level measuring sensors (30)

7/31/2019 Level Measuring Sensors (30)

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LEVEL MEASURINGSENSORS

A Presentation

of

Industrial Instrumentation


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Presented to:

Dr. Ing. Naveed Ramzan

Presented By:

Yasir Javed

2007-Chem-30


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LEVEL MEASURING SENSORS

Level measuring sensors detect the level ofsubstances 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 alake).

In open tank or closed tank


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CLASSIFICATIONOF LEVEL MEASURINGSYSTEMS

Liquid or Solid Level Measurement

Point or Continuous Level Measurement *

Electromechanical or Electromagnetic Level

Measurement Contacting or Non-Contacting Level Measurement


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GENERAL CONSIDERATIONSIN LEVELMEASURING 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 ANDDETECTION SYSTEMS

Point level measurement sensors are used to marka single discrete liquid heighta preset levelcondition.

They are:

Floats

Displacers

Conductivity Probes

Vibrating fork Resonance


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FLOATS

A float operated liquid level control

operates on the basic buoyancy

principle, which states, "a body (float)

immersed in a liquid is buoyed upwardby 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 liquidlevel moves.


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ADVANTAGESAND DISADVANTAGES

The Advantages of this method are that it isrelatively simple, suitable for various products, andvery accurate.

The Disadvantages are that it requires a certainamount of mechanical equipment, especially inpressure vessels.


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DISPLACERS

Displacer level sensors are force balance devicesin accordance with Archimedes' principle whichstates thatthe buoyant force acting on an objectequals the weight of the fluid displaced.


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The obvious Advantage of this method is that theabsolute level does not change.

The Disadvantages are that it is dependent onrelative density and requires a significant amount ofmechanical equipment. Cost of installation fordisplacers is high and many refineries are nowreplacing them due to the inaccuracies experienced

under process density changes especially oninterface duties.


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CONDUCTIVITY

The principle for this form of level measurement isthat the presence of a product will cause a changein the resistance between two conductors.


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The Advantages of this method are simple,inexpensive and suitable for dual or multiple pointcontrol.

The Disadvantages are probe cannot becomecontaminated with grease or other deposits and haslimited suitability for products of varyingconductivity.


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VIBRATING FORKS RESONANCE

Frequency changes which shift tuning fork Piezocrystal which oscillates forks and Solution causes areduction in resonant frequency Second crystaldetects.


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Advantages:

No moving parts, thus no maintenance.

Can be mounted in any position.

Small and can be mounted in a 1-in or smaller

mounting boss.

Disadvantages:

They are prone to material buildup between the

tines. Build up material can cause inaccurate reading.


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CONTINUOUS LEVEL MEASUREMENTANDDETECTION SYSTEMS

Continuous level sensors are more sophisticatedand can provide level monitoring of an entiresystem.

They include;

Radar Systems

Ultrasonic Systems

RTD

Magnetostrictive


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RADAR

Extremely short microwave impulses with lowemitted power in the 6.3 GHz frequency range areemitted by the antenna system to the measuredproduct, reflected by the product surface andreceived again by the antenna system.


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ADVANTAGEAND DISADVANTAGE

The Advantage of radar is its broad applicability

on most liquids and measurement independent ofpressure, temperature, vapor, and (to a degree)product dielectric constant.

The Disadvantage is that the measurement maybe lost because of heavy agitation of the liquid orthe formation of foam.


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ADVANTAGESAND DISADVANTAGE

The main Advantages of ultrasonic level

instrumentation are that the transducer does notcome into contact with the process material, theyhave no moving parts and a single top of vesselentry makes leaks less probable than fully wetted

techniques. The main Disadvantage is that there are various

influences that affect the return signal. Things suchas powders, heavy vapors, surface turbulence,

foam and even ambient noise can affect thereturning signal. Temperature can also be a limitingfactor in many process applications.


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TIME-DOMAIN REflECTOMETRY TDR

The principle of this technology is to launch anextremely short microwave impulse on awaveguide, 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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The Advantages of TDR level measurements are

disregard of the presence of vapors, steam, dust,gas layers, buildup, temperature changes, pressurechanges, acoustic noise, changing density of thematerial to be measured, changing dielectricconstant of the material to be measured, and

changing conductivity of the material to bemeasured.

Some Disadvantages of this measuring system

are that it does come in contact with the processand is an intrusive form of measurement. Thesystem is not suitable for highly agitated liquids.


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MAGNETOSTRICTIVE

A magnetostrictive system consists of amagnetostrictive 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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Advantages are high accuracy and independence

from material characteristics such as conductivityand dielectric constants.

Disadvantages are limitations to relatively cleanliquids and anything that would cause the float to

hang up on the probe.


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