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Two Categories of Measurement
• Direct measurements include – Sight glasses– Dipsticks– Floats.
• Indirect include – Differential Pressure– Sonic– Nuclear Radiation– RF Admittance(Those that make a measurement of some other
variable, which is only related to level)
Two Types of Level Measurements....
• Point Level– High level & spill prevention alarms,– Low level & pump protection alarms,– Pump controls to turn pumps on & off
at certain distinct levels.
• Continuous Level- Process control - Inventory control - Inventory management
Point Level Applications
mPm
mPm
m P m
mPm
mPm
High Level
Low Level
Leak Detection
Pump Protection
Filling Line
Interface Between Fluids
Will often have several combinations of sensors for sophisticated needs
Continuous LevelComposition Change
Point Level
Considerations – Process Conditions
• Pressure• Temperature• Agitation• Corrosion• Foaming• Explosion Hazards• Dangerous or Lethal Materials
Considerations –Material changes
• Material Density• Chemical Composition• Material Buildup• Electrical Properties
Some Level Technologies
Bubblers Conductivity Differential Pressure (D/P) Displacers Floats Gap Switches Load Cells Microwave
NuclearPaddle WheelsPlumb Bobs
(Yo Yo’s)RadarRF AdmittanceTuning ForksUltrasonic
Basic Sight Glasses
CLOSE-LOOP SIGHT GLASSSCALE
OPEN-END SIGHT GLASS
(A) OPEN OR VENTED VESSEL (B) PRESSURIZED VESSEL
Sight Gauge
• Typical Price: $300 to $2500• Maximum Temperature: 700°F (370°C)• Accuracy: 0.25" (6mm) • Application comments:
– Excellent for thin fluids – Limited for interfaces, thick fluids, and foams– Must have same temperature as tank – Foam and boiling problems – Opaque coatings cause incorrect readings.
Tape Floats (& Servos)
• Typical Price: $$1000 to over $2500• Maximum Temperature: 300°F (150°C)• Accuracy: 1" (3mm)• Application comments:
– Excellent for continuous measurement of thin fluids– Limited with thick– Can ignore foams– Servo plumb bob is suitable for solids and interface– Mechanical hang up is biggest problem.
Float
• Typical Price: $300 to over $2500 for continuousmeasurement;
$100 to $1000 for point level switches• Maximum Temperature: 500°F (260°C)• Accuracy:
0.1 to 3% span for continuous; 1" for switches • Application comments:
– Excellent for continuous measurement of thin fluids– Limited for interfaces and thick fluids– Ignores foam, but can be used to measure it– Point level switches are fair for thick fluids– Moving parts limit most designs to clean service– Only density-adjusted floats can detect interfaces
Application Box Score Color Codes
Point Level
Continuous
Liquids Granulars Slurries Interface
Liquids Granulars Slurries Interface
Green -OK Yellow- Caution
Clear- Not OK
Float Trade-offs• Advantages
– Unlimited tank height– Can achieve high accuracy– Low cost if not remote reading
• Disadvantages– Moving parts exposed to fluids– Limited pressure rating– Not very good in agitated vessels– Not suitable for granulars– High Maintenance
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Air Bubble System
INDICATOR / RECORDER
AIR FILTER / REGULATOR
FROM AIR ORGAS SUPPLY
VISUAL FLOW INDICATORLEVEL
That pressure is related to change in height of liquid in vessel, AND density
PRESSURE
Bubbler
• Typical Price: $300 to $2500• Maximum Temperature: No limit• Accuracy: 0.5 to 1% span assuming constant density
• Application comments: – Excellent for continuous level of thin fluids– Both conductive and insulating fluids– Will ignore foam– High maintenance– Needs clean gas supply
Bubbler Trade-offs• Advantages
– Low Cost– Simple Design– Easy to Install
• Disadvantages– Susceptible to plugging– Not suitable for closed or
pressure vessels– Requires frequent
maintenance– Requires compressed air
supply
pressure gauge
air in
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Diaphragm• Typical Price:
– $300 to $1500 for continuous – $100 to $300 for point level switches
• Maximum Temperature: – 350°F (175°C)
• Accuracy: 1 to 3% span for continuous,assuming constant density; 1 to 2" (25 to 50mm) for switches
• Application comments: – Continuous measurement is limited in thin fluids, – air in thick, will ignore foam; – Submerged sensors need reference to
atmospheric pressure;– Infrequently used switches suitable for granular
solids only.
