Transcript Gas Detection Systems

Gas Detection Systems
Sensor Operating Theory
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Gas/Vapor Hazards•
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Flammable/Combustible
Toxic
Oxygen-deficiency
Oxygen-enrichment
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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
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Gas Sensing Technologies
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Combustibles:
Hydrogen Sulfide:
Oxygen:
Carbon Monoxide:
Chlorine:
Sulfur Dioxide:
Nitrogen Dioxide:
Catalytic and Infrared
Electrochemical & MOS
Electrochemical
Electrochemical
Electrochemical
Electrochemical
Electrochemical
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Key Gas Detection Definitions
• Measurement range:
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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
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Combustible Gases-Lower &
Upper Limits*
*No differentiation between the terms “Explosive” and “Flammable” as applied to the lower and
upper limits of flammability
Clean Air
Too rich for ignition
Too lean for ignition
50%
Typical Combustible
Gas Detector Range
of Measurement
Combustibles
Lower
Explosive
Limit (LEL)
Upper
Explosive
Limit (UEL)
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Flammable Limits & Volumetric
Equivalents
Gas Type
Methane
Propane
Hydrogen
Hexane
Methane Gas: 100% LEL=
75%
50%
25%
10%
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L.E.L.
5%
2.1%
4.0%
1.1%
U.E.L.
15%
9.5%
75%
7.5%
5% by volume in air
3.75
2.5%
1.25%
.50%
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Catalytic Sensor Operation
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DET-TRONICS
ALARM
INCREASE
SETUP DECREASE
ACCEPT
CAL/RESET
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
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Catalytic Sensor Operation
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Flammable
gas
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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
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Catalytic Sensor Operation
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Catalytic Sensor Limitations
FAIL TO DANGER
•Poisoning of Catalyst
•Plugging of Flame Arrestor
•Read Low @ High Gas Levels
•Frequent Calibration
•Service Life - Variable (Months)
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Infrared Combustible Detectors
reference
absorption band
• Principal of Operation
• Gases are Transparent
to Visible Light BUT...
• Gases are Strongly
Opaque to Infrared
Light at Some
Wavelengths
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active absorption band
Methane
Propane
Point Infrared Detector
• Principle of Operation
IR Transparent Windows
Flammable
hydrocarbon
gas
IR Source
Optical Beamsplitter
Permeable Gas
Cell
Reference
Signal Detector
Measurement
Signal Detector
Optical Filters
Microprocessor
and
Electronics
Gas
Concentration
(LEL)
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Fail-Safe Operation
Active
Reference
Clean
Air
Active
Failed
Sensor
IR Signal Level
Active
Active
Reference
Failed
IR-Source
IR Signal Level
Active
IR Signal Level
Reference
Failed Microprocessor = 0 mA
IR Signal Level
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IR Signal Level
Reference
Hydrocarbon
Gas
Dirty
Optics
and
Clean
Air
Reference
Open Path IR Gas Detector
• Instrumental Layout
10 - 60 m
Optical Beamsplitter
IR Source
Measurement
Signal Detector
Flammable
hydrocarbon
gas
Optical Filters
Microprocessor
and
Electronics
Reference
Signal Detector
Gas
Concentration
(LEL)
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Drivers for Optical Technology
• Safety Driven Design Requirements
– FAIL SAFE
• No Unrevealed Sources of Failure
– HIGH AVAILABILITY
• Long Service Life
• Low and Easy Maintenance
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Electrochemical Toxic Sensor
Theory
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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
Capillary Entrance
O-ring
Membrane
Sensing Electrode
Reference Electrode
Electrolytic fluid
Counter Electrode
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Electrochemical Toxic Sensor
Operation
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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.
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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
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Gas Detector Calibration
Requirements
• Catalytic Sensors:
• DEC IR Detectors:
Field Calibration required for accuracy
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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