Level · PDF fileOpen Vessel Level Measurement XMTR L H P atm ... Hybrid Inventory System 19...

Level

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Contents

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Topics: Slide No:• Pressure 03 - 28 Transmitter, HTG & Hybrid System• Other Technology (Float, Capacitance, 29 - 48

Displacer, Servo, Nucleonic, Laser &Ultrasound)

• Exercise 49 - 53

Pressure Transmitters

In open vessel a pressure transmitter mounted near the bottom of the tank will measure the pressure corresponding to the height of the fluid above it.

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Open Vessel Level Measurement

XMTR

HL

Patm

Phead

Patm

What happen to Atmospheric pressure? Cancelled Off

Phigh - Plow = Phead

Plow = Patm

Phigh = Phead + Patm


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What happens when the liquid level drop below the sensor ?

Transmitter mounted above the tap in an open vessel

XMTR

HL0%

100%

0% hg

The 0% has to be at least at the same level as the transmitter sensor or below the tapping point..

The sensor will not be able to sense any pressure change !!


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“Zero Suppression” is often applied to compress the range of the transmitter OR to cancel the effects of the liquid head in the pipe connecting the transmitter to a tank when the transmitter is mounted below the vessel connection.

XMTR

HL

Actual ZeroSuppressed

4mA

20mA Max. Level

Min. Level

XMTR

HL

Actual Zero Suppressed

4mA

20mA Max. Level

Min. Level

At min. level the High side pressure is high than the low side pressure


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“Zero Elevation” is often applied to cancel the effects of the head caused by the seal fluid in the reference leg (low side) of a transmitter measuring level in a pressurized vessel.

XMTR

Actual Zero Elevated

4mA

20mA Max. Level

Min. Level

L H At min. level the low side pressure is high than the high side pressure


Dry leg: no fluid in low side impulse piping, or legIf the gas above the fluid does not condense, the piping for the low side of the transmitter will remain empty.

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Closed Tank Level Measurement (Dry Leg)

XMTR

HL

Phigh = Ptop+PheadPlow = Ptop

Ullage or Vapor

Phead

Phigh - Plow = Phead


Wet leg pressure is additive to pressure on low side of the transmitter.If the gas above the liquid condenses, the piping for the low side of transmitter will slowly fill with liquid. To eliminate this potential error, the pipe is filled with a convenient reference liquid.

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Closed Tank Level Measurement (Wet Leg)

XMTRHL

Phead

Ptop= Ullage

Phigh =Phead+Ptop

Phigh - Plow = Phead - Pwet leg

Pwet

Plow=Pwet leg+Ptop


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• Bottom-mount technology: potential leakage• Often requires 2 taps• Variable density creates errors• Temperatures beyond 600 F• High vacuum applications are tricky• Highly corrosive processes limit life• Abrasive processes can damage diaphragms• Liquids Only

Limitations


Consists of air supply, pressure regulator, flow meter, transmitter & extended tube.Can be used for very corrosive applications.Tank vented.

Air is bubbled through the tube at a constant flow rate. The pressure required to maintain flow is determined by the vertical height of the liquid above the tube opening times the specific gravity.

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Pin(flow=const)

Bubbler System

Pressure to maintain flow = Phead

S.Gf

H

Phead = H * S.Gf

TXR


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• Allows dp to be a top down measurement

• No process contact with transmitter

• Open or low pressure • Control of Air supply is

important for accuracy

Gauge Pressure Transmitter

Air Supply

ValveApplication of Bubbler system:


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Reliable, Simple, Easy to Use, Well Understood, Flexible Uses:

Differential PressureTransmitter

Diaphragm Seals extend limitations due to process conditions such as:

high temperaturescorrosionviscous materialssuspended solids pluggingsanitary needs

Remote Seals


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•CPI / HPIBatch reactors digestersfractionatorsdistillation column bottoms and reflux drumsseparators surge drums reservoirs intermediate storage...

•Powerdrum level dearators...

•Pulp & PaperHeadboxStock TanksChemical Storage tanksEvaporatorsLow concentration liquor tanks...

