different techniques of level measurments
TRANSCRIPT
DIFFERENT TECHNIQUES OF LEVEL MEASURMENTS
Prem BabooSr. Manager (Prod)National Fertilizers Ltd. IndiaF.I.E., Institution of Engineers (India)Technical Advisor & an Expert for www.ureaknowhow.com
CONTENTS1.INTRODUCTION .2.PHYSICAL TECHNIQUE .3.PRESSURE TECHNIQUE .4.BUOYANCY TECHNIQUE .5.CAPACITANCE TECHNIQUE .6.RADAR TECHNIQUE .7.CONDUCTIVITY TECHNIQUE8.ULTRASONIC TECHNIQUE .9.POWER SONIC TECHNIQUE .10.RADIATION TECHNIQUE .
INTRODUCTION .
• What is a level ?
• WHY IT IS MEASURED ?
• WHY ITS MEASUREMENT IS IMPORTANT
LEVEL
THE LEVEL MAY BE EXPRESSED IN TERM OF PRESSURE EXERTED OVER A DATUM LEVEL OR IN TERM OF THE LENGTH OF THE
LIQUID COLUMN.
WHY IT IS MEASURED ?TO ENSURE THAT RIGHT AMOUNT OF LIQUID/SOLID ARE ADDED TO
THE VESSEL AT RIGHT TIME AND FOR SAFE OPERATION.
WHY ITS MEASUREMENT IS IMPORTANT ?
LEVEL MEASUREMENT IS ONE OF THE IMPORTANT PARAMETER IN ANY PROCESS INDUSTRY LIKE OUR FERTILIZER PLANT.THE LEVEL HAS A SIGNIFICANT EFFECT ON PROCESS QUALITY, CONTROLLABILITY , PROCESS STABILITY AND OPTIMISATION . HENCE PRECISE, ACCURATE AND RELIABLE LEVEL MEASUREMENT IS NECESSARY . A WIDE VARIETY OF LEVEL MEASURE MENT TECHNIQUES ARE AVAILABLE TO MEET THE DIVERSE LEVEL REQUIREMENTS OF THE PROCESS INDUSTRY EVOLVED OVER THE YEARS.
PHYSICAL TECHNIQUE
A.
DIRECT METHOD.
B.
INDIRECT METHOD.
GLASSES
DRAIN
LEVEL GAUGE
I/S VALVES
DRAIN VALVE
CHECK VALVE
CHECK VALVE
TECNOMATIC (I) PR. LTD.
CHECK VALVE.Level gauges have auto shut off ball check valves generally. In the event of glass failure or gasket burst, pressure difference between vessel and gauge causes ball check to close entry of process fluid in to the gauge chamber, Thus reducing spillover of valuable process fluid. The extended tip of trim prevents ball to shut off gauge during startup.
SOME SAFETY HINTS • Assembling and disassembling of gauge glass must
be done with the help of “ TORQUE WRENCH” and in the sequence as shown in figure.
• The glass should not be in contact with any metal surface. Temperature differences at points of contact will set up high loading and may break glass.
• In steam service-glasses should be protected by mica shields to reduce steam erosion and retighten gauge after attains process temperature.
• While taking in line 1st isolation valve should be crack open & care must be taken to slowly warm up the gauge glass and anyone should not be in front of level gauge.
LEVEL GAUGE
ASSEMBLING &DISASSEMBLINGSEQUENCE
TRANSPARENT GLASS MODULE
• Material : Borosilicate Glass
GASKET• Material :Asbestos (High Temp & Press).
• Teflon optional.
• Thickness : 1~ 2 mm Approx.
MICA• It is provided to protect the Glass
from heat
INDIRECT METHOD1.MAGNETIC LG:= WHEN LOCAL LEVEL INDICATION IS REQUIRED ON CORROSIVE &
TOXIC / DANGERS MATERIAL .THE USE OF GLASS OR GASKETS AND THREADED CONNECTIONS IS NOT DESIRABLE FOR SUCH APPLICATIONS MAGN ETIC LEVEL GAUGES ARE USED.A FLOAT WITH PERMANENT MAGNET INSIDE A SEALED CHAMBER AND INDICATOR WAFERS ARE ATTRACTS TOWARDS MAGNET WHICH ARE IN GLASS TUBE OUT SIDE THE CHAMBER,AS LEVEL GOES UP AND DOWN IN THE VESSEL.
2.FLOAT ACTUATED LG:- IT IS EMPLOYED IN OPEN VESSELS .THE FLOAT RESTS ON THE
SURFACE OF LIQUID,SUPPORTEDLY BY BUOY- ANT FORCE.THE FLOAT IS CONNECTED TO A LIGHT WE- IGHT CABLE,THE OTHER END OF WHICH IS CONNECTED TO A COUNTER WEIGHT. COUNTER WT ALSO ACTS AS INDICATOR AND MOVES AGAINST A VERTICAL SCALE.
MAGNETIC LEVEL GAUGE
FLOAT ACTUATED LEVELGAUGE
PRESSURE TECHNIQUCE .
1.
STATIC PRESSURE ACTUATED MECHANISMS .
2.
DIFFERENTIAL PRESSURE ACTUATED MECHANISMS .
1.STATIC PRESSURE ACTUATED MECHANISMS .
Pressure is defined as the force per unit area
WHERE THE LIQUID IS REQUIRED IN AN OPEN TANK,THE PRESSURE DUE TO
LIQUID MAY BE FOUND BY USING WITH A PG/LT CALIBRATED IN TERMS OF
LEVEL,DIRECTLY MOUNTED AT 2” OR 3” ABOVE THE BOTTOM OF TANK.
