Gas Detection Systems Theory and Principles

Gas/Vapours Hazards- Flammable / Combustible Toxic Oxygen-deficiency Oxygen-enrichment

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

Gas Measurement Techniques Combustibles: Catalytic and Infrared Hydrogen Sulphide: Electrochemical & MOS Oxygen: Electrochemical Carbon Monoxide: Electrochemical Chlorine: Electrochemical Sulphur Dioxide: Electrochemical Nitrogen Dioxide: Electrochemical

Typical Components Sensor(s), depends on # of channels Controller-Based Detection System: Sensor(s), depends on # of channels Transmitter, Junction Box for each Field Wiring Controller, Mounting Cage or Enclosure Calibration Gas Accessories

System Configuration & Selection Identify Gas Hazard Types! Identify Customer Priorities- Life or Process Safety? Interference/Background Gases? Area Classification & Environment? Manned or Unmanned? Expected Service Life? Long Term Plans?

Gas Detector Design/Performance Standards Gas Detector Approval Agencies: Factory Mutual Canadian Standards Association CENELEC IEC others Other bodies providing recommendations: Instrument Society of America National Fire Protection Association Industrial Risk Insurers

Level of Safety Attained Varies by User and Project Requires Gas Hazard and Application Research Know Sensing Technology Capabilities and Limitations Establish Test, Maintenance, & Calibration Schedules VERY SAFE UNSAFE

Combustible Gas Types Hydrocarbon & Hydrogen Gas Types are most common- H C Methane Hydrogen H

Key Gas Detection Definitions Gas measurement ranges: % by volume, LEL, trace Combustible Gases: Hydrocarbon or not? Toxic Gases: PPM / PPB measurement range Flashpoint / Vapour Pressure Vapour Density Aerobic/Anaerobic Sensor, Transmitter, Controller Unitized, Controller-based Calibration

Combustible Gas Grouping Grouped by: ease of ignition amount of energy released Typical class 1 gases per NEC 500 (IEC/CENELEC): Group A (IIC): acetylene Group B (IIB + H2): hydrogen Group C (IIB): ethylene Group D (IIA): propane mining (I): methane Class II groups include: metal, coal, grain dusts and fibers

Combustible Gases-Lower & Upper Limits* *There is 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 Combustibles 50% Lower Explosive Limit (LEL) Upper Explosive Limit (UEL) Typical Combustible Gas Detector Range of Measurement

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

Gas Sensor Locations and Placement Sensors must contact gas of interest Know the gas hazard properties and application details: vapor density? flashpoint/vapor pressure? interferents? Identify the most likely leak scenario and event sequence Identify high risk leak sources & gas accumulation areas Prioritize leak risk areas Identify problems, i.e vibration, heat, moisture, sensor poisons, barriers, etc.

Sensor Area of Coverage Absolute area of coverage undefined by approval bodies UL suggests 900 square foot space or less as start only Evaluate specific properties of the gas and application Smoke generator test useful to identify indoor air current patterns and outdoor prevailing winds

Point Detector Area of Coverage Area covered by one sensor is not consistent in all cases UL suggests 900 feet2 space or less as start only Evaluate specific properties of the gas & application Smoke generator test useful to identify indoor air current patterns and outdoor prevailing winds 30 34 30

How Gas Detectors Respond A Point Detector measures the concentration at the point where it is located and reads % of LEL. the detector must be in the cloud to detect it...

Point Detection Limitation Fundamental Weakness of Point Detection Wind Direction Point Detector

Principles of Gas Detection 1. The gas concentration is largest somewhere in the middle. 2. The concentration decreases towards the edges 3. The shape is made irregular and/or elongated by air currents 4. Gas cloud dispersion behaviour outdoors is difficult to predict with any degree of certainty

Catalytic Sensor Theory of Operation Gas or vapor of interest must contact sensing element Traditional method of measuring flammable gases Only option for hydrogen gas detection Used w/ transmitter module to provide 4-20 ma signal output Destructive measurement technique Typical service life of 3-6 years

Catalytic Sensor Theory of Operation Two beads arranged in wheatstone bridge Active element (bead) catalyzes the gas molecules Passive reference bead has identical R to active bead Heat changes active R Unbalanced bridge provides linear mV signal output Constant E or I power source required Must calibrate on start-up &routinely for accuracy

Catalytic Sensor Theory of Operation Stainless sinter provides ex-proof design Compensates for humidity and temperature changes Thermal barrier prevents heating of reference bead Unsuitable for low oxygen applications (<10% O2) Can be poisoned by external airborne substances Possible ambiguous reading above 100% LEL Always install w/ sinter pointing downwards

Catalytic Gas Sensor Poisons/Inhibitors Poisons affect catalytic sensor response & longevity Erosion, impervious covering, or plugging active sites Impact depends on poison type, level, time of exposure Known catalytic sensor poisons: silicone oils, greases, resins (RTV adhesive) halogens (halon, chlorine, fluorine, bromine, freon) phosphate esters tetraethyl lead trichlorobenzene acid and PVC vapors other corrosive materials

Background Combustible Gases Background combustible gases common in oil/gas refineries and industrial plants Zero “drift” (+ve and -ve) is common symptom Affects catalytic sensor service life Non-destructive measurement technique (IR) may be recommended

Electrochemical Toxic Sensor Theory Specific to toxic gas type (H2S, CO, SO2, NO2, CL2 ) Consists of sensing, reference, and counter electrodes Diffusion thru capillary results in oxidation/ion reduction Microampere current flow is generated; converted to 4-20 ma Not intended for applications with high levels of target gas, or flues, stacks, or scrubbers Capillary Entrance O-ring Membrane Electrolytic fluid Sensing Electrode Reference Electrode Counter Electrode

Non-Dispersive IR Absorption Theory IR absorption “fingerprints” vary by chemical structure Spectra “Fingerprint” comparison of materials shown at left Beer-Lambert law defines the absorption physics

Methane and Butane Absorption Spectra

IR Detector Theory of Operation Signal strength measured within absorption & reference bandwidths Optical bandpass filters used to define channels Loss of reference signal means optics problem

InfraRed Gas Detector System Elements Flammable hydrocarbon gas Optical Beamsplitter Measurement Signal Detector IR Source Optical Filters Microprocessor and Electronics Reference Signal Detector Gas Concentration (LEL)

Infrared Hydrocarbon Gas Detectors Ratio is related to gas concentration Not suitable for hydrogen (H2) detection Transmittance vs. concentration is non-linear Algorithms required to linearize signal output