•Food and BeverageFermentorsstorage tanksaging tanks brew kettles...

And many more!

Typical Pressure Applications

Hydrostatic Tank Gauging (HTG)

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It is basically a method for measuring mass.

A local processor handles all the real-time calculations for transmitter algorithms and equations for MASS, DENSITY, LEVEL and VOLUME.

Top transmitter PT only used on non-atmospheric tanks to compensate for ullage pressure differences.

Middle transmitter PM located at a specific distance “H” above PB to calculate liquid density (PB-PM)

Bottom transmitter PB located at the base of the tank measures static head.

RTD measures tank contents temperature for reference correlations.


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H

HBT

PB

PM

PT

Volume =

Level =

Mass =

Density =

(PB - PT) x Area

(PB - PM ) / H

Mass / Density

(PB - PT) /Density + HBT

A system approach to tank inventory

TT

Effective Level Measurement Options:


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Top Pressure Transmitter

Middle Pressure Transmitter

Temperature Sensor

Bottom Pressure Transmitter

(1 unit per tank)

All Smart transmitters are in multiAll Smart transmitters are in multi--drop mode drop mode -- Digital signalsDigital signals onlyonly

(can take up to 14 SAMs)

(can take up to 31 AIMs)

[Handles tank calculaton]

(obtain data from SAM & convert to std MODBUS outputs)

Smart Application Module (SAM)

HARTCommunicator Application Interface

Module (AIM)

SCADA SCADA PackagePackageRS-485 MODBUS

Communications Network

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Advantages of HTG Multi-Parameter MeasurementsMASS: Inventory and BillingDENSITY: Quality ControlVOLUME: Inventory & BillingLEVEL: Tank Capacity

Continuous Density Measurement Improved Accuracy High Reliability Low Maintenance Diagnostic Easy Installation Non-Intrusive No Moving Parts


Hybrid Inventory System

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Temperature Sensor


(1 unit per tank) (can take up to 14 SAMs)

(can take up to 31 AIMs)

[Handles tank calculaton]

(obtain data from SAM & convert to std MODBUS outputs)

Smart Application Module (SAM)

HARTCommunicator Application Interface

Module (AIM)

SCADA SCADA PackagePackageRS-485 MODBUS

Communications Network

When the Middle transmitter is removed & the Top Pressure When the Middle transmitter is removed & the Top Pressure Transmitter is replaced by a Radar Gauge, then the whole system Transmitter is replaced by a Radar Gauge, then the whole system will be known as will be known as Hybrid Inventory SystemHybrid Inventory System

Radar Gauge

Hybrid Inventory System

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• LEVEL is calculated by Radar GaugeL = H - D

• VOLUME is computed by Radar GaugeThrough Strapping Table relationship level to volume

• DENSITY is computed by the systemS.G = Head Pressure / Level Head pressure measured by bottom

pressure transmitter• MASS is computed by the systemMass = Density X Volume

• CORRECTING for Density & Volume back to standard values is computed



Temperature Sensor


Radar Gauge

H

L

D

Measurement Options:

Advantages of Hybrid System

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Highly accurate Multi-Parameter MeasurementsMASS: Inventory and BillingDENSITY: Quality ControlVOLUME: Inventory & BillingLEVEL: Tank Capacity

Radar can be installed without removing the tank from service

Radar unit can be used with or without a stilling well

Process conditions up to 375°F (190°C) and 150 psi (10 bar)

Optional average temperature measurement Good for density-stratified products

Hybrid Inventory Systems

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Radar is an Electromagnetic Wave

Wavelength, Meters

10 -11

10 7

10 5

10 3

10 1

10 -5

10 -3

10 -1

10 -9

10 -7

10 -13

10 2

10 18

10 16

10 14

10 12

10 10

10 8

10 6

10 4

10 20

10 22

Frequency Cycles/second

gamma rays

x-rays

ultra violet

visible light

------ultra high freq----------

----------TV broadcasting------------------FM Radio-------------

-------low frequency------------

Radar,

3-30 GHz

Microwave oven,

2 - 10 GHz

Cellular, pager,

300-3000 MHz

Electromagnetic Spectrum

Radio Detection And Ranging

--------super high freq---------

Radar Gauge

Radar Gauge

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• Pulse » Measures range ( distance )» Transmits a pulse and measure time until echo

is received» Accuracy depends on ability to measure time Radar signals travel at the speed of light. Must measure in picoseconds ( x10-12 ) ! Cost-effective electronics do not exist to do this

accurately !