PRESSURE CONVERSION TABLEatm bar Kg/cm2 lb/in2 mmwc In of
H2OkPa
atm 1.0133 1.0332 14.70 760.0 407.14 101.33
bar 0.9869 1.0197 14.504 750.0 401.81 100.0
Kg/cm2 0.9678 0.9807 14.22 735.5 394.05 98.07
lb/in2 0.0680 0.0689 0.0703 51.7 27.69 6.89
mmwc 1.316 X 10(-3)
1.333 X 10(-3)
1.360 X 10(-3)
1.934X 10(-2)
0.536 0.133
In of H2O
2.456 X 10(-3)
2.489 X 10(-3)
2.538 X 10(-3)
3.611 X 10(-2)
1.867 0.249
kPa 9.869 X 10(-3)
1X10(-2 1.020 X 10(-2)
0.145 7.5 4.018
STATIC PR.METHOD LOCAL
LOCAL INDICATORPG
PRESSURE= h (SG)WHERE, h IS HEAD.SG IS SPECFIC GRAVITY.
STATIC PR.METHOD REMOTE
SUPPLY / OUT-PUT FORREMOTE
LT
PRESSURE= h (SG)WHERE, h IS HEAD.SG IS SPECFIC GRAVITY.
+-
STATIC PRESSURE ACTUATED MECHANISMS .
AIR SUPPLYAFR
PURGE TUBE
ROTA-METER
FEED BACK PRES
24VDC & O/P WIRES
PT
PG
PURGE TUBE
AIR SUPPLY
STATIC PR.PURGE METHOD. WITH A PURGING GAS/AIR BLOWN
THROUGH THE STAND PIPE WHICH GOES DOWN ALL MOST TO THE BOTTOM OF THE TANK ABOUT 2” OR 3” ABOVE THE SEDIMENTS,THE PRESSURE IN THE STAND PIPE WILL BUILD UP UNTILL IT IS EQUAL TO THAT DUE TO THE LIQUID ABOVE THE LEVEL OF THE BOTTOM TIP OF THE PIPE. THE PRESSURE IN THE BUBBLER PIPE IS SLIGHTLY GREATER THAN THE HEAD OVER THE BUBBLER PIPE.
2.DP. ACTUATED MECHANISMS
DIFFERENTIAL PRESSURE METHOD ARE USED FOR
LEVEL MEASUREMENT BY USING D.P. CELLS .
STRAIN BASED D.P. CELLS
•
Figure 3-4: Strain-Gage Based Pressure Cell
CAPACITANCE BASED D.P. CELL
•
Figure 3-5: Capacitance-Based Pressure Cell
DIAPHRAM • P1
• P2
DIAGPHRAGM This is a widely used elastic element as a pressure
transducer. As used in gauge diaphragm ,are thin piece of material which will move when force is applied to one or the other or both side .the motion of diaphragm can be transmitted by the suitable linkage to make it indicating device. There are two classes of diaphragm depending upon physical characteristics , and those made from materials which are having no spring characteristic. The diaphragm depending on another spring are called slack diaphragm.
CAPSULEThe capsule pressure gauge is nothing but it is combined two diaphragm back to back The efficiency and accuracy is than flat ,corrugate dished type spring diaphragm. The large deflection is produced by cascading arrangement of capsules. The linkage and pointer mechanism is same as flat, corrugated and dished type diaphragm.
BELLOWS• The basic advantage of using bellows other
diaphragm or or bourdon is that it developes larger force to operate linkage in mechanical gauge or mechanical transducer.It can be used for gauge,absolute and differential pressure measurements.The materials used for fabricating bellows are stainless steel, brass,phosphor bronze,monel.
BELLOW• Link Accessory
• Pressure
LIQUID HAVE CONDENSIBLE VAPOURS
SUPPLY / OUT-PUT FORREMOTE
CONDENSATE POT SLIGHTLYHIGH THAN IMPULSE CONNECTION
LP
HP
LT
-+
BY USE OF DPT MOST OF THE LIQUID UNDER PRESS,HAVE
CONDENSIBLE VAPOURS , SO TO PREVENT IN-ACCURACIES DUE TO CONDENSATE IN IMPULSE LINE CONNECTING THE D.P.CELL TO THE TOP OF THE TANK. A CONDENSING POT IS USED , IT IS HIGH PRESSURE SIDE OF D.P.CELL TO A CONSTANT LIQUID HEAD+ VAPOUR PRESSURE & LOWER PRESS.SIDE TO THE VARIABLE HEAD + VAPOUR PRESS. NOW D.P.CELL MEASURES THE DIFF, PRESS IN TERMS OF LEVE OF THE VESSEL .
SUPPLY / OUT-PUT FORREMOTE
+ -
LP
HP
LIQUID HAVE NON-CONDENSIBLE VAPOURS
X
X
DP OF A STEAM DRUM
DP OF A STEAM DRUM
DP=H1(SG1)+H2(SG2)-H3(SG3)
DP=0.03H1+0.76H2-0.99H3
CRYOGENIC MATERIAL WHEN PROCESS MATERIAL IS LIQUID LIKE NITROGEN THE VESSEL
IS USUALLY SURROUNDED BY THERMALLY INSULATED & EVACUATED COLD BOX. HERE THE LP SIDE OF DIRECT ACTING DP CELL IS CONNECTED TO VAPOUR SPACE ABOVE THE LIQUID NITROGEN. AS LIQUID NITROGEN APPROACHES THE HP SIDE OF DP CELL WHICH IS AT AMBIENT TEMP. OUTSIDE THE COLD BOX ,ITS TEMP. REACHES THE BOILING POINT OF NITROGEN. IT WILL BOIL & FROM THAT POINT ON THE CONNECTING LINE WILL BE FILLED WITH NITROGEN VAPOUR . THIS CAN CAUSE NOISE IN THE LEVEL MEASURE MENT TO PROTECT AGAINST THIS, THE LIQUID PORTION OF THE CONNECTING LINE SHOULD BE SLOPED BACK TOWARDS THE TANK & CROSS SECTION OF THE LINE BE LARGE ( ABOUT INCH IN Dia) TO MNIMIZE THE TURBULENCE CAUSED BY THE SIMULTANEOUS BOILING & RECONDENSING OCCURING AT THE LIQUID VAPOUR INTERFACE.
