Chapter 3

Level Measurement

ObjectivesAfter finish this lesson, you will be ableto:

• Define the level• Differentiate direct measurement &

indirect measurement• Apply the factor for selecting level

measurement device

• Differentiate between open & closed tank measurement

• Understand the principle, operation, advantages & disadvantages of devices

Level Measuring Devices

Differential Pressure (DP)

Principles

• Known hydrostatic

• Based on height of liquid head (pressure)

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

• Each sides exposed same pressure system pressure cancelled &

differential pressure indicates liquid level

• Problems – vapor may condensate & fill LP tap – can affect accuracy

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

DP Disadvantages

• Affected changes density – used fixed specific gravity liquid

• Require constant head maintain at reference leg

• Susceptible to dirt / scale entering tube

Dip StickPrinciples

• Oldest form• Essential

stick/rod• Commonly used –

oil level & fuel quantity

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• Dip stick withdrawn & level read at the last interface contact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

Lead Line

Principles• Same way dip

stick• Steel measuring

tape attach weight

• More capable than dip stick

Operations

• Measuring clear liquid – applied special chemicals

• This can darken/change color• Chemical not react with the process

Advantages

• Compact, easy• Accuracy 0.1% up to range 20 ft• Accurate• Reliable• Dependable

Disadvantages

• Only measure open system• Safety precaution – caustic & toxic

process• Not possible for continuous

representation

Sight GlassesPrinciples• Visual

measurement• Transparent tube

of glass/plastic mounted outside vessel

• Level inside vessel = level sight glass

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

• 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

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

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

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

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

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

25

Typical Installatio

ns of Magnetic

Gauge

Operations

• 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 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

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

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

• 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

Cable

FloatDrum indicator

Figure 3.13: Float and spring-loaded drum

• 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

• Float & Tape commonly used reliable, little maintenance &

calibration

• Usage : local indication & auxiliary / backup

• Limited to liquid-gas interface

• Not accurate for foaming liquid

THANK YOU

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

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

• 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

• Dielectric varies with level

• Changes in capacitance proportional with level

• If the wall nonconductive, a single probe contain 2 electrode can be used -horizontally

• 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

• Changes capacitance direct function of dielectric constant

• Dielectric constant air & most gases = 1

• Temperature cause dielectric constant of liquid vary (T , K ↓)

Advantages

• Simple design

• No moving parts

• Minimal maintenance

• Availability of corrosive resistant probe

Disadvantages

• Temperature will effect error reading

• Probe coated with a conductive material, errors may occur

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

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

• 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

• 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

Advantages

• Low cost• Simple design• No moving parts• Effective for many water-based

material

Disadvantages

• System must be conductive• Only point detection can be

measured• Possibility sparkingprohibitive for explosion & flammable

substances

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)

• 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

Operations

• Sonic level measurement dependent on sound wave striking live medium & reflecting wave

• Dead media adsorb most sound energy

surfaceonincidentenergy SoundabsorbedenergySoundd

• 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

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

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

Lead line

LT

Sight glass

Reflex sight glass

dipstick

Magnetic gage

Ultrasonic

Capacitance

Magnetic FloatCable Float

Sight glass

Side mounting Float

THANK YOU