Radar Techniques

Radar Gauge

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• FMCW: Frequency Modulated Continuous Wave» Does NOT calculate time-of-flight» Evaluates the phase difference between the transmitted

and return signal» Plotting these phase differences against the transmitted

signal yields a result proportional to distance

Radar Techniques

Radar Gauge

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• Non Contact, Non Intrusive• Tolerates Wide Range of Process Conditions

» Corrosive Processes» High Temperatures» Changes in Vapor Space» Variable Density» Variable Dielectric» Viscous or Sticky Products

• Low Maintenance• No Special Licenses Required• Can measure long distances• Liquids, pastes, solids

Advantages of Radar Gauge

Radar Gauge

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• Sensors can be completely removed from process by use of a window made out of a nonmetallic material, such as Teflon, Ryton, Ceramic

• Sensors can be removed from the process without opening the vessel

Radar Application Considerations

• Cost• May not work with processes with low dielectric constant• May not work in applications with large amounts of

turbulence• Process connections tend to be large (>4” flanges)

Radar Limitations

Radar Gauge

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•Pulp & Paper»High Density Storage»Color tanks»Bleach tanks»Hydropulpers»Retention tanks»Black liquor tanks

•Pharmaceutical»Batch reactor»Chemical storage

•Power»Slurries

•Chemical»Polymers»Latex»High temp»LPG tanks»Butane sphere»Batch reactors»Two-phase sludge»Cyclohexane

•Minerals»Steel Scale Holding Tanks

And More!

Possible Applications

Radar Gauge

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Both technologies

Top down, non contact

Easy to install

Good for abrasive materials, slurries

Not affected by changing fluid properties: density, conductivity, dielectric

Radar vs. Ultrasonic Gauge

Differences:RadarFull vacuum to several hundred psiWide temperature limitsCan handle steam, fog, vaporsCan handle some foams and agitationCan be used with windows

UltrasonicVery slight vacuum to about 100 psiNarrow temp band (<200 F)Is greatly affected by changes in vapor spaceSignal is lost in foam and agitation

Similarities:

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Gas Temp (oC) RADAR ULTRASOUNDmillion m/s m/s

Dry Air 0 299.91 331.8100 299.94 386

Water vapor 100 299.10 404.8

Carbon Dioxide 0 299.85 250.050 299.87 279.0

Ammonia 0 299.93 415.0

Acetone 0 297.64 223.0

Source: Instrument Engineer’s Handbook, Liptak

Radar vs. Ultrasonic Gauge

Radar Gauge

Other technologiesFloat Mechanism

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Tank

Isolat

ing

Valve

Float

Drain Valves

Indicator

IndicatorExternal Still pipe to guide the float

Float

Internal Still pipe to guide the float

Float-operated gauge level-indicator, indicates liquid level in cone or flat roof unpressurised tanks.Recommended for use on tanks storing water, fuel, oil, chemicals or other liquid products where operations do not require extreme accuracy.

Other technologiesCapacitance Probe

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C = KEoAd where

K = dielectric constant of materialEo = permitivity of vacuumA = Area of plates (probe)C = capacitance (pF)d = distance between plates

d

k

• A capacitance instrument measures amount of capacitance between two plates of a capacitor.

• The capacitance of a capacitor increases if a dielectric is placed between the plates

• Circuit applies high frequency signal to probe


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Nonconductive Fluid

d

Conductive Fluid

NonconductiveCoating

• Process fluid is the dielectric barrier

• Tank Wall forms second plate• The variation of dielectric is the

measurement

• Process fluid is the second plate• Insulation on probe is dielectric• The variation of the plate size is

the measurement

How Capacitance varies with process fluid?

