PowerPoint Presentation

Chapter 3:Level Measurement

1ObjectivesDefine the levelDifferentiate direct measurement & indirect measurementApply the factor for selecting level measurement deviceDifferentiate between open & closed tank measurementUnderstand the principle, operation, advantages & disadvantages of devices

223IntroductionMeasured at position of interface between phase (height)Direct & indirectVertical measurement surface to reference pointPressure at base height hydrostatic pressure

DP (differential pressure capsule) is most common usedLiquid level measuring devices are classified into two groups: (a) direct method (b) indirect method

P = gh4Direct Level MeasurementEmploys physical properties fluid motion, buoyancy, optical, thermal, electrical propertiesPosition interface monitored visuallySimplicityNot require compensation for changes level causes by temperature changes.Indirect Level MeasurementInvolves converting measurement other quantityTemperature affected accuracy of indirect level measurement (changes density affected pressure)Compensate sensitive specific gravity & temperatureOpen Tank MeasurementSimplest applicationHP base of tankLP vent to atm0% level 4 mA100% level 20 mA

Phigh = Patm + gHPlow = Patm

Differential Pressure = Phigh Plow = gH

Closed Tank MeasurementHP base of tankLP top of tankConnected to DP

Phigh = Pgas + gHPlow = Pgas

Differential Pressure= Phigh Plow = gH

The effect of gas pressure is cancelled and only pressure due to hydrostatic head of the liquid is sensed.Level Measuring DevicesDifferential Pressure (DP)Principles

Known hydrostaticBased on height of liquid head (pressure)

Operations

the top & bottom tank is connected to DPthe top LPthe bottom HPsystem sensed both high & low sidehigh side sensed pressure exerted by height of liquid

each sides exposed same pressure -system pressure cancelled & differential pressure indicates liquid levelproblems vapor may condensate & fill LP tap condensate liquid produces its own static head - can affect accuracy

easy to install, has wide range, faster response timemodification - can measure hard to measure fluids (viscous, slurries, corrosive) simple & accuratecalibration is simple, no special tools requiredDP Advantagesaffected changes density used fixed specific gravity liquidrequire constant head maintain at reference legsusceptible to dirt / scale entering tubeDP DisadvantagesDip StickPrinciples

oldest formessential stick/rodcommonly used oil level (internal combustion engines)& fuel quantity(underground storage tanks

Operations

lowered vertically into tank until reach reference pointbottom tank reference point to ensure correct depthdip stick withdrawn & level read at the last interface contactreading indicate level

Lead LinePrinciples

same way dip sticksteel measuring tape attach weightmore capable than dip stick

Operations

measuring clear liquid applied special chemicals to the sticks or line.this can darken/change color when immersed in liquid.chemical not react with the processonly measure open systemsafety precaution caustic & toxic processnot possible for continuous representationDisadvantagesAdvantagesCompact, easyAccuracy 0.1% up to range 20 ftAccurate, reliable, dependableSight GlassesPrinciples

visual measurementtransparent tube of glass/plastic mounted outside vessellevel inside vessel = level sight glass

Operations

Principle - equal pressure on the surfaces of two connected columns causes liquid to seek same levelone leg represent process, another transparent tubefluctuates; level changes- true representation of the process level

tube have a scale indicator for direct readingcan be adapted to open / closed tankclosed-tank sight glass used in both pressurized & atmospheric processApplication on pressurized vessels - boiler drum, evaporators, condensers, distillation column & stillsLow & high pressure process

Low pressure sightglass

clear round tubefitted between service valveequipped with ball checks, inserted within valve chamber shut off flow if leak/ruptureBall checks permit free flow of fluid when level changing

High pressure sightglass

Contain liquid under high pressure reflex sight glass usedIs armored; permit it tolerate higher temp & pressureViewing glass install in protective high-pressure housing1 inch thickGage Glass

Principles

similar sight glassglass covered ports in vessel observed easilyhave scale mounted on the tankOperations

simple and reliabilitytransmitter fail; it still can be usedLimited functionDisadvantages : very often inaccessible - difficult to measurevulnerable to breakage release to environmenthot, corrosive, caustic - spills could be seriouscareful when install at the risk breakage is possible shut down process for repair glass

25

Magnetic Gauges Principles

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 pipeMagnet balance by counterweight; fall & rise easily

27Operations

ring-shaped floating magnet fits around the outside of the tube & follows any change in the liquid level.

the magnet inside the tube follows the floating magnet & moves the indicator along calibrated scaletank sealed liquid not leave the tankcannot used to measure liquid if floating is stick to tubetube material should withstand the temperature, pressure & chemical action30

Typical Installations of Magnetic GaugeFloat Device

Principles

reliable of other visual sensorsfloat move by level changesmovement convey a level measurementrises & falls with level; position sensed outside the vesselCableFloatWeight/levelindicatorOperations

common uses ; connected to a pulley by a chain / flexible cablerotating pulley, turn, connected to indicating devicefloat moves, counterweight moves along scale

perforated tape can be used to drive a dial indicatoras the raise / lower in level, weight will also raise / lower by pulley arrangementinstalling scale on the tube / guide, level can be determineapplication: closed tank / atmospheric pressure