Differential Pressure (D/P)
• Advantages– User familiarity– Easy installation/calibration– Reasonable Instrument Cost
• Disadvantages– Affected by product density– Bottom Mounted– Valving and piping increase cost– Multiple Fugitive Emission points
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Trade-offsSealed System Diaphragm
Differential Pressure
• Typical Price: $300 to $2500 for continuous measurement; $100 to $1000 for point level switches
• Maximum Temperature: 350°F (175°C)• Accuracy: 0.25 to 1% span for continuous, assuming
constant density; 1 to 4" (25 to 100mm) for switches • Application comments:
– Excellent for continuous measurement of both thin and thick liquids and ignores foam;
– Risk of plugging by slurries requires diaphragm seals, repeaters, purging, or sealing legs;
– Point level switches are limited to clean liquids with constant specific gravity.
Interface Measurement with Displacers
(COURTESY OF MASONEILAN-DRESSER INDUSTRIES)
2
10
4
3POUNDS
2
10
4
3POUNDS
2
10
4
3POUNDS
1.4 lb 1.2 lb 1.0 lb
14-in. WATERLEVEL
7-in. WATERLEVEL
0-in. WATERLEVEL
2.25 in.DIAMETER14 in. LONG
(a) (b) (c)
DISTILLATE
WATER
WATERWATER
DISTILLATE
DISTILLATE
Displacer• Typical Price:
– $1000 to $2500 for continuous – $100 to $1000 for point level switches
• Maximum Temperature: 850°F (450°C)• Accuracy: 0.25 to 1% span for continuous,
assuming constant density;
0.2 to 0.5" (5 to 13mm) for switches• Application comments:
– Excellent for continuous measurement of thin fluids,– Only fair for interfaces and limited for thick fluids,– Ignores foam; – Point level switches are excellent for thin fluids,
but only fair for thick fluids and locating interfaces; – Neither are recommended for sludge or slurries;
vacuum and high viscosity can cause dynamic instability.
Displacer Trade-offs• Advantages
– Limited motion– Few moving parts
• Disadvantages– Calibration changes with density– Bottom mounted– Buildup on displacer
can cause calibration shift– Valving increases cost– High cost with long spans
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
AC
Factors that Affect CapacitanceDistance Between Plates
AC
AC
Distance
Capacitance
Distance
Capacitance
Factors that Affect CapacitanceDielectric of the Insulator
AC
Dielectric
Capacitance
Dielectric
Capacitance
AC AC
The Tank and Probe form a Capacitor
C =k A
d
Where:C= Capacitance in pFk= Dielectric Constant of material A= Area of the platesd= Distance between platesk air = 1
d
C
k oil = 2
A
The Tank and Probe form a Capacitor
C =k A
d
Where:C= Capacitance in pFk= Dielectric Constant of material A= Area of the platesd= Distance between plates
k air = 1
A
d
Ck oil = 2
Cut Monitor
Detects Change in Dielectric Constant of material… …such as contamination
getting into your product
Measuring with Changing Dielectric
Two Probes – one references “K”
Single Probe with two sensors – lower one measures “K”
RF Admittance Trade-offs
• Advantages– By far the most versatile technology– Wide range of process conditions– Works with all types of materials– On-off or continuous measurement
• Disadvantages– Application Sensitive [ALL TYPES ARE]– Insulating granular measurements
require special considerations– Changing dielectric measurements
requires special transmitter and sensor
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Capacitance / Radio Frequency• Typical Price: $300 to over $2500 for continuous measurement;
$100 to over $1000 for point level switches• Maximum Temperature: 2,000°F (1100°C)• Accuracy: 0.5 to 3% span for continuous;
0.125 to 2" (3 to 50mm) for switches• Application comments:
– Excellent for thin fluids & for measuring interfaces.– Fair to excellent in thick fluids and slurries.– Can ignore organic foam but can measure both aqueous
and organic foams.– As switch is generally very good for powder, chunky, or
sticky solids, but limited as continuous measurement of these.– As continuous measurement also cannot measure interface
between conductive layers or between liquids & solids.– As a switch, conductive coatings produce false high
without guard-type probe.– Short insertions can be a problem.