Level is proportional to dielectric change

Level is proportional to plate area change


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• Limitation– Change in Dielectric creates error– Coating on probe by product creates errors– With non metallic tanks or tanks without vertical

walls, addition of reference probe is required– Calibration can be difficult especially since one

cannot “bench calibrate”– Changing vapor space can affect output


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•Pulp & PaperSewage levelLiquor tanksBulk solids

•ChemicalInterface: fatty acid/water, oil/waterCarbon blackSeparators

•Food & BeverageStorage silos

•Oil & GasWater bottomWater cut

Potential Applications

Other technologiesDisplacers

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Angular movement is then converted to electrical or pneumatic output.

pounds

0

pounds

0Based on Buoyance Force

The displacer is buoyed up by a force proportional to the weight of the liquid it displaces

Vertical movement of the displacer is converted to angular movement by mechanical linkages

Buoyant force increases as level rise


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Liquid LevelMeasurement

InterfaceMeasurement

DensityMeasurement

Good for short span measurement


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Benefits:• Simple, Reliable.• Good for Interface measurements.• Good for Density measurements.• Unaffected by Agitation.• Tolerates High Temperatures and pressures.• Point or Continuous .

Limitations:– Does not tolerate viscous, dirty, or sticky fluids– Variable density causes errors in level measurement– Typically used for smaller spans (cost effective)– Must be installed carefully– Intrusive & Contact


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• Typical Applications:Oil and Water interfaceOil and Gas SeparatorsStripper Reflux Drum LevelDehydration UnitsEffluent SeparatorsAbsorption TowersCondensate Discharge AccumulatorsDensity and Interface Measurements

Other technologiesServo Gauging

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Uses a combination of a displacer and a spring balance

The servo motor strives to obtain an equilibrium between the displacer and the balance. Any change in level will cause a change in equilibrium.Advantages:

Very precise (1 mm accuracy) Can measure level, interface relatively low cost

Limitations: Intrusive Mechanical linkages

Cable

Storage Drum

Balance Detector

ServoMotor

Displacer

Other technologiesNucleonic Gauging

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Gamma rays are emitted from the source. The presence or absence of the gamma rays is measured by the detector.

Nucleonic level switches use radioisotope sources sized to provide measurable radiation at the detector when no product material is present between source and detector.

DetectorGamma Source

Single Point System


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Nucleonic level transmitters use the same radioisotope sources, but respond to the total absorption of gamma rays as they pass from the source to detector.

The amount of radiation reaching the detector is inversely proportional to the amount of material in the vessel.

Detector

Source

Continuous System


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– Unaffected by:• High temperatures• High Pressures• Corrosive Materials• Abrasive Materials• Viscous Materials• Agitation• Clogging/Plugging

– Point and Continuous– Liquids and Solids– Interface (based on H2

density)

Advantages:

– Large density changes can create errors

– Layer of coating on vessel walls create errors

– Licensing Required– Leak Checks required– Cost

Limitations:


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•ChemicalDistillation TowerBatch ReactorStorage TanksResin Bed levelHydrocracker reactor

•Pulp & PaperDigester LevelWood Chip BinsBleach TowerConsistencyEffluent WasteSlurriesLiquor concentrates

•RefiningFractionator TowerSurge TanksCoke Drum InterfaceDesalter

•Food and BeverageHopper LevelBlending Vats

•Mining Crusher LevelStorage silosSlurries

•UtilitiesSO2 / Lime scrubberFly ash Slurries

Typical Applications

Other technologiesLaser 43

glass windowlaser device

• Function: Uses infrared light to send a focused beam towards surface. Time of travel and reflection is measured.

• Narrow, focused beam: good for applications with space restrictions.

