Engineer’s Class Visit To March, 2012 Samuel M. Herb, PE

Some Applications

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)

Level / Inventory Measurement How far from full How far from empty How much material

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 High Level Low Level Leak Detection Pump Protection Filling Line

Additional Point Level Applications

Interface Between Fluids Will often have several combinations of sensors for sophisticated needs Continuous Level Composition 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  Nuclear  Paddle Wheels  Plumb Bobs (Yo Yo’s)  Radar  RF Admittance  Tuning Forks  Ultrasonic

Dip Stick Level Measurement

Basic Sight Glasses

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.

Float & Cable Arrangements

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 continuous measurement; $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 LiquidsGranularsSlurriesInterface LiquidsGranularsSlurriesInterface 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 Interface Liquids Granulars Slurries Liquids Granulars Point Level Slurries Interface

Air Bubble System 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 Interface Liquids 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 Interface Liquids Granulars Slurries Liquids Granulars Point Level Slurries Interface Trade-offs Sealed 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)

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 Interface Liquids Granulars Slurries Liquids Granulars Point Level Slurries Interface

Capacitor Probe LT C PROBE ONE: PLATE OF CAPACITOR METAL WALL: OTHER PLATE OF CAPACITOR

It Starts With The Capacitor AC Conductive Plate Dielectric Insulating Material

AC Factors that Affect Capacitance Distance Between Plates AC Distance Capacitance Distance Capacitance

Factors that Affect Capacitance Area of Plates AC Area Capacitance Area Capacitance AC

Factors that Affect Capacitance Dielectric of the Insulator AC Dielectric Capacitance Dielectric Capacitance AC

The Tank and Probe form a Capacitor C = k A d Where: C= Capacitance in pF k= Dielectric Constant of material A= Area of the plates d= Distance between plates k air = 1 d C k oil = 2 A

The Tank and Probe form a Capacitor C = k A d Where: C= Capacitance in pF k= Dielectric Constant of material A= Area of the plates d= Distance between plates k air = 1 A d C k oil = 2

Capacitance Limitations Changing Density Liquids that Coat

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”

BUT: RF Admittance Uses RF signal to overcome limitations of Capacitance

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 Interface Liquids 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; 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 Interface Liquids 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

Level Measurement by Weight

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 Interface Liquids 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 Interface Liquids 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 Interface Liquids 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 Interface Liquids 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 Interface Liquids Granulars Slurries Liquids Granulars Point Level Slurries Interface Trade-offs Reflects 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 Interface Liquids 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

Continuous Radioactive Measurement

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 vessel non-intrusive –Suitable for high temperature, pressure, and corrosive materials –Non contact Disadvantages –High Cost –Requires licenses, approvals, and inspections –People fear radiation exposure Continuous Interface Liquids 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 HEAT Dissipates

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 Signals L e v e l C h a n g e s

Magnetostrictive Level Sensor FLOAT Product Level Electronics within Probe

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

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 Interface Liquids 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 Interface Liquids 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; (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, conducting thick 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

Remote Configuration

PC Configuration Complete

Real World uses Multiple Sensors Point Levels Continuous

Summary: Level Instrumentation By surface detection By interface detection By hydrostatic pressure By weight By golly…