BY USE OF DPT (PURGE)
FOR HIGH VISCOSIVE, SUSPENDED PARTICLES & SLURRIES LIQUIDS PURGE TYPE DIFFERENTIAL PRESSURE TRANSMITTER ARE USED PURGE MEDIA MAY BE WATER OR GAS / AIR .
PURGE IS AIR OR GAS
PURGE ROTAMETER
DIFF,PRESS TRANSMETER
LP SIDE
HP SIDE
A-FIG
DPT
x
x
x x
x xx
x x
DRAIN VALVE
PURGE IS WATER
PURGE ROTAMETER
DIFF,PRESS TRANSMETER
HP SIDE
LP SIDE
B-FIG
DPT
x
x
x x
x xx
x x
DRAIN VALVE
BUOYANCY TECHNIQUCE
1.
FLOAT TYPE LEVEL TROL.
2.
FLOAT TYPE LEVEL INDICATOR.
3.
FLOAT TYPE LEVEL SWITCH.
1.FLOAT TYPE LT.FLOATS ARE MOTION BALANCE DEVICES
THAT MOVE UP & DOWN WITH LIQUID LEVEL ,DUE TO WEIGHT VARIATION OF FLOATS IN ACCORDANCE WITH BUOYANT FORCE ACTING ON FLOATS & WEIGHT VARIATION OF FLOATS IS EQUAL TO FLUID DISPLACED.
PRINCIPLE• ARCHIMEDES PRINCIPLE STATED THAT ,WHEN AN
OBJECT IS WHOLLY OR PARTIALLY IMMERSED IN A FLUID, IT EXPERIENCES A BUOYANT FORCE (OR UPTHRUST) WHICH IS EQUAL TO WEIGHT OF FLUID DISPLACED BY THE OBJECT.
• THE TENDENCY OF A BODY TO FLOAT OR TO RISE WHEN SUB- MERGED IN FLUID IS CALLED ,BUOYANCY.
• 100 CUBIC INCH DISPLACER WILL GENERATE A BUOYANT FORCE OF 3.6 POUND.
FLOAT CHAMBER
0% LEVEL
100% LEVEL
DRAIN VALVE
LEVELTROL
WORKING NOW WHEN THERE IS NO LIQUID IN THE FLOAT
CHAMBER,THE ENTIRED WEIGHT OF FLOAT ATTACHED TO THE TORQUE ARM CAUSES A
TURING MOMENT ON TORQUE TUBE EQUAL TO TORSIONAL STRESS SET UP IN THE TORQUE ROD
IT IS ZERO SIGNAL TO THE TRANSDUCER ATTACHED AT OTHER END OF TORQUE ROD. FILL
UP THE LIQUID/WATER UPTO 100%. ANGULAR DISPLACEMENT TO OTHER END OF TORQUE ROD IS PROPORTIONAL TO LOSS OF THE WEIGHT OF
FLOAT AND IS AMPLIFIED BY TRANSDUCER.
NOTE:- SEE SG OF LIQUID IF CALIBRATE BY WATER
DENSITY
MATERIAL DENSITY(g/cm3)
ICE 0.92
WATER 1
GLYCERINE 1.26
IRON 7.8
MERCURY 13.6
GOLD 19.3
AMMONIA 0.587
UREA 1.15
CONDENSATE 0.83
DISPLACERS
• For 1.st and 2.nd series standard displacers are made of AISI 316 stainless steel.
• For 3.rd and 4.th series displacers are made of solid Aluminum 99.5%.
• Special Teflon displacers are available for all rating and for temperature below 150 Deg C.
• Level range: 14”, 32”, 48”,60”,72”,84”,96” and 120”.On request all sizes.
• For displacer SG limits are defined.
FEATURE OF LEVEL TROL• LT ARE MORE ACURATE & RELIABLE
FOR CLEAN LIQUID OF CONSTANT DENSITY &TEMPERATURE.
• BY FLOAT CHAMBER,IT IS CALIBRATE / REPAIR WITHOUT DISTURBING PROCESS.
• FLOAT LENGTH AVAILABLE ARE 14”, 32”,48”,60” AND EVEN UPTO 60 feet.
LIMITATION OF LT
• LT IS NOT SUITED FOR SLURRY, SLUDGE SERVICES BECAUSE COATING OF DISPLACER CHANGE ITS VOLUME & THEREFORE ITS BUOYANT FORCE.
• CHANGE IN DENSITY / TEMP OF PROCESS MATRIAL CAUSE ERROR.
• COSTLY AS COMPARED.
WHY DENSITY CAUSE AN ERROR ?
WEIGHT = MASS *ACCELERATION DUE TO GRAVITY.
Ie, 1 NEWTON = 1 kg * 9.8 m/s.sq.
MASS = VOLUME * DENSITY.
SO,
WEIGHT =
VOLUME * DENSITY * ACCELERATION. DUE TO GRAVITY
2.FLOAT TYPE LEVE INDICATOR.
3.FLOAT TYPE LEVEL SWITCH.
3.FLOAT TYPE LEVEL SWITCH.
