201309 cld 20202 chapter 2-level measurement devices
DESCRIPTION
notesTRANSCRIPT
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Chapter 3
Level Measurement
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ObjectivesAfter finish this lesson, you will be ableto:
• Define the level• Differentiate direct measurement &
indirect measurement• Apply the factor for selecting level
measurement device
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• Differentiate between open & closed tank measurement
• Understand the principle, operation, advantages & disadvantages of devices
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Level Measuring Devices
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Differential Pressure (DP)
Principles
• Known hydrostatic
• Based on height of liquid head (pressure)
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Operations
• The top & bottom tank is connected to DP
• The top – LP• The bottom – HP• System sensed both high & low side• High side – sensed pressure exerted
by height of liquid
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• Each sides exposed same pressure system pressure cancelled &
differential pressure indicates liquid level
• Problems – vapor may condensate & fill LP tap – can affect accuracy
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DP Advantages• Easy to install
• Has wide range
• Faster response time
• Modification can measure hard to measure fluids (viscous, slurries, corrosive) • Simple
• Accurate
• Calibration is simple, no special tools required
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DP Disadvantages
• Affected changes density – used fixed specific gravity liquid
• Require constant head maintain at reference leg
• Susceptible to dirt / scale entering tube
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Dip StickPrinciples
• Oldest form• Essential
stick/rod• Commonly used –
oil level & fuel quantity
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Operations
• Lowered vertically into tank – until reach reference point
• Bottom tank – reference point to ensure correct depth
• Dip stick withdrawn & level read at the last interface contact
• Reading indicate level
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Lead Line
Principles• Same way dip
stick• Steel measuring
tape attach weight
• More capable than dip stick
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Operations
• Measuring clear liquid – applied special chemicals
• This can darken/change color• Chemical not react with the process
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Advantages
• Compact, easy• Accuracy 0.1% up to range 20 ft• Accurate• Reliable• Dependable
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Disadvantages
• Only measure open system• Safety precaution – caustic & toxic
process• Not possible for continuous
representation
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Sight GlassesPrinciples• Visual
measurement• Transparent tube
of glass/plastic mounted outside vessel
• Level inside vessel = level sight glass
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Operations
• Principle - equal pressure on the surfaces of two connected columns causes liquid to seek same level
• One leg represent process, another transparent tube
• Fluctuates level changes
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• Tube have a scale – indicator for direct reading
• Can be adapted to open / closed tank
• Closed-tank sight glass used in both pressurized & atmospheric process
• Application on pressurized vessels - boiler drum, evaporators, condensers, distillation column & stills
• Low & High pressure process
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Low Pressure SightGlass
• Clear round tubefitted between service valve
• Equipped with ball checks, inserted within valve chamber – shut off flow if leak/rupture
• Ball checks permit free flow of fluid when level changing
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High pressure sightglass
• Contain liquid under high pressure – reflex sight glass used
• Is armored; permit it tolerate higher temp & pressure
• Viewing glass – install in protective high-pressure housing
• 1 inch thick
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Gage GlassPrinciples• Similar sight glass• Glass covered ports
in vessel – observed easily
• Have scale mounted on the tank
• Disadvantage: shut down process for repair glass
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Operations
• Simple • Reliability• Transmitter fail it still can be used• Limited function
• Disadvantages :Very often inaccessible - difficult to measureVulnerable to breakage – release to
environmentHot, corrosive, caustic; spills could be serious
• Careful when install at the risk breakage is possible
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Magnetic GaugesPrinciples
• Used the attraction between 2 magnet to follow a liquid level (1 magnet enclosed in a float ring & 1 inside a nonmagnetic tube – magnet follower)
• Tube made from nonmagnetic metal / PVC pipe
• Magnet balance by counterweight fall & rise easily
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25
Typical Installatio
ns of Magnetic
Gauge
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Operations
• Ring-shaped floating magnet fits around the outside of the tube & follows any change in the liquid level.
