, most             The chemical composition of the          abrasiveness of the solids and the
aqueous solutions are well suited        liquid slurry to be metered will be a    concentration of the solids in the
for use with a magmeter. Depending       determining factor in selecting the      slurry. The abrasiveness of a slurry
on the individual flowmeter, the         flowmeter with the proper design         will affect the selection of the
liquid conductivity must be above        and construction.                        construction materials and the use
the minimum requirements                                                          of protective orifices. Abrasive
specified. The conductivity of the       Operating experience is the best         slurries should be metered at
liquid can change throughout             guide to selection of liner and          6 ft/sec or less in order to minimize
process operations without               electrode materials, especially in       flowmeter abrasion damage.
adversely affecting meter                industrial applications, because, in     Velocities should not be allowed to
performance, as long as it is            many cases, a process liquid or          fall much below 4 ft/sec, since any
homogeneous and does not drop            slurry will be called by a generic
below the minimum conductivity           name, even though it may contain
                                                                                  solids will tend to settle out. An ideal
                                                                                  slurry installation would have the
threshold. Several factors should be     other substances which affect its        meter in a vertical position. This
taken into consideration concerning      corrosion characteristics. Commonly      would assure uniform distribution of
liquids to be metered using              available corrosion guides may also      the solids and avoid having solids
magnetic flowmeters. Some of             prove helpful in selecting the proper    settle in the flow tube during
these are:                               materials of construction.               no-flow periods.
1. All water does not have the same                                               Consideration should also be given
   conductivity. Water varies greatly    Velocity
                                                                                  to use of a protective orifice on the
   in conductivity due to various ions   The maximum (full scale) liquid          upstream end of a wafer-style
   present. The conductivity of “tap     velocity must be within the specified    magnetic flowmeter to prevent
   water” in Maine might be very         flow range of the meter for proper       excessive erosion of the liner. This
   different from that of “tap water”    operation. The velocity through the      is especially true since Tefzel liner
   in Chicago.                           flowhead can be controlled by            have excellent chemical resistance,
2. Chemical and pharmaceutical           properly sizing the meter. It isn’t      but poor resistance to abrasion. In
   companies often use deionized or      necessary that the flowhead be the       lined or non-conductive piping
   distilled water, or other solutions   same line size, as long as such          systems, the upstream protective
   which are not conductive enough       sizing does not conflict with other      orifice can also serve
   for use with magnetic flowmeters.     system design parameters.                as a grounding ring.
                                         Although the meter will increase
3. Electrical conductivity is a          hydraulic head loss when sized
   function of temperature.              smaller than the line size (because      Sludges and
   However, conductivity does not        the meter is both obstructionless        Grease-Bearing Liquids
   vary in any set pattern for all       and of short lay length), the amount     Sludges and grease-bearing liquids
   liquids as temperature changes.       of increase in head loss is negligible   should be operated at higher
   Therefore, the temperature of the     in most applications. The amount of      velocities, about 6 ft/sec minimum,
   liquid being considered should        head loss increase can be further        in order to reduce the coating
   always be known.                      limited by using concentric reducers     tendencies of the material.