TYPICAL L/S APPLICATION
RADAR TECHNIQUE .
• HISTORY :FIRST TIME(1925),A.TAYLOR & LEO YOUNG OF U.S.NAVY USED RADAR TO MEASURE THE HEIGHT OF EARTH’S IONOSPHERE.
• 1934,THEY USED FOR NAVY SHIPS.
• 1935,ENGLAND USED IT TO DETECT AIRCRAFT.
• IT USED AS LEVEL GAUGE IN 1976.
PRINCIPLE.THE PRINCIPLE OF WORKING
RADAR TYPE LEVEL TRANS MITTER IS BASED UPON THE
TECHNOLOGY TDR (TIME DOMAIN REFLECTOMETRY).
RADAR STAND FOR ?
RADARRA-STANDS FOR RADIO.
D-STANDSFOR DETECTION.
A- STANDS FOR AND.
R-STANDS FOR RANGING.
COMPONENTS.
1. RADAR SENSORS CONSIST OF TRANSMITT- ER,AN ANTENNA,A RECEIVER WITH SIGNAL PROCESSOR. TRANSMITTER IS MOUNTED ON TOP OF VESSEL.
2. ITS SOLID STATE OSCILLATOR SENDS OUT AN ELECTROMAGNETIC WAVES (USING A SELECTED CARRIER FREQUENCY AND WAVE- FORM) AIMED DOWN AT THE SURFACE OF THE PROCESS FLUID IN VESSEL. THE FREQUENCY USED IS TYPICALLY 10 GHz.
WORKING.THE ANTENNA IS OF TWO TYPES PARABOLIC DISH AND HORN TYPE.THE SIGNAL RADIATED TOWARDS THE SURFACE OF THE PROCESS LIQUID.A PORTION IS REFLECTED BACK TO THE ANTENNA,WHERE IT IS COLLECTED & ROUTED TO THE RECEIVER.HERE, A MICROPROCESSOR CALCULATES THE TIME OF FLIGHT AND HENCE THE LEVEL OF PROCESS LIQUID.
TIME OF FLIGHT
TIME OF FLIGHT .
RADARPULSE
PULSE SEQUENCE
TIME FOR RECEIVER
TIME OF FLIGHT• THE TIME OF FLIGHT IS THE PERIOD
BETWEEN THE TRANSMISSION OF RADAR PULSE AND THE RECEPTION OF THE RETURN ECHO. IT IS DETERMINED BY THE RADAR DETECTOR,WHICH IS SIMULTANEOUSLY EXPOSED TO SENT AND REFLECTED SIGNAL. DETECTOR O/P IS BASED ON THE DIFFERENCE.
FOR EXAMPLE• IF THE IMPULSES ARE EMITTED BY
ANTENA SYSTEM AS IMPULSE PACKETS WITH PULSE DURATION OF 1/n SECOND AND IMPULSE BREAKS OF 278 ns, THIS CORRESPONDS TO A PULSE FREQUENCY OF 3.6 MHz.
t
t
TIME TRANSFORMATION
THE LEVEL.
THE DEPTH OF VAPOUR SPACE IS AS
WHERE “T” IS TIME OF FLIGHT AND “C” IS SPEED OF RADAR SIGNAL & EQUAL TO VELOCITY OF LIGHT.
THE LEVEL IS L = E-d
‘d’= T/2c
THE RADAR SIGNALS REACT TO TWOELECTRICAL PRIMARY QUANTITIES:-
1. THE ELECTRICAL CONDUCTIVITY OF THE SUBSTANCE.
2. THE DIELECTRIC OF THE SUBSTANCE.
ALL PRODUCTS , WHICH ARE ELECTRICALLY CONDUCTIVE OR HAVE A DIELECTRIC CONSTANT GREATER THAN 1.5 ,HAVE SUFFICENT GOOD REFLECTION CHARACTERISTICS FOR RELIABLE & ACCURATE RADAR LEVEL MEASUREMENT.
DESIGN
• THE LARGER THE ANTENNA’S Dia,GREATER THE SIGNAL STRENGTH.
• THE DRAWBACK OF SMALL ANTENNA INCLUDE HIGHER BEAM SPREADING & THE CORRESPONDING INCREASED THE CHANCE OF REFLECTION FROM OBSTACLE WITH IN THE TANK & ON +VE SIDE,THERE IS A GREATER CHANCE THAT THE EMITTED BEAM WILL BE REFLECTED BACK TO DETECTOR,ALIGNMENT OF SENSOR IS NOT CRITICAL.
• CONTACT & NON-CONTACT ALSO AVAILABLE
FEATURES OF GWR ( LT )• LEVEL IS NOT AFFECTED BY MEDIA VARIABLES SUCH AS
PRESSURE,DENSITY,PH,VISCOSITY.• EASY BENCH CONFIGURATION-NO NEED FOR LEVEL
SIMULATION.• TWO WIRE,INTRINSICALLY SAFE LOOP POWERED • 360 DEGREE ROTATABLE HOUSING CAN BE DIS-
MANTLED WITHOUT DEPRESSURING THE VESSEL VIA QUICK CONNECT / DISCONNECT PROBE COUPLING,SO ON LINE MAINTANCE IS POSSIBLE.
• HART DIGITAL COMMUNICATION.• PROBE DESIGN UPTO 400 DEG–C / 350 KG/CM2• SATURATED STEAM APPLICATION UP TO 110 KG/CM2 &
320 DEG-C.• CRYOGENIC APPLICATION UPTO -150 DEG –C• ACCURACY UP TO 0.01% OF THE SPAN.
LIMITATION• REFLECTIVE PROPERTIES OF PROCESS MATERIAL
AFFECT RETURNED RADAR SIGNAL STRENGTH SO IF DUST PARTICLE SIZE INCREASES,OR IF THE FOAM & DUST GETS THICK,IT WILL NO LONGER DETECT THE LIQUID LEVEL. INSTEAD THE DUST OR FOAM LEVEL WILL BE MEASURED.
