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Engineer’s Class Visit To

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

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Engineer’s Class Visit To

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

  2. Some Applications

  3. 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)

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

  5. 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

  6. High Level Filling Line Low Level Pump Protection Leak Detection Point Level Applications

  7. Additional Point Level Applications

  8. Interface Between Fluids Composition Change Continuous Level Point Level Will often have several combinations of sensors for sophisticated needs

  9. Considerations – Process Conditions • Pressure • Temperature • Agitation • Corrosion • Foaming • Explosion Hazards • Dangerous or Lethal Materials

  10. Considerations –Material changes • Material Density • Chemical Composition • Material Buildup • Electrical Properties

  11. Some Level Technologies • Nuclear • Paddle Wheels • Plumb Bobs (Yo Yo’s) • Radar • RF Admittance • Tuning Forks • Ultrasonic • Bubblers • Conductivity • Differential Pressure (D/P) • Displacers • Floats • Gap Switches • Load Cells • Microwave

  12. Dip Stick Level Measurement

  13. Basic Sight Glasses

  14. 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.

  15. Float & Cable Arrangements

  16. 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.

  17. 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

  18. Application Box Score Color Codes Green -OK Yellow- Caution Point Level Liquids Granulars Slurries Interface Continuous Liquids Granulars Slurries Interface Clear- Not OK

  19. 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 Point Level Slurries Liquids Granulars Interface Slurries Continuous Liquids Granulars Interface

  20. Air Bubble System PRESSURE That pressure is related to change in height of liquid in vessel, AND density

  21. 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

  22. Bubbler Trade-offs pressure gauge • 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 air in Point Level Slurries Liquids Granulars Interface Slurries Continuous Liquids Granulars Interface

  23. 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.

  24. Trade-offs Differential Pressure (D/P) Sealed System Diaphragm • 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 Point Level Slurries Liquids Granulars Interface Slurries Continuous Liquids Granulars Interface

  25. 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.

  26. Interface Measurement with Displacers (COURTESY OF MASONEILAN-DRESSER INDUSTRIES)

  27. 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.

  28. 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 Point Level Slurries Liquids Granulars Interface Slurries Continuous Liquids Granulars Interface

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

  30. It Starts With The Capacitor AC Conductive Plate Conductive Plate Dielectric Insulating Material

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

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

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

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

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

  36. Capacitance Limitations • Changing Density • Liquids that Coat

  37. Cut Monitor Detects Change in Dielectric Constant of material… …such as contamination getting into your product

  38. Measuring with Changing Dielectric Two Probes – one references “K” Single Probe with two sensors – lower one measures “K”

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

  40. 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 Point Level Slurries Liquids Granulars Interface Slurries Continuous Liquids Granulars Interface

  41. 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.

  42. Trade-offs Conductivity Switch Uses Ions in Material • Advantages • Low Cost • Disadvantages • Conductive, non-coating liquids only • Insulating coating – – – + – + + – + – Point Level Slurries Liquids Granulars Interface Slurries Continuous Liquids Granulars Interface

  43. 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

  44. Level Measurement by Weight

  45. 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 Point Level Slurries Liquids Granulars Interface Slurries Continuous Liquids Granulars Interface

  46. Trade-offs Tuning Fork Material Dampens Vibration • Advantages • No Calibration • Unaffected by electrical and physical properties • Disadvantages • Unpredictable with foam • Product Buildup • Moving Parts Point Level Slurries Liquids Granulars Interface Slurries Continuous Liquids Granulars Interface

  47. 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.

  48. Rotating Paddle Switch Material slows/stalls paddles • 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.

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