• Non contacting: uses a window• Accuracy: +/- 1 cm• Works best in cloudy, shiny liquids or

solids• May pass through surfaces of clear, still

fluids• Cannot tolerate dust, fog, steam or

vapors• High cost• Alignment is critical

Other technologiesUltrasonic Gauging

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• Advantages:Non ContactNo element contaminationCan be used for liquids and solidsTolerates Many Process Conditions:

Sound Waves

A sound pulse(9 to 160 kHz) is transmitted and reflects off the surface back to the transceiver. The true reflected echo pulse is extracted and the time interval between transmission and reception is evaluated electronically.

The higher the level the faster echo reflected

Varying Density Corrosive Processes Viscous Product Varying Dielectric Sludge Buildup


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• Conditions of the vapor space impact speed of signal travel and thus, the measurement

• Changes could be due to: temperature dust vapor composition stratification of the vapor

• Some units have temperature compensation

• Gas blankets can be used to provide uniform vapor space condition

Sound Waves

Application Considerations


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• Process surface conditions can affect signal return . Surface must have ability to reflect signal.

• Heavy agitation and foam may cause signal to be absorbed

• Vortex in fluid can misdirect signal• In open, outdoor installations,

wind can blow signal off coarse• Stilling wells can be used to isolate

the surface and contain signal.

Application Considerations


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Limitations– not suitable for vacuum service– Cannot tolerate high temperatures (>200 F)– Foam interferes with signal– Agitation may distort signal– Internal obstacles can create false echoes– Nearby equipment could generate frequencies

that will cause errors– Vapor pressure limited to 50 psi


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•ChemicalDistillation chamberCorrosivesSlurriesLatex PVCWaxes

•Food and BeverageDearating vesselalcohol fermenterbaking batterchocolatedairy productsgrain storage

•Cryogenic systems (point level)

•Waste waterClarifierSettling tanksReservoirsFlood controlSludge levels

•Pulp & PaperBlack liquor w/ solids

•PharmaceuticalEmulsionsLotions

•MarineFuel or ballast water indicationBilge alarm

Typical Applications

Exercise

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1. Which has the best accuracy on 6 meter High Water tank? A. Hollow or glass fill float with mechanical gauge (1 inch) B. “Servo” Gauge (1 mm) C. Pressure Transmitter ±0.1% of F.S [ ]

2. An inground reservoir is 5 meters deep. Which of the following method(s) will be suitable to measure and transmit the level without having to dig a hole to reach the bottom of the reservoir or the scour main. (Answer Yes [Y] or No [N])

A. Differential Pressure [ ] B. Servo Level Gauge [ ] C. Capacitive Probe [ ] D. Nucleonic Gauging [ ] E. Ultrasonic Gauging [ ] F. Radar Gauging [ ] G. Bubbler System [ ]

Exercise

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3. For HTG, why is a 2nd Pressure Transmitter added to tank in the middle ?

4. Which one of the following tank gauging system is based on Mass? (A) Radar (B) Nucleonic (C) Servo Balance (D) HTG [ ]

5. Which of the following statement about Radar Gauge is NOT True ? (A) Top-down mounting (B) Can handle agitated & sticky process fluid (C) Can be used on a tank with non-metallic internal surface. (D) Can handle process with deep vacuum [ ]

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10’

5’ Assuming SG is = 1.1

What is the volume?____________ft3

What is the density of this fluid?___________#/ft3

What is the mass?__________pounds

What is the pressure level reading? ________in H2O

water = 62.4 # / ft3

ExerciseExercise

6.

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10’

5’Now, suppose the SG changes to 1.05 and the level does not change

What is the volume?____________ft3

What is the density of this fluid?___________#/ft3

What is the mass?__________pounds

What is the pressure level reading? _______in H2O

water = 62.4 # / ft3

ExerciseExercise

7.

Exercise

Pmeasured = 150 inH2Os.g. = 1.5

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Pin(flow=const)

What is the fluid level in the tank?

8.

Reference A Smith-Corripio [1997]. Principles and Practice

of Automatic Process Control 2nd Edition, John Willey & Sons

Ogata [2010]. Modern Control Engineering 5th

Edition, Prentice Hall Rosemont [2002]. Fundamental Control Training http://www.isa.org

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