• INTERNAL PIPING,DEPOSITS ON THE ANTENNA,MULTIPLE REFLECTIONS OR REFLECTION FROM THE WALL CAN ALL INTERFERE WITH THE PROPER OPERATION OF A RADAR SENSOR.
• CANNOT BE USED IF LIQUID SURFACE NOT FLAT.
CAPACITANCE TECHNIQUE . • A CAPACITOR CONSISTS OF TWO CONDUCTORS
PLATES SEPARATED BY AN INSULATOR CALLED AS DIELECTRIC CONSTANT.
• THE WORD CAPACITANCE IS DERIVED FROM THE WORD CAPACITY.THE CAPACITY MEANS HOW MUCH QUANTITY CAN BE STORED / PLACED IN THE VESSEL,TANK . IN CASE OF THE CONDUCTOR WE ARE TAKING ABOUT HOW MUCH CHARGE CAN BE STORED/PLACED.
• IF WE GIVE THE +“Q” UNIT OF CHARGE TO THE CONDUCTOR, ITS POTENTIAL”V” WILL RISE. ACTUAL RISE OF THE POTENTIAL DUE TO THIS CHARGE WILL BE DEPEND ON THE SIZE OF THE CONDUCTOR AND ITS POSITION RELATIVE TO ANY NEGHBOURING CONDUCTOR.
Q = CV OR C = Q / V
*CAPACITY = CHARGE / POTENTIAL
THE CAPACITANCE UNIT IS THE FARAD,DEFINED AS THE POTENTIAL CREATED WHEN ONE VOLT.BATTERY IS CONCETED TO A CAPACITOR CAUSES THE STORAGE OF ONE COULOMB OF ELECTRIC ENERGY.
+ -
PLATES
DISTANCE=D
DIELETRIC (AIR)
VOLTS
FACTORS EFFECTING THE CAPACITY OF THE CONDENCER ARE AS:-
1.AREA OF THE PLATES.
2.DISTANCE BETWEEN THE PLATES.
CAPACITY OF THE CONDENCER(C) IS DIRECTLY PROPORTIONAL TO THE AREA 0F THE PLATE AND INVERSELY PROPORTIONAL TO THE DISTANCE BETWEEN THE PLATES.
C = K A / D WHERE “C” IS THE CAPACITANCE (FARAD) “A” IS AREA OF THE PLATES( METRE Sq ) “K” IS THE DIELECTRIC CONSTANT . “D” IS THE DISTANCE BETWEEN THE PLATES(METRE)CONSEQUENTLY;# THE LARGER THE AREA OF THE PLATES AND THE CLOSER THE PLATES , THE HIGHER THE CAPACITANCE.# THE HIGHER THE DIELECTRIC CONSTANT, THE HIGHER THE CAPACITANCE.
WORKINGIN LEVEL MEASUREMENT,AREA OF THE PLATE ( ELECTRODE ) AND DISTANCE BETWEEN ELECTRODES IS FIXED AND REMAINS CONSTANT. HENCE FROM ABOVE DISCUSSION CAPACITANCE IS DIRECTLY PROPORTINAL TO THE DIELECTRIC CONSTANT OF THE MATERIAL BETWEEN THE PLATES AND VARIES WITH RESPECT TO CHANGE IN LEVEL. THE BEST LEVEL MEASUREMENT ARE OBTAINED WHEN THE CHANGE IN CAPACITANCE ARE HIGH, MEANS CHANGE BY 0% AND 100% LEVEL.
PHYSICAL CAP. PROBE
Dead length
MEASUREMENT IS MADE BY APPLY AN RF ( BY THE ELECTRONIC CIRCUIT ) BETWEEN THE CONDUCTIVE PROBE AND VESSEL WALL .THE RF SIGNAL RESULTS IN A CURRENT FLOW THROUGH THE DIELECTRIC PROCESS MATERIAL IN THE TANK FROM THE PROBE TO THE VESSEL WALL.WHEN THE LEVEL IN THE TANK DROPS & THE PROBE IS EXPOSED TO THE VAPOUR / AIR,THE DIELECTRIC CONSTANT OF VAPOUR / AIR IS LESS AS COMPAR- ED TO THE PROCESS LIQUID.THIS CAUSE A DROP IN CAPACITANCE READING & DROPS IN CURRENT FLOW.THE CHANGE IS DETECTED BY THE ELECTRONIC CIRCUITRY AND TRANSLATED IN TO A SCALED ANALOG SIGNAL 4-20mA
24VDC,4-20mALOOP POWERED
FOAM CAUSE ERROR
DIELECTRIC STBILITYCRITICAL
MEDIALEVEL
MEDIABUILDUP
NOW DITANCE BETWEEN TWO CAPACITANCE PLATES ABOVE WATER SURFACE(CHAMBER & PROBE) IS LARGE,SO “CA” IS SMALL. CONVERSELY ,THE DISTANCE BETWEEN PLATES BELOW WATER SURFACE( PROBE & WATER) IS SMALL AND THEREFORE,THE “CB” WILL BE LARGER COMPARED WITH “CA”.THE NET RESULT IS THAT ANY RISE IN THE WATER LEVEL WILL CAUSE AN INCREASE IN CAPACITANCE THAT CAN BE MEASURED IN TERMS OF LEVEL.