Conductivity Switch
• Advantages– Low Cost
• Disadvantages– Conductive,
non-coating liquids only– Insulating coating
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Uses Ions in Material
++
++–
–
–
–
––
Trade-offs
Conductivity Switch
• Typical Price: $100 to $1000• Maximum Temperature: 1800°F (980°C)• Accuracy: 0.125" (3mm) • Application comments:
– Excellent for conductive thin fluids– Fair for detecting interface– Limited for thick fluids and slurries or solids– Can detect conductive process materials– Thin insulating coatings produce false lows
while conductive thick fluids can show false highs– Ignores organic foams but can measure aqueous foams
Load Cell Trade-offs• Advantages
– Non- Intrusive– Mass measurement– Ok for liquids, granulars, and slurries– Hi Accuracy in weight
• Disadvantages– Best specified when vessel is ordered– Usually requires free standing structure– Only weight measurement...not level– May be affected by ice, snow, and
wind loading
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Tuning Fork
• Advantages– No Calibration– Unaffected by electrical
and physical properties• Disadvantages
– Unpredictable with foam– Product Buildup– Moving Parts
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Material Dampens Vibration
Trade-offs
Vibrating Switch
• Typical Price: $100 to $1000• Maximum Temperature: 300°F (150°C)• Accuracy: 0.25" (6mm)• Application comments:
– Excellent for powders and chunky solids.– Fair with thick fluids and limited with thin. – Ignores foam.– Excessive material build-up can prevent operation.– Sensitive to mechanical shock.
Rotating Paddle Switch
• Typical Price:
$100 to $300• Maximum Temperature:
500°F (275°C)• Accuracy: 0.5" (13mm)• Application comments:
– Excellent for powders
and fair for chunks of solids;– Limited to detection of dry,
non-corrosive, low pressure solids.
Material slows/stalls paddles
Paddle Wheel Trade-offs
• Advantages– Low Cost?
• Disadvantages– Very High Maintenance– Mechanical (Moving Parts)– Only for Granular Materials
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Plumb Bobs (YoYo’s) Trade-offs
• Advantages– Level Measurement independent of density– Wide range of granular size– Independent of moisture
• Disadvantages– Mechanical... moving parts– Higher maintenance– Plumb bob (weight) sometimes trapped
by the filling process– Moderate to high cost
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Weight on Flexible Cable
Radar
• Advantages– Hi accuracy– Ignores Vapor space changes
• Disadvantages– FCC Site License needed– Cannot measure interface– High cost
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Trade-offsReflects from surface
Radar
• Typical Price: $1000 to over $2500• Maximum Temperature: 500°F (260°C)• Accuracy: 0.1 to 1% span• Application comments:
– Excellent for continuous measurement of conducting fluids, aqueous slurries, and powders;
– Limited for insulating fluids, aqueous foam, chunks or sticky solids;
– Low dielectric materials limit range; – Condensation or crystallization on antenna
can cause errors.
Laser• Typical Price: Over $2,500• Maximum Temperature: 300°F (150°C)• Accuracy: 0.25" (6mm) • Application comments:
– Excellent for continuous measurement of thick fluids, chunks and sticky solids;
– Limited for thin fluids, locating interfaces, working with foam, and powders.
