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7/27/2019 Level Measurement.pptx
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PROCESS
INSTRUMENTATION
A Presentation
on
Level measurement
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Presented to:
Madhusudhan Sir
Presented By:
Nitish.G.R
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LEVELMEASUREMENT
Level measurement is defined as the determination
of position of an interface between the two media.
These media may be solids, fluids or thecombination of solid and fluid.
Many methods are available for the measurement
of level with its own advantages and disadvantages
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IMPORTANCEOFLEVELMEASUREMENT
The measurement of level is of great importance in
the industrial field, particularly when control is also
necessary.
Most of the industrial process require the amount of
material in a tank or closed vessels.
In many situations it is not possible to view theinterior of equipments directly.
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LEVEL MEASURING SENSORS
Level measuring sensors detect the level of
substances that flow, including liquids, slurries,
granular materials, and powders.
The substance whose level to be measured can be
Inside a container or in its natural form (e.g. a river or a
lake).
In open tank or closed tank
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CLASSIFICATIONOF LEVEL MEASURING
SYSTEMS
Liquid or Solid Level Measurement
Point or Continuous Level Measurement *
Electromechanical or Electromagnetic LevelMeasurement
Contacting or Non-Contacting Level Measurement
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GENERAL CONSIDERATIONSIN LEVEL
MEASURING SENSOR SELECTION
Density and viscosity
Chemical composition
Ambient temperature Process temperature
Process pressure
Regulated environment
Process agitation
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POINT/SWITCH LEVEL MEASUREMENT AND
DETECTION SYSTEMS
Point level measurement sensors are used to mark
a single discrete liquid heighta preset level
condition.
They are:
Floats
Displacers
Conductivity Probes
Vibrating fork Resonance
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FLOATS
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A float operated liquid level control operates on the
basic buoyancy principle, which states, "a body
(float) immersed in a liquid is buoyed upward by a
force equal to the weight of the
displaced liquid". As a result, floats ride on the
liquid surface partially submerged and move the
same distance the liquid level moves.
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ADVANTAGESAND DISADVANTAGES:
The Advantages of this method are that it is
relatively simple, suitable for various products, and
very accurate.
The Disadvantages are that it requires a certain
amount of mechanical equipment, especially in
pressure vessels.
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DISPLACERS
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DISPLACERS
Displacer level sensors are force balance devices
in accordance with Archimedes' principle which
states thatthe buoyant force acting on an object
equals the weight of the fluid displaced.
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ADVANTAGESAND DISADVANTAGES:
The obvious Advantage of this method is that the
absolute level does not change.
The Disadvantages are that it is dependent on
relative density and requires a significant amount of
mechanical equipment. Cost of installation for
displacers is high and many refineries are now
replacing them due to the inaccuracies experienced
under process density changes especially on
interface duties.
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CONDUCTIVITYPROBES
The principle for this form of level measurement is
that the presence of a product will cause a change
in the resistance between two conductors.
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ADVANTAGESAND DISADVANTAGES:
The Advantages of this method are simple,
inexpensive and suitable for dual or multiple point
control.
The Disadvantages are probe cannot become
contaminated with grease or other deposits and has
limited suitability for products of varying
conductivity.
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VIBRATING FORKS RESONANCE
Frequency changes which shift tuning fork Piezo
crystal which oscillates forks and Solution causes a
reduction in resonant frequency Second crystal
detects.
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ADVANTAGESAND DISADVANTAGES:
Advantages:
No moving parts, thus no maintenance.
Can be mounted in any position.
Small and can be mounted in a 1-in or smallermounting boss.
Disadvantages:
They are prone to material buildup between thetines.
Build up material can cause inaccurate reading.
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CONTINUOUS LEVEL MEASUREMENTAND
DETECTION SYSTEMS
Continuous level sensors are more sophisticated
and can provide level monitoring of an entire
system.
They include;
Radar Systems
Ultrasonic Systems
RTD
Magnetostrictive
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RADAR
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It is a Continuous level measurement system
Extremely short microwave impulses with low
emitted power in the 6.3 GHz frequency range areemitted by the antenna system to the measured
product, reflected by the product surface and
received again by the antenna system.
Guided wave radar is used to measure the level of
bulk solids and powders.
Micro-pulses at the speed of light are transmitted
and reach the material surface and reflect back to
the GWR.
Time is calculated and it is converted into level of
the solid surface.
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ADVANTAGES:
Response is instantaneous.
Reading is not affected by the dust.
Accuracy is very high.
It is used to measure the level up to 115 feet. Can be used for high temperature and high
pressure.
Installation is very easy.
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DISADVANTAGES:
Purchasing cost is high.
Pulses can be damaged by the large particles.
Not used for sanitary applications.