AS A LEVEL SWITCH
CAPACITANCE DEVICES ARE ALSO MOSTLY USED FOR ON-OFF SERVICES AS LEVEL SWITCHES, FOR SUCH FUNCTION CAPACITANCE PROBES ARE MOUNTED IN HORIZONTIAL POSITION SO THAT LARGE PLATE AREA IS EFFECTIVELY USED. SUCH LEVEL SWITCHES ARE USED IN ON-OFF CONTROL TO A PUMP OR LOW & HIGH ALARM.
CHANGE IN CAPACITANCE,”C”=(K2- K1)*A / D WHERE K1,IS DIELECTRIC CONSTANT OF VAPOUR/AIR AND K2,IS DIELECTRIC CONSTANT OF PROCESS MATERIAL. “D”IS THE DISTANCE BETWEEN PLATES.
LEVEL SWITCH
USED IN UREA STORAGE
APPLICATION OF CAPACITANCE LEVEL SENSORS
FEATURES OF CAP. (LG); 1. ONLY ONE PROBE IS USED AS COMPARED TO
HYDRASTEP. 2. COST WISE CHEAP THAN OTHER ie RADAR ETC. 3. USED IN LARGE SIZE / DIAMETERS VESSEL , BY USING
STILLING WELL. 4.TWO WIRE INTRINSICALLY SAFE LOOP POWERED
LEVEL TRANSMITTER WITH 24VDC SUPPLY & 4-20mA 5. USED FOR SOLIDS BY FLEXIBLE PROBES. 6.EASY TO CALIBRATE & REPAIR IF PROBE CHAMBER IS
USED AND REPEATABILITY IS 1.0%. 7. ELECTRONIC CARD ASSEMBLY CAN BE MOUNTED AT
A DISTANCE 100 ft. 8. 8.A HIGH SENSITIVE DESIGN (0.5pF) ALSO AVAILABLE &
RESPONSE TIME IS 100 ms.
DIELECTRIC CONSTANT VALUES FOR
PROBE DESIGN THE MOST COMMON PROBE DESIGN IS STAIN-
ESS STEAL ROD OF ¼” OR ½” DIA, SUITABLE FOR NON-CONDUCTIVE/NON-CORROSIVE MATERIAL.THE PROBE IS INSULATED FROM HOUSING & BIN WALL BY AN LOW DIELECTRIC INSULATOR,SUCH AS NYLON OR RYTON.THESE POLYMERS HAVE MAX. OPERATING TEMP 175-350 DEGREE-C. CERAMIC CAN BE USED FOR HIGHER TEMP APPLICATION OR IF ABRASION RESISTANCE IS REQUIRED. FOR APPLICATION WHERE THE PROCESS MATERIAL IS CONDUCTIE & CORROSIVE,THE PROBE MUST BE COATED WITH TEFLON,KYNAR & POLYETHYLENE.
LIMITATION• DIELECTRIC CONSTANT CHANGE WITH TEMPERATURE ABOUT
0.1% PER DEGREE – C .• INSTALLATION IS IMPOSSIBLE IF VESSEL IS AGITATE.• CHEMICAL CHANGES OF THE MEASURED MATERIAL EFFECT THE
DIELECTRIC CONSTANT SO THE LEVEL.• IN SOLID MEASUREMENT VARIATION IN PARTICLES SIZE EFFECT
THE DIELECTRIC CONSTANT SO THE LEVEL .• PRODUCT COATING ON THE PROBE MAY EFFECT ACCURATE
LEVEL MEASUREMENT .• THE MINIMUM DIFFERENTIAL CAPACITANCE REQURIRED FOR
LEVEL MEASUREMENT SHOULD BE HIGH AS 10 pf • INSULATION HAS BROKEN DOWN ie AGE FACTOR AND DUE THE
EFFECT OF TEMPERATURE.
ULTRASONIC TECHNIQUE .
HISTORY.THE ORIGIN OF ULTRASONIC LEVEL
INST,GOES BACK TO ECHOMETERS USED IN MEASURING THE DEPTH OF WELL BY FIRING A BLANK SHELL AND TIMING THE
RETURN OF ECHO.
PRINCIPLETHESE OPERATE ON THE
PRINCIPLE OF SONAR DETECTOR.
SONAR STAND FOR ?
SONAR.
• SO-STANDS FOR SOUND.
• N-STANDS FOR NAVIGATION.
• A- STANDS FOR AND.
• R-STANDS FOR RANGING.
THE VELOCITY OF ULTRASONIC VARIES WITH BOTH THE SUBSTANCE THROUGH WHICH IT TRAVEL & WITH THE TEMP,OF SUBSTANCE.SO,THE SPEED OF SOUND IS TO BE USED IN LEVEL MEASURING, THE SUBSTANCE THROUGH WHICH IT TRAVEL MUST BE WELL KNOWN & TEMP- VARIATION MUST BE MEASURE & COMPENSATED.
TYPES OF ULTRASONIC
A
RETURNED ECHO TIMING.
B
SIGNAL ABSORPTION.
RETURNED ECHO TIMING.
SENSOR ASSEMBLY
SENSOR ASSEMBLY
SIGNAL ABSORPTION
SENSOR ASSEMBLY
WORKING
THE ULTRASONIC LEVEL SENSOR ASSEMBLY CONSIST OF SEPARATE TX & RECEIVER ELEMENTS.A SINGLE TRANSDUCER IS CYCLED ON & OFF AT REGULAR INTERVALS TO LISTEN FOR RELECTED ECHO.
FREQUENCY RANGE
FREQUENCY RANGE OF ULTRASONIC IS 20~45 KHz FOR LEVEL GAUGES (SLIGHTLY HIGH THAN AUDIBLE SOUND WHICH IS 9~10 KHz) ARE GENRATED BY ELECTROSTATIC & PIEZOELECTRIC TRANSDUCERS.
