Magnetic flow meters are
widely used to measure the flow rate of conductive liquids in process
applications. Recent technological developments have been made them relatively
easy to install and often more economical than other designs. A primary
advantage of this type of instrument is that magnetic flow meters provide
virtually unobstructed flow. Because the flow path is relatively unobstructed,
they are well suited to use with high viscosity fluids that contain solids.
Important application includes flow measurement of sludge in sewage treatment
plant, quarries in mining operations, and liquid metals in various industrial
processes.
In addition magnetic flow
meters do not create pressure drop. Velocity is measured directly and thus variations
in density do not affect their accuracy. Further these instruments are free of
Reynolds number constraints; they may be used for measuring the velocity of
liquids with any flow profile.
In general magnetic flow
meters are accurate reliable measurement devices that do not intrude into the
system. Often, these instruments can be maintained without shutting down the
process. Further, since they produce an electrical output this type of flow
meter is compatible with electronic control system.
PRINCIPLE
OF OPERATION
Magnetic flow meters are the
first type of flow meters to be considered for highly corrosive application and
for application involving flow measurements of erosive slurries and conductive
fluids. These types of meters use Faradays Law of Electromagnetic Induction for
making flow measurement.
Faraday's Law of
Electromagnetic Induction states that whenever a current carring conductor
moves through a magnetic field of known field strength, a voltage is induced in
the conductor which is proportional to the relative velocity between conductor
and the magnetic field.
In the case of magnetic flow
meter electrically conductive flowing liquid works as the conductor. The
induced voltage is given as,
E = CBLV
Where: B – Strength of
magnetic field.
C – Dimensional constant
L – Length of the conductor
E – Induced voltage
V – Velocity of the flowing
fluid
To convert velocity measured
into volumetric flow
Q = VA
Where: Q – Volumetric flow
rate
V – Fluid velocity
A – Area of cross section of
flow meter
CONSTRUCTION
AND WORKING
The magnetic flow meter
consists of an electrically insulated or non conducting pipe with a pair of
electrodes mounted inside the pipe wall opposite to each other. There will be a
constant magnetic field across the pipe. The fluid under measurement acts as
the conductor. The direction of magnetic field and fluid velocity should be
mutually perpendicular to each other.
As the liquid passes through
the flow meter an EMF is induced in the liquid which is detected by the pair of
electrodes mounted in the inner wall of the flow meter pipe. This induced
voltage is proportional to the velocity of the flowing fluid. The measurement
of magnetic flow meters is independent of viscosity, density, temperature and
pressure.
Some magnetic flow meters
are available with removable electrodes. This option allows the electrodes to
be inspected periodically and cleaned if necessary, without dismantling the
meter. Ultrasonic cleaning system are also available to move accumulated non
conductive material from electrodes with opening system.
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