CableFloatDrum indicatorFigure 3.13: Float and spring-loaded drummore sophisticated; a steel tape & spring-loaded drumlevel raises / lowers, drum winds in the steel tape / lets it outa window spring-loaded drum housing allows operator view graduation; determine leveladvantage: reading can be at ground levelfloat & tape commonly used; reliable, little maintenance & calibrationUsage : local indication & auxiliary / backuplimited to liquid-gas interfacenot accurate for foaming liquidCapacitanceelectrical device that store energy & electricityconsists 2 plates separates by insulating material; dielectricconnecting plates to power supply, electrons attracted from one plate to the othercapable storing & holding the charge until discharge

Operations

one side of process container acts one plate & immersed electrode is other sidedielectric; air / material inside vesselarea & distance between 2 plates is fixed valuesDielectricDielectricPlate 2Plate 2Plate 1Plate 1 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

Dielectric varies with levelchanges in capacitance proportional with level

if the wall nonconductive, a single probe contain 2 electrode can be used -horizontallyterminal of electrode connected to measuring deviceinterface passes between electrodes, capacitance changes because the different in dielectric constantconductive material; insulate probe is used (teflon-coated)continuous measurement, probe install verticallychanges capacitance direct function of dielectric constantdielectric constant air & most gases = 1temperature cause dielectric constant of liquid vary (T , K )44Barrel (side mount) and probe (top mount) formatsBasic full/empty type level indication set by mounting positionAccurate switching even with heavy residue build-upHigh reliability due to no moving partsCan often be used with liquids and powder or granular product

simple design, no moving parts, minimal maintenanceavailability of corrosive resistant probe

AdvantagesDisadvantagestemperature will effect error readingprobe coated with a conductive material, errors may occurConductivityfor liquid interface of relatively high conductivityinclude water-based material: brine solution, acids, caustic, certain beverageslimited to alarm devices & on/off system

Principles

2 electrodes positioned in a tank (extends min & max level)Cable grounded & functions as common electrodeStilling well provided to ensure interface not disturbed & prevent false measurement

terminals electrode connected to relays; transmit signals to display / control deviceextends min level, conductivity path established through grounded tanklevel falls below electrode, path interrupted, showed gas / vapor is nonconductive

ControllerHigh-levelprobeLow-levelprobelevel condition activate device to sound alarm control to operate pump / feed controller that adjust levelcurrent will flow through electrodes & tankrelay detect high level

50 Uses stainless steel probes (1m) to provide sensing for up to 3 switch pointsCan only be used with conductive fluidsUsed in conjunction with a level control relay to set & monitor switching pointsOnce installed switching points can be easily adjusted via relay controls

low costsimple designno moving partseffective for many water-based materialAdvantagesDisadvantagesSystem must be conductiveOnly point detection can be measuredPossibility sparking; prohibitive for explosion & flammable substancesUltrasonic & Sonic SensorsPrinciples

Measuring device not contact with process materialmeasures distance from one point in vessel to level interfaceoperate on the echo principlesdifferent ultrasonic & sonic; operating range (US: 20kHz & S: 10kHz /below)

54 Ultrasonic sound pulses are reflected by the surface of the liquid Non-contact level sensing of water, emulsions, oils, chemicals and effluent Provides proportional indication of level via a 0/4-20mA or 0-10V output Typical range 0.25 to 10m Provides accurate level indication, but can be affected by dust or foam Can be used with solids, but range reduced by 50%can be used continuouslysonic conductivity precise; depend the density & characteristic of surfacesound waves tend to pass through most gases.waves lost kinetic energy through friction liquid more reflectivegas more adsorb

Operations

sonic level measurement dependent on sound wave striking live medium & reflecting wavedead media adsorb most sound energy

value d; depend the frequency sound wave.influenced by surface porosity, material thickness, rigiditycontinuous, based on time elapses from generation of sound wave to detection of the reflected wave

Level measurement troubleshootingConnectionsDP cell sensing line connection is reversedHigh operating pressure & low hydrostatic pressure; easy to occurActual level , indicated level Over-pressuring3 valve manifolds on DP cell prevent over-pressuring & easy to removeFail immediately / diaphragm become distortedLow / high reading

Obstructed sensing linesSmall diameter; clogged with particulate inaccurate readingSluggish response to level changesTo overcome: periodic draining & flushingDraining sensing linesTo remove debris / particulate settle on the bottom vessel / in lineClose tank; remove condensate prevent fluid pressure build up in LP impulse lineLeaking / drained wet leg false high level indication

Lead lineLT

Reflex sight glass

CapacitanceMagnetic FloatCable FloatSight glass

Side mounting FloatUltrasonic63End