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• The magnet inside the tube follows the floating magnet & moves the indicator along calibrated scale
• Tank sealed – liquid not leave the tank
• Cannot used to measure liquid if floating is stick to tube
• Tube material should withstand the temperature, pressure & chemical action
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Float DevicePrinciples
• Reliable of other visual sensors
• Float move by level changes
• Movement convey a level measurement
• Rises & falls with level; position sensed outside the vessel
Cable
FloatWeight/levelindicator
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Operations
• Common uses connected to a pulley by a chain /
flexible cable
• Rotating pulley, turn, connected to indicating device
• Float moves, counterweight moves along scale
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• Perforated tape can be used to drive a dial indicator
• As the raise / lower in level, weight will also raise / lower by pulley arrangement
• Installing scale on the tube / guide, level can be determine
• Application: closed tank / atmospheric pressure
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Cable
FloatDrum indicator
Figure 3.13: Float and spring-loaded drum
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• More sophisticated a steel tape & spring-loaded drum
• Level raises / lowers, drum winds in the steel tape / lets it out
• A window spring-loaded drum housing allows operator view graduation; determine level
• Advantage: reading can be at ground level
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• Float & Tape commonly used reliable, little maintenance &
calibration
• Usage : local indication & auxiliary / backup
• Limited to liquid-gas interface
• Not accurate for foaming liquid
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THANK YOU
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Capacitance• Electrical device that store energy &
electricity• Consists 2 plates separates by
insulating material dielectric• Connecting plates to power supply,
electrons attracted from one plate to the other
• Capable storing & holding the charge until discharge
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Operations• One side of process
container acts one plate & immersed electrode is other side
• Dielectric air / material inside vessel
• Area & distance between 2 plates is fixed values
Dielectric
Dielectric
Plate 2
Plate 2
Plate 1
Plate 1
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• Amounts of charge stored determined by 3 factors; area, distance, type of dielectric
• Relationship
where:C = capacitance; farad (F)K = dielectric constantA = area of platesD = distance between plates
DKAC
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• Dielectric varies with level
• Changes in capacitance proportional with level
• If the wall nonconductive, a single probe contain 2 electrode can be used -horizontally
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• Terminal of electrode connected to measuring device
• Interface passes between electrodes, capacitance changes because the different in dielectric constant
• Conductive materialinsulate probe is used (teflon-coated)
• Continuous measurement, probe install vertically
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• Changes capacitance direct function of dielectric constant
• Dielectric constant air & most gases = 1
• Temperature cause dielectric constant of liquid vary (T , K ↓)
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Advantages
• Simple design
• No moving parts
• Minimal maintenance
• Availability of corrosive resistant probe
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Disadvantages
• Temperature will effect error reading
• Probe coated with a conductive material, errors may occur
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Conductivity• For liquid interface of
relatively high conductivity
• Include water-based material
brine solution, acids, caustic, certain beverages
• Limited to alarm devices & ON/OFF system
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Principles
• 2 electrodes positioned in a tank (extends min & max level)
• Cable grounded & functions as common electrode
• Stilling well provided to ensure interface not disturbed & prevent false measurement
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• Terminals electrode connected to relays
transmit signals to display / control device
• Extends min level, conductivity path established through grounded tank
• Level falls below electrode, path interrupted, showed gas / vapor is nonconductive
Controller
High-levelprobe
Low-levelprobe
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• Level condition activate device to sound alarm – control to operate pump / feed controller that adjust level
• Current will flow through electrodes & tank
• Relay – detect high level
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Advantages
• Low cost• Simple design• No moving parts• Effective for many water-based
material
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Disadvantages
• System must be conductive• Only point detection can be
measured• Possibility sparkingprohibitive for explosion & flammable
substances
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Ultrasonic & Sonic Sensors
Principles
• Measuring device not contact with process material
• Measures distance from one point in vessel to level interface
• Operate on the ECHO principles• Different ultrasonic & sonic; operating
range (US: 20kHz & S: 10kHz /below)
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• Can be used continuously• Sonic conductivity precisedepend the density & characteristic
of surface• Sound waves tend to pass through
most gases.• Waves lost kinetic energy through
friction • Liquid more reflective• Gas more adsorb
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Operations
• Sonic level measurement dependent on sound wave striking live medium & reflecting wave
• Dead media adsorb most sound energy
surfaceonincidentenergy SoundabsorbedenergySoundd
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• Value ddepend the frequency sound wave.
• Influenced by surface porosity, material thickness, rigidity
• Continuous processbased on time elapses from generation of
sound wave to detection of the reflected wave
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Level measurement troubleshooting
1. Connections– DP cell sensing line connection is reversed– High operating pressure & low hydrostatic
pressure; easy to occur– Actual level , indicated level ↓
2. Over-pressuring– 3 valve manifolds on DP cell – prevent
over-pressuring & easy to remove– Fail immediately / diaphragm become
distorted– Low / high reading
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3. Obstructed sensing lines– Small diameter; clogged with particulate –
inaccurate reading– Sluggish response to level changes– To overcome: periodic draining & flushing
4. Draining sensing lines– To remove debris / particulate settle on
the bottom vessel / in line– Close tank; remove condensate – prevent
fluid pressure build up in LP impulse line– Leaking / drained wet leg – false high level
indication
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Lead line
LT
Sight glass
Reflex sight glass
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dipstick
Magnetic gage
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Ultrasonic
Capacitance
Magnetic FloatCable Float
Sight glass
Side mounting Float
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THANK YOU