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Gas Detection Systems

Gas Detection Systems. Sensor Operating Theory. Gas/Vapor Hazards-. Flammable/Combustible Toxic Oxygen-deficiency Oxygen-enrichment. Gas Detection Objectives . Measure gas before hazardous concentration is present Provide outputs for Emergency Action

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Gas Detection Systems

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  1. Gas Detection Systems Sensor Operating Theory

  2. Gas/Vapor Hazards- • Flammable/Combustible • Toxic • Oxygen-deficiency • Oxygen-enrichment

  3. Gas Detection Objectives • Measure gas before hazardous concentration is present • Provide outputs for Emergency Action • Suitable for extreme environments (arctic, tropic, etc) • Complement to Flame Detection equipment • Cost-effective protection

  4. Gas Sensing Technologies • Combustibles: Catalytic and Infrared • Hydrogen Sulfide: Electrochemical & MOS • Oxygen: Electrochemical • Carbon Monoxide: Electrochemical • Chlorine: Electrochemical • Sulfur Dioxide: Electrochemical • Nitrogen Dioxide: Electrochemical

  5. Key Gas Detection Definitions • Measurement range: gas concentration at full scale sensor output • Unit of Measurement: %LEL, PPM, or %volume • Flashpoint: minimum temperature of liquid where enough vapor is given off to sustain a fire or explosion • Vapor Density: tendency of a pure gas of vapor to rise or sink in clean air • Calibration: process of matching transmitter signal to sensor measurement

  6. Clean Air Too rich for ignition Too lean for ignition Combustibles 50% Lower Explosive Limit (LEL) Upper Explosive Limit (UEL) Typical Combustible Gas Detector Range of Measurement Combustible Gases-Lower & Upper Limits* *No differentiation between the terms “Explosive” and “Flammable” as applied to the lower and upper limits of flammability

  7. Flammable Limits & Volumetric Equivalents Methane Gas: 100% LEL= 5% by volume in air 75% = 3.75 “ 50% = 2.5% “ 25% = 1.25% “ 10% = .50% “

  8. Catalytic Sensor Operation • Contact-based • Destructive measurement • Traditional method • Only option for hydrogen gas detection • Used w/ transmitter module to provide 4-20 ma signal output • Typical life of 3-6 years • 2 year mfgr warranty

  9. Flammable gas Catalytic Sensor Operation • Active Element - Hot wire resistor coated with catalytic material • Reference Element - Same as active but without catalyst • Catalytic Oxidation of flammable gas generates heat & changes resistance of Active Element • Explosion is prevented by Flame Arrestor • Constant U or I power source required • Must calibrate on start-up & routinely for accuracy

  10. Catalytic Sensor Operation

  11. Catalytic Sensor Limitations • FAIL TO DANGER • Poisoning of Catalyst • Plugging of Flame Arrestor • Read Low @ High Gas Levels • Frequent Calibration • Service Life - Variable (Months)

  12. reference absorption band active absorption band Infrared Combustible Detectors • Principal of Operation • Gases are Transparent to Visible Light BUT... • Gases are Strongly Opaque to Infrared Light at Some Wavelengths Methane Propane

  13. Flammable hydrocarbon gas Point Infrared Detector • Principle of Operation

  14. Fail-Safe Operation

  15. Flammable hydrocarbon gas Open Path IR Gas Detector • Instrumental Layout 10 - 60 m Optical Beamsplitter Measurement Signal Detector IR Source Optical Filters Microprocessor and Electronics Reference Signal Detector Gas Concentration (LEL)

  16. Drivers for Optical Technology • Safety Driven Design Requirements • FAIL SAFE • No Unrevealed Sources of Failure • HIGH AVAILABILITY • Long Service Life • Low and Easy Maintenance

  17. Capillary Entrance O-ring Sensing Electrode Membrane Reference Electrode Counter Electrode Electrolytic fluid Electrochemical Toxic Sensor Theory • Specific to toxic gas type (H2S, CO, SO2, NO2, CL2 ) • Consists of sensing(working), reference, and counter electrodes • Vapor diffusion into capillary results in oxidation/ion reduction • The signal (current) from the working electrode is converted to a voltage by operational amplifier

  18. Electrochemical Toxic Sensor Operation • All provide 4-20 ma signal output • 0-100 ppm range typical • Carbon monoxide ranges: 0-100, 0-500, or 0-1000 ppm- must specify • Nitrogen dioxide range: 0-20 ppm • Hydrophobic filter provides humidity protection • All designed to meet X-proof req. except CL2, SO2 sensors (I.S. barrier required) • All designed for calibration by a transmitter, controller or external control system.

  19. Traditional Sensor Calibration • Matching transmitter output to raw sensor signal in clean air (zero) & mid-scale (span) conditions • Manual or Automatic depending upon transmitter type • Adjustments necessary if drifting has occurred • Requires compressed, accurate calibration gases • Always required @ start-up & after sensor replacement • Every 30-90 days thereafter typical

  20. Gas Detector Calibration Requirements • Catalytic Sensors: Field Calibration required for accuracy • DEC IR Detectors: Routine calibration not required; Factory calibrated for methane only • Electrochemical Sensors: Field Calibration required for accuracy • DEC Open Path IR System: Routine calibration not required; Factory calibrated for methane only

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