AN ELECTROSTATIC TRANSDUCERS
IS MADE UP OF THIN ,FLEXIBLE GOLD PLATED PLASTIC FOIL , STRECHED OVERAN ALLUMINIUM BACK PLATE & HELD UP IN PLACE BY LEAF SPRING ( USED FOR CLEAN ENVIRONMENT ).
PIEZOELECTRICTRANSDUCERS
UTILISE CERAMIC OR POLYMER CRYSTAL VIBRATED AT THEIR NATURAL FREQUENCY. THEASE ARE MORE RUGGED.
FEATURES OF ULTRASONIC LEVEL GAUGE
• SELF CALI BRATION FEATURE .• ACCURACY IS WITH IN 5mm UPTO 30 ft .• EASY BENCH CONFIGURATION-NO NEED FOR
LEVEL SIMULATION.• TWO WIRE,INTRINSICALLY SAFE LOOP POWERED • 360 DEGREE ROTATABLE HOUSING CAN BE DIS-
MANTLED WITHOUT DEPRESSURING THE VESSEL VIA QUICK CONNECT / DISCONNECT PROBE COUPLING,SO ON LINE MAINTANCE IS POSSIBLE.
• HART DIGITAL COMMUNICATION.• BETTER FOR DIRTY / FOAM OR LARGE GRAIN SIZE.
CALIBRATIONTARGET
LIMITATION• TEMP OF PROCESS MATERIAL AFFECT
ULTRASONIC SIGNAL ,SO THE LEVEL(COMPENSATION IS REQD.)
• AFFECTED BY COMPOSITION OF THE VAPOUR SPACE.
• RESPONSE TIME IS LESS THAN RADAR.
• ACCURACY IS LIMTED UPTO 30 ft.
SOUND’S VELOCITY.MEDIA TEMP VELOCITY
AIR 0 DEG-C 332 m/s
AIR 20 DEG-C 344 m/s
AIR 100 DEG-C 392 m/s
WATER 20 DEG-C 1498 m/s
SEA-WATER 20 DEG-C 1543 m/s
HYDROGEN 0 DEG-C 1284 m/s
CONDUCTIVITY TECHNIQUE
HYDRASTEP SYSTEM
• THE HYDRASTEP SYSTEM HAVE BEEN DESIGNED AS ELECTRONIC ALTERNATIVES TO CONVENTIAL VISUAL WATER GAUGES ON BOILERS,GIIVING MORE RELIABLE & SAFER WATER LEVEL. THE SYSTEM IS BASED ON SIGNIFICANT DIFFERENCE IN RESISTIVITES OF WATER & STEAM OVER THE RANGE 100 DEG-C TO 560 DEG-C .
• THE CELL CONSTANT IS DETERMINED BY THE LENGTH & DIAMETER OF ELECTRODE TIP & WATER COLUMN BORE.
• BY CHOSING THE CELL CONSTANT IN WATER LESS THAN 100 K ohms & GREATER THAN 10 M ohms.
Hyderastep Electrode
POWER SONIC TECHNIQUE
RADIATION TECHNIQUE;
• IT IS BASED ON THE PROPERTIES OF GAMMA RAYS, THAT THEY COULD PASS THROUGH A SEEMINGLY SOLID, IMPENETRABLE MASS OF MATTER & GAMMA RAYS LOST SOME OF THEIR INTENSITY.
• GAMMA RAYS WERE AFFECTED BY THE SPECIFIC GRAVITY ,TOTAL THICKNESS OF OBJECT & BY DISTANC BETWEEN GAMMA RAYS SOURCE & DETECTOR.
BASED ON PHYSICAL LAW OF ATTENUATION OF NUCLEAR RADIATION AS IT PASSES THROUGH
MATTER. THE ABSORBTION OF GAMMA RADIATION FOLLOWS AN EXPONENTIAL LAW.
I=I0e-μρd
A BEAM OF RAYS WITH THE ORIGINAL INTENSITY I0 IS WEAKENED AS IT PASSES THROUGH MATTER HAVING THICKNESS “d” AND DENSITY “ρ”,
THE MASS WEAKENING CO-EF μ DEPENDS
ON THE TYPE OF RADIATION SOURCE USED.
SOURCE;
COBALT-60 HAVE ENERGY LEVEL
=1.33 MeV.
CESIUM-137 HAVE ENERGY LEVEL
=0.56 MeV.
ROD SOURCEROD SOURCE• TYPE
CONSTRUCTED AS ROD.
• LENGTH1040MM IN ONE PART AND INACTIVE PART AT THE
BOTTOM 800 MM.
