Gas Detection Systems

advertisement
Gas Detection Systems
Sensor Operating Theory
1
Vi verner liv, miljø og verdier...
Gas/Vapor Hazards•
•
•
•
Flammable/Combustible
Toxic
Oxygen-deficiency
Oxygen-enrichment
2
Vi verner liv, miljø og verdier...
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
3
Vi verner liv, miljø og verdier...
Gas Sensing Technologies
•
•
•
•
•
•
•
Combustibles:
Hydrogen Sulfide:
Oxygen:
Carbon Monoxide:
Chlorine:
Sulfur Dioxide:
Nitrogen Dioxide:
Catalytic and Infrared
Electrochemical & MOS
Electrochemical
Electrochemical
Electrochemical
Electrochemical
Electrochemical
4
Vi verner liv, miljø og verdier...
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
5
Vi verner liv, miljø og verdier...
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)
6
Vi verner liv, miljø og verdier...
Flammable Limits & Volumetric
Equivalents
Gas Type
Methane
Propane
Hydrogen
Hexane
Methane Gas: 100% LEL=
75%
50%
25%
10%
=
=
=
=
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%
“
“
“
“
7
Vi verner liv, miljø og verdier...
Catalytic Sensor Operation
•
•
•
•
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
8
Vi verner liv, miljø og verdier...
Catalytic Sensor Operation
•
•
•
•
•
Flammable
gas
•
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
9
Vi verner liv, miljø og verdier...
Catalytic Sensor Operation
10
Vi verner liv, miljø og verdier...
Catalytic Sensor Limitations
FAIL TO DANGER
•Poisoning of Catalyst
•Plugging of Flame Arrestor
•Read Low @ High Gas Levels
•Frequent Calibration
•Service Life - Variable (Months)
Vi verner liv, miljø og verdier...
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
Vi verner liv, miljø og verdier...
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)
Vi verner liv, miljø og verdier...
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
Vi verner liv, miljø og verdier...
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)
Vi verner liv, miljø og verdier...
Drivers for Optical Technology
• Safety Driven Design Requirements
– FAIL SAFE
• No Unrevealed Sources of Failure
– HIGH AVAILABILITY
• Long Service Life
• Low and Easy Maintenance
Vi verner liv, miljø og verdier...
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
Capillary Entrance
O-ring
Membrane
Sensing Electrode
Reference Electrode
Electrolytic fluid
Counter Electrode
17
Vi verner liv, miljø og verdier...
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.
18
Vi verner liv, miljø og verdier...
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
19
Vi verner liv, miljø og verdier...
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
20
Vi verner liv, miljø og verdier...
Download
Related flashcards

Symbols

28 cards

Electrical engineering

45 cards

Electric power

25 cards

Signal processing

22 cards

Electrical components

22 cards

Create Flashcards