Flow Reference Section

Viscosity                                 The magnitude of this error signal is     power lines, in order to preserve
Viscosity does not directly affect        a function of the number of turns         signal integrity.
the operation of magnetic                 in the loop, and the change               In comparison, dc pulse magmeters
flowmeters, but, in highly viscous        in magnetic flux per unit time. In a      have a pulse frequency much lower
fluids, the size should be kept as        magnetic flowmeter, the electrode         (typically 5 to 10% of ac line
large as possible to avoid excessive      wires and the path through the            frequency) than ac meters. This
pressure drop across the meter.           conductive liquid between the             lower frequency eliminates noise
                                          electrodes represent a single turn        pickup from nearby ac lines,
Temperature                               loop. The flow-dependent voltage is       allowing power and signal lines to
                                          in phase with the changing                be run in the same conduit and thus
The liquid’s temperature is generally     magnetic field; however, flow-            simplifying wiring. Wiring is further
not a problem, providing it remains       independent voltage is also               simplified by the use of integral
within the mechanism’s operating          generated, which is 90°out of phase       signal conditioners to provide
limits. The only other temperature        with the changing magnetic field.         voltage and current outputs. No
considerations would be in the case                                                 separate wiring to the signal
of liquids with low conductivities        The flow-independent voltage is
                                          therefore an error voltage which          conditioners is required.
(below around 3 micromhos per
centimeter) which are subject             is 90° out of phase with the desired
                                          signal. This error voltage is often       Power
to wide temperature excursions.
Since most liquids exhibit a              referred to as quadrature. In order       By design, ac magnetic flowmeters
positive temperature coefficient          to minimize the amount of                 typically have high power
of conductivity, the liquid’s minimum     quadrature generated, the electrode       requirements, owing to the fact
conductivity must be determined           wires must be arranged so that they       that the magnetic field is constantly
at the lower temperature extreme.         are parallel with the lines of flux of    being powered. Because of the
                                          the magnetic field.                       pulsed nature of the dc pulse
Advantages of the                         In ac field magmeters, because            magmeter, power is supplied
DC Pulse Style                            the magnetic field alternates             intermittently to the magnetic field
                                          continuously at line frequency,           coil. This greatly reduces both
From the principles of operation,         quadrature is significant. It is          power requirements and heating
it can be seen that a magnetic            necessary to employ phase-                of the electronic circuitry, extending
flowmeter relies on the voltage           sensitive circuitry to detect and         the life of the instrument.
generated by the flow of a                reject quadrature.
conductive liquid through its
magnetic field for a direct indication    It is this circuitry which makes
of the velocity of the liquid or slurry   the ac magnetic meter highly
being metered. The integrity of this      sensitive to coating on the
low-level voltage signal (typically       electrodes. Since coatings
measured in hundreds of                   cause a phase shift in the
microvolts) must be preserved so as       voltage signal, phase-
to maintain the high accuracy             sensitive circuitry leads to
specification of magnetic flowmeters      rejection of the true voltage
in industrial environments. The           flow signal, thus leading
superiority of the dc pulse over the      to error.
traditional ac magnetic meters in         Since dc pulse magmeters
preserving signal integrity can be        are not sensitive to phase
demonstrated as follows:                  shift and require no phase-
                                          sensitive circuitry, coatings
Quadrature                                on the electrodes have
                                          a very limited effect on
Some magnetic flowmeters employ           flowmeter performance.
alternating current to excite the
magnetic field coils which generate       Wiring
the magnetic field of the flowmeter
(ac magnetic flowmeters). As a            In ac magnetic flowmeters,
result, the direction of the magnetic     the signal generated by flow
field alternates at line frequency,       through the meter is at line
i.e., 50 to 60 times per second.          frequency. This makes these
If a loop of conductive wire is           meters susceptible to noise
located in a magnetic field, a            pickup from any ac lines.
voltage will be generated in that         Therefore, complicated wiring
loop of wire. From physics, we can        systems are typically required
determine that this voltage is 90°        to isolate the ac flowmeter
out of phase with respect to              signal lines from both its own
the primary magnetic field.               and from any other nearby
                                                                           Figure 3: Vertical installation of inline meter
In traditional ac magnetic
flowmeters, it is necessary after
installation of the meter to “null”
or “zero” the unit. 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 OMEGA® 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         Figure 4: Horizontal installation of in-line meter
voltage is measured during this
period, it is considered extraneous                                              electromagnetic fields in the same
noise in the system and is             the tee fitting supplied with the
                                       meter, or by drilling a tap into the      area as the magmeter. Grounding is
subtracted from the signal voltage                                               achieved by connecting the piping
generated when the magnetic field      line to accept the fitting that comes
                                       with the meter.                           system and the flowmeter to a
is on. This feature insures high                                                 proper earth ground system.
accuracy, even in electrically noisy   Prior to installation of the meter, the
                                       following recommendations and             Unfortunately, this is not always
industrial environments.                                                         done properly, resulting in
                                       items of general information should
Installation                           be considered.                            unsatisfactory meter performance.
                                                                                 In conductive piping systems, a
OMEGA magnetic flowmeters are
designed for easy installation.
                                       First, before installing a magmeter,
                                       it is important to consider location.
                                                                                 “third wire” safety ground to the
                                                                                 power supply and a conductive path
FMG600 Series Magmeters are            Stray electromagnetic or                  between the meter and the piping
ideal substitutes for the flanged      electrostatic fields of high intensity    flanges are typically all that is
spool type meter, which are heavier    may cause disturbances in normal          required. In non-conductive or lined
and significantly more expensive.      operation. For this reason, it is         piping systems, a protective
The thin wafer style of the FMG600     desirable to locate the meter away        grounding orifice must be supplied
Series allows them to be slipped       from large electric motors,               to provide access to the potential of
between standard flanges, without      transformers, communications              the liquid being metered. Dedicated
the need to cut away pipe to make      equipment, etc., whenever possible.       or sophisticated grounding systems
room for the meter. Furthermore,       Second, for proper and accurate           are not normally required. Detailed
the low weight of the meter means      operation, it is necessary that the       information concerning proper
that, in many cases, no additional     flowmeter be installed so that the        flowmeter grounding is provided
pipe supports are required after       pipe will be full of the process liquid   with the owner’s manual that comes
meter installation. Recommended        under all operating conditions.           with each flowmeter.
piping configurations include          When the meter is only partially
the installation of by-pass piping,                                              Finally, the position of the flowtube
                                       filled, even though the electrodes        in relation to other devices in the
cleanout tees and isolation valves     are covered, an inaccurate
around the flowmeter                                                             system is also important in assuring
                                       measurement will result.                  system accuracy. Tees, elbows,
(Figures 3 and 4).
                                       Third, for magnetic flowmeters,           valves, etc., should be placed at
With insertion-style magmeters,        grounding is required to eliminate        least 10 upstream and 5
even greater reductions in weight      stray current and voltage which may       downstream pipe diameters away
and cost have been achieved.           be transmitted through the piping         from the meter to minimize any
Installation is accomplished by        system, through the process liquid,       obstructions or flow disturbances.
threading the piping system into       or can arise by induction from

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