Microwave
• Advantages– Flush mounted– Unaffected by physical
and electrical properties• Disadvantages
– Conductive coatings may cause problems
– Need emitter and receiver (2
installations)
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Interrupted Line of Sight
Trade-offs
Microwave Switch• Typical Price: $300 to over $1000• Maximum Temperature: 400°F (200°C)• Accuracy: 0.5" (13mm)• Application comments:
– Excellent for conducting fluids and detecting interfaces– Limited insulating fluids and powders and chunks– Ignores organic foams and can measure aqueous foams– Fluids with low dielectric constant and thick coatings
are a problem– Microwave signals are absorbed almost entirely by water
causing problems with products that are water-based or have water content
Radiation Point Measurement
TWO GM-8GAMMASWITCHES
LOW LEVEL ALARMACTUATED WHEN BOTTOMSWITCH DETECTS INCREASEDRADIATION DUE TO FALLOF LIQUID
Radiation [Nuclear]
• Typical Price: over $2500• Maximum Temperature: No limit• Accuracy: 1 to 2% span for continuous;
0.25 to 1" (6 to 25mm) for switches • Application comments:
– Excellent for continuous measurement of most fluids and solids,– Limited only in aqueous foams and slurries;– As a switch, is limited in detecting interfaces and aqueous
slurries,and can ignore foams but also can measure them;
– Requires NRC license and source disposal can be a problem;– Heavy coatings can limit reliability
Nuclear Trade-offs• Advantages
– Can be mounted external to vesselnon-intrusive
– Suitable for high temperature,pressure, and corrosive materials
– Non contact• Disadvantages
– High Cost– Requires licenses, approvals,
and inspections– People fear radiation exposure
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Thermal Dispersion• Typical Price: $100 to over $2500• Maximum Temperature: 850°F (450°C)• Accuracy: 1 to 3% span for continuous,
assuming constant density;
0.5" (13mm) for switches• Application comments:
– Excellent for thin fluids, – Fair for thick; – Ignores foams, but can measure them; – Foams detection is limited by thermal conductivity, – Interface detection by differential thermal conductivity.
How much HEATDissipates
AD
R
TO BREATHER EQUALIZER
BASE STRIP
RESISTANCEHELIXUNSHORTED
LIQUID SURFACE
HELIX SHORTEDBELOW SURFACE
Resistance Tape Level Sensor
Increased Level Squeezes Tape &Shorts Resistance
Resistance Tapes
• Typical Price: $1000 to $2500• Maximum Temperature: 225°F (110°C)• Accuracy: 0.1 to 1% span• Application comments:
– Excellent for continuous measurement of thin fluids,– Limited with thick– Fair in aqueous slurries– Will ignore foams– Limited temperature and pressure range– Large specific gravity changes affect accuracy
Magnetostrictive
• Typical Price: $1000 to $1,500• Maximum Temperature: 300°F (150°C)• Accuracy: 0.020% span or 0.030”
(0.762mm); whichever greater• Application comments:
– Excellent for continuous measurement of thin fluids
– Limited for thick fluids– Can be used with density-adjusted float
to locate interfaces– Ignores foams
Sig
nal
s
Le
vel C
ha
ng
es
Sound• Can be audible or not (Ultrasonic is not)• Created by Piezo-electric crystal
Measurement
Influences• Temperature 0.3% per degree F• Change in materials & gases• Stratified atmospheres
• Time of flight measurement• Distance = (Velocity * Time)/2• Will not work in vacuum
Sound Reflections• Source of echo is boundary area of two different materials• Smooth surfaces produce best echoes• Poor echo produced by
– Foam– Rough Surfaces– Agitation
Lost Echo• Scattering of sound by excessively rough surfaces • Acousticly “soft” surface, such as foaming• Exceeding the calibration range
Beam Angle
• Sound is not a parallel beam• The longer the distance,
the larger the sound footprint• Typical Angle of 12 degrees• Every foot of range
increases footprint by 1.25”
1 ft
1.25”
Ultrasonic Measurement
RECEIVER ANDAMPLIFIER
TIMED GAINCONTROL
WAVESHAPING
LOGIC ANDDISPLAY
GENERATOR ANDTRANSMITTER
TIMINGGENERATOR
TRANSDUCERS
TRANSMITTED BURST
RECEIVED BURST (ECHO)
ELAPSED TIME PROPORTIONALTO DISTANCE
OBJECT BEINGSENSED
Ultrasonic Ignores Obstructions
Software mapping techniques used to "blank out" or suppress unwanted interference signals creating picture of a "perfect" tank.