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ULTRASONICLEVELMEASUREMENT
Ultrasonic level measurement technique is used to
measure the level of solid without contacting the
solid.
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Ultrasonic level transmitter sends
sound waves into the vessel where
solid is placed and is reflected back
to the ultrasonic level transmitter.
Ultrasonic transmitters work on the principle of
sending a sound wave from a peizo electrictransducer to the contents of the vessel.
Software enhances the signal.
The device measures the length of time it takes for
the reflected sound wave to return to the
transducer.
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ADVANTAGES :
It is non-contact sensor.
It compensates the temperature changes automatically.
Can be used for temperature less than 3000F.
It can be used for less than 260 feet.
Response time is less.
Installation is easy.
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DISADVANTAGES:
It is used for pressure less than 40psi.
Reading is changed due to the change in the angle
of repose.
It is not suitable for light weight material due to the
absorbing of sound waves signal.
Heavy dust affects the performance.
Uneven shape of solids affects the performance.
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TIME-DOMAIN REFLECTOMETRY TDR
The principle of this technology is to launch an
extremely short microwave impulse on a
waveguide, which can be a cable or rod.
This waveguide, or probe,
contains the signal in a
close area around the
cable or rod.
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ADVANTAGESAND DISADVANTAGES:
The Advantages of TDR level measurements are
disregard of the presence of vapors, steam, dust, gas
layers, buildup, temperature changes, pressure
changes, acoustic noise, changing density of the
material to be measured, changing dielectric constant ofthe material to be measured, and changing conductivity
of the material to be measured.
Some Disadvantages of this measuring system are that
it does come in contact with the process and is anintrusive form of measurement. The system is not
suitable for highly agitated liquids.
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MAGNETOSTRICTIVE
A magnetostrictive system consists of a
magnetostrictive wire in a probe and a donut-
shaped float containing a permanent magnet. The
float is the only moving part and travels up and
down the probe.
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ADVANTAGESAND DISADVANTAGES:
Advantages are high accuracy and independence
from material characteristics such as conductivity
and dielectric constants.
Disadvantages are limitations to relatively clean
liquids and anything that would cause the float to
hang up on the probe.
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OTHERS METHODS FOR LEVEL
MEASUREMENT
Dip Stick
Weight and cable
Laser. Thru wave radar
Load cells.
Strain gauges
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DIPSTICK
Simple and cheap
Can be used with any wetmaterial and not affected by
density.
Can not be used withpressurized tanks
Visual indication only(electronic versions areavailable)
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WEIGHTANDCABLELEVELSENSOR
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WEIGHTANDCABLELEVELSENSOR
Weight and cable level sensors have been used for
many years to measure the level of the solids in
storage vessels.
The instrument lowers a weight on the cable and
touches the solid surface and rises to the sensor.
The distance it travels is measured and converted
into level of the solid.
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ADVANTAGES:
For high temperature system.
It is very easy to install.
Less cost.
It is not affected by the angle of repose. It can be used for dusty particles.
Not affected by the process condition.
It can be used to measure the level up to 150 feet.
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DISADVANTAGES:
Response time is high.
For dusty application regular maintenance is
required.
It is used for pressure less than 30psi.
Accuracy is moderate for greater than 30 feet.
It has moving parts so parts may wear and require
maintenance.
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LASERLEVELSYSTEM
It is used to measure the
level for very difficult
applications.
Laser is a narrow beam
that does not scatter on
reflection.
It is used for long ranges.
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ADVANTAGES:
No calibration is required.
It can measure level up to 250 feet.
Reading is not affected by the angle of repose. No effect of process conditions (temperature,
pressure).
Installation is easy.
Response is instantaneous. High accuracy.
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DISADVANTAGES:
Purchasing cost is very high.
It is not suitable for high dust application
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THRU-AIRRADARSENSOR
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It operates on the same principle of GWR (Guided
Wave radar) sensor. But it more desirable due to
following advantages over GWR.
It is non-intrusive.
It can operate at higher temperature and pressure
than guided wave radar level sensor.
It is used to measure the level up to 130feet.
Response is instantaneous than GWR
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DISADVANTAGES:
It is used for those type of materials having
dielectric constant greater than 1.8.
Cleaning is required for dusty applications.
Purchasing cost is high.
Proper installation and set up is required.
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LOADCELLS
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Load cells operate on the principle that when loads
are applied on them than they change their
resistance, capacitance and inductance.
But mostly are designed to change their resistance.
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ADVANTAGES:
It is not intrusive system.
Can be used for extreme conditions.
Un-affected by the dust. It can be used for dangerous materials.
Highly accurate.
Ranges from 100 lbs to 1,000,000 lbs.
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DISADVANTAGES:
Purchasing cost is high.
Calibration is required.
Require extra attention to get accuracy.
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THANKYOU