• MATERIALCOBALT 60
• RANGE 35 mCurie
SHIELDINGSHIELDING
THE SHEILDING CONSISTS OF A LEAD CYLINDER THE SHEILDING CONSISTS OF A LEAD CYLINDER WITH A RADIATION EXIT SLOT, THE CYLINDER IS WITH A RADIATION EXIT SLOT, THE CYLINDER IS WELDED IN TO A STEEL TUBE THAT IS SHIELDED WELDED IN TO A STEEL TUBE THAT IS SHIELDED WITH A STEEL COVER. THE CYLINDER IS WITH A STEEL COVER. THE CYLINDER IS MOUNTED IN SUCH A STRUCTURAL FRAME MOUNTED IN SUCH A STRUCTURAL FRAME THAT BY TURNING IT THRO’ 90 DEG ADDITIONAL THAT BY TURNING IT THRO’ 90 DEG ADDITIONAL SHIELDING IS PROVIDED FOR EFFECTIVE SHIELDING IS PROVIDED FOR EFFECTIVE RADIATION BEAM BY THE STRUCTURAL FRAMERADIATION BEAM BY THE STRUCTURAL FRAME
SCINTILLATION COUNTERSCINTILLATION COUNTER
THE RADIATION DETECTOR CONSISTS OF A SCINTILLATION COUNTER WITH HIGH SPECIFIC SENSITIVITY TO GAMMA RADIATION
THE NaI CRYSTAL CONVERTS THE GAMMA QUANTA THE NaI CRYSTAL CONVERTS THE GAMMA QUANTA EMITTED BY THE RADIO ACTIVE SOURCE IN TO LIGHT EMITTED BY THE RADIO ACTIVE SOURCE IN TO LIGHT FLASHES,THE QUANTITY OF WHICH IS PROPORTIONAL FLASHES,THE QUANTITY OF WHICH IS PROPORTIONAL TO THE INCIDENT RADIATION INTENSITY.THE CRYSTAL TO THE INCIDENT RADIATION INTENSITY.THE CRYSTAL IS OPTICALLY COUPLED WITH A PHOTOMULTIPLIER IN IS OPTICALLY COUPLED WITH A PHOTOMULTIPLIER IN WHOSE PHOTOSENSITIVE LAYER THE LIGHT FLASHES WHOSE PHOTOSENSITIVE LAYER THE LIGHT FLASHES RELEASE ELECTRONS. RELEASE ELECTRONS.
AN HV SUPPLY APPLIED TO THIS LAYER ACCELARATES AN HV SUPPLY APPLIED TO THIS LAYER ACCELARATES THE ELECTRONS TOWARDS THE ANODE WHERE THEY THE ELECTRONS TOWARDS THE ANODE WHERE THEY RELEASE FURTHER ELECTRONS AS THEY IMPINGE ON A RELEASE FURTHER ELECTRONS AS THEY IMPINGE ON A DYNODE SYSTEM. THUS HIGH AMPLITUDE PULSES ARE DYNODE SYSTEM. THUS HIGH AMPLITUDE PULSES ARE PRODUCED AND SUBSIQUENTLY CONVERTED IN TO PRODUCED AND SUBSIQUENTLY CONVERTED IN TO STANDARD PULSES OF APPROX. 10V BY A SERIES STANDARD PULSES OF APPROX. 10V BY A SERIES AMPLIFIER. AMPLIFIER.
THE GAUGETHE GAUGETHE LEVEL GAUGE LB-323 IS A MICROPROCESSOR THE LEVEL GAUGE LB-323 IS A MICROPROCESSOR CONTROLLED SYSTEM THAT ALSO SUPPLIES THE CONTROLLED SYSTEM THAT ALSO SUPPLIES THE OPERATING & CONTROL VOLTAGE TO OPERATING & CONTROL VOLTAGE TO SCINTILLATION COUNTER.THE STANDARD PULSES SCINTILLATION COUNTER.THE STANDARD PULSES SUPPLIED BY THE DETECTOR IS PROCESSED BY SUPPLIED BY THE DETECTOR IS PROCESSED BY THIS SYSTEM AND CALCULATES THE AVERAGE THIS SYSTEM AND CALCULATES THE AVERAGE COUNT RATE AND COMPUTES FINAL RESULTS ON COUNT RATE AND COMPUTES FINAL RESULTS ON THE BASIS OF PRE PROGRAMMED PARAMETERS. THE BASIS OF PRE PROGRAMMED PARAMETERS. IT GIVES AN OUTPUT OF 4-20 mA .IT GIVES AN OUTPUT OF 4-20 mA .
10 75
COMPUTE
MEASURE
CODE
ENTER
LOCKED
PROBE FAILED
I
O
POWER SWITCH
CODE DISPLAY LVEL DISPLAY
FRONT PANEL OF LB-323
THE HALF LIFE OF COBALT-60(USED AT ¾-LT-2)IS 5.3 YEARS.THIS MEANS THAT,IN 5.3 YEARS,THEACTIVITY OF A 100 milliCurieCOBALT-60 SOURCE WILL REDUCED TO 50 (mCi) milliCurie . ONE mCi IS DEFINED AS THE RATE OF ACTIVITY OF ONE milligram OF RADIUM 226. WHEN USED FOR LEVEL MEASUREMENT THE CONTINUOUS LOSS OF SOURCE STRENGTH REQUIRE NOT ONLY CONTINUOUS COMPENSAT -ION BUT EVENTUALLY,THE SOURCE MUST BE REPLACED. THIS MEANS NOT ONLY THE EXPENSE OF PURCHASING ANEW SOURCE , BUT ALSO THE COST OF DISPOSING OF OLD ONE.
FEATURES• Non contact, continuous level measurement • independent of pressure, temperature and chemical
properties of the product • use of rod sources enables linearization of measuring
signal even at the beginning of the measuring range • adaptable for all shapes of vessel (cylindrical, conical, etc) • simple to calibrate (only one measuring point is needed) • source design for output signal proportional to product
level or volume • measuring range of several meters possible (more than 10
m) • automatic decay compensation • minimal source activity through the use of scintillation
counters • indication in % level or millimeters
RADIATION EXPOSURE LIMITS• UP TO 0.2 Sv
• Up to 1.0 Sv
• Up to 2.0 Sv
• More than 6 Sv
• No Effect evident
• Slight changes of blood structures but no serious damage is likely to occur
• Radiation hangover, vomiting, serious illness possible, good chances of recuperation
• No chance of survival
• The above limits are for the whole body exposure to radiation for a short time , but permanent exposure to radiation will cause much damage due to regenerative capacity of living
organism.