Ultrasonic Trade-offs• Advantages
– Non-contact technology– No moving parts– Easy Calibration
• Disadvantages– More Position sensitive
than other technologies– Can be affected by vapors,
structural members, foam– Limited temperature &
pressure range– Won’t work in a vacuum
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Ultrasonic Gap Switches
• Advantages– No calibration needed– No moving parts– Easy to install– Can ignore foam
• Disadvantages– Fooled by a plugged gap– Won’t detect aerated products
Continuous
InterfaceLiquids Granulars Slurries
Liquids Granulars
Point Level
Slurries Interface
Material fills gap & blocks sound
Ultrasonic
• Typical Price: $100 to $2500• Maximum Temperature: 300°F (150°C)• Accuracy: 0.25 to 3% span for continuous;
0.125 (3mm) for switches • Application comments:
– Excellent for thin fluids– Ignores foams– Continuous measurement is fair for thick fluids
but switches are limited in that application– With continuous, dust, dew in vapor space
hurts performance and range is limited by foam and angled or aerated solids
– With switches, air bubbles and solid particles in the liquid will produce a "low" signal.
Micropulse TDR
• Sends million pulses per second along length of wire or rod.
(Time Domain Reflectometry)
• Impedance change reflects signals.
• Level = half time of travel
TDR Probe styles
• Single probes – (Rod or Cable)
Easier to install – lower price.• But they have larger pulse field
– 12” radius, must be kept away from
internal obstructions in vessel.– Good sensitivity – Good accuracy (to K > 2 – 3)– Best ability to ignore material build-up
12”
TDR Probe styles
• Twin probes – (Rod and Cable)better sensitivity to lower “K” materials due to a concentrated electromagnetic field.– 4” radius, which can easier ignore
internal structures in vessel.– Good with lower “K” materials down to
about 2 and slightly below (K > 1.7)– Not good for coating build-up that may
bridge elements
4”
TDR Probe styles
• Coaxial probes – (only Rod style)
Best ability to accurately sense the lowest “K” materials (down to K > 1.5)
- Not affected by vessel obstructions.Signal fully contained.
- Not good with viscous or coating fluids.- Not suitable for solids/granular mtrls.- Length limits to 20 feet.
10mm Rod X 1” ID
316SS tube 1.28” OD
Time Domain Reflectometry (TDR)
• Typical Price: $1000 to over $2500• Maximum Temperature: 400°F (200°C)• Accuracy: 0.1 to 2% span• Application comments:
– Excellent for continuous measurement of thin fluids, conductingthick fluids, aqueous slurries, powders, and solid chunks
– Limited for sticky solids and locating interfaces – Ignores organic foams and can measure aqueous foams– Long nozzles are a problem– Range and accuracy on insulating material is greater
with high dielectric constant– Can have significant dead zones depending on mounting.
Level Measurement-Common Problems
• Plugged lines• Liquid/foam• Liquid agitation• Others
– Dry leg/wet leg– Zero elevation/suppression
Summary: Level Instrumentation• By surface detection• By interface detection• By hydrostatic pressure• By weight• By golly…
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