designing a gas detection system (1)

Designing a Gas Detection System

Honeywell ProprietaryHoneywell.com*

Why Gas Detection?Safeguard Life and Property.Provide Early Warning of Hazardous Conditions.Provide Opportunity for Evacuation and Notification from Re-entryProvide Time for Intervention and Correction.Trigger Facility Protection Systems.Ventilation, Water Mist, Fire Suppression.Satisfy Local Fire Code and Provide Insurability.Address Real and Perceived Safety Concerns.Note: Gas Detection is Recommended Practice, Required by Code, or Required by Law.


Points to ConsiderUnderstand the applicationIdentify potential danger pointsEstablish design goalsDetermine gas characteristicsProfile the plant and potential release scenariosOther elements in selecting gas detection systemsActual placement of detectionIndoorsOutdoors


Understand The ApplicationThe gases to be monitoredToxic (STEL, TLV, TWA)Combustible (LEL, UEL)Exposure limitsDensity and Other Gas PropertiesLocal and federal regulationsUniform Fire CodeCode of Federal RegulationsLocal Fire Marshal


Identify Potential Danger PointsRelease Points sensors should be placed as close as possible to potential leak sources. Seals and flanges, fittings and weldsExpansion joints and gasketsEngine combustionStorage, loading and unloading areasRunoff areasDecomposing materialsReceptor Points a gas detection notification system should protect any person, property or equipment that may come in contact with harmful gases.Wind directionVentilation systemsRun off areasConfined spacesCommunities and facilities


Establish Design GoalsInitiate a response based on an early warning of a potential problem.

Notification or annunciation method of warningVentilation control Process shutdownEvacuation and emergency responseAmount of confinement - over pressurization and accumulationRun-up distance speed of flame increases with distanceAmount of congestion or obstaclesFuel quantity and mixingMargin of safety distance between leak source and receptorsPlant safety processInsurance requirements


Determine Gas CharacteristicsLEL, UEL, ToxicityVapor densityDensity differences with temperatureCryogenic liquids, flammable liquidsLow density gases displacing ambient density gases (helium vs. oxygen)Gases under pressure will condense in areas where vented firstGases changing composition dry iceToxicity vs. flammability (MTBE 40 PPM 1.6% LEL)Hydrolyzed (BF3, F2)Pyrolyzed (NF3)Flash point - the lowest temperature at which a liquid can form an ignitable mixture in air near the surface of the liquid. The lower the flash point, the easier it is to ignite the material.Rate of evaporation and dispersion characteristicsGas mixing (fuel, oxygen, ignition source)Oxygen enriched environments


Gas HazardsThere are three main types of gas hazard

Flammable Risk of fire and or explosion, e.g. Methane, Butane, PropaneToxicRisk of poisoning, e.g. Carbon Monoxide, Hydrogen Sulfide, ChlorineAsphyxiantRisk of suffocation, e.g. Oxygen deficiency, Nitrogen, Carbon Dioxide


Flammable RiskFire TriangleThree factors are always needed to cause combustion: A source of ignitionOxygenFuel in the form of a gas or vapour


Flammable RiskThe operation of a cars choke illustrates an important part of flammable gas hazardsAll flammable gases are only ignitable over their flammable rangeFlammable gases tend to be measured in percentage of their explosive Limit (%LEL)L.E.L. (lower explosive limit)U.E.L. (upper explosive limit)0% v/v gas 100% v/v airtoo leanflammable rangetoo rich100% v/v gas 0% v/v air


Toxic RiskSome gases are poisonous and can be dangerous to life at very low concentrations.Some toxic gases have strong smells like the distinctive rotten eggs smell of H2SOthers are completely odourless like Carbon Monoxide


Toxic RiskThe measurement most often used for the concentration of toxic gases is parts per million (ppm).For example 1ppm would be equivalent to a room filled with a total of 1 million balls and 1 of those balls being red. The red ball would represent 1ppm.


Profile the Plant and Potential Release ScenariosGas sensors should be placed to ensure that a quantity of gas will past by them in all normal release scenarios.

Identify physical features of plantIdentify ventilation tracksIdentify escape routesProtect entrances to areasMark escape routesIdentify wind directions


Other ElementsAccessibility for calibration and maintenanceWiring and installationEnvironmental conditionsEMI and RFIAlarm levelsExposure limitsOxygen levels some toxic gas electrochemical sensors require a minimum oxygen level to function. All catalytic bead combustible detectors require oxygen to work.Be aware of poisoning and inhibiting factors


Interior Detector Placement Guidelines Operate Detectors Within their Temperature Limits. Use Sample Draw or Duct Mount Configurations When NeededWater, Moisture, Dust and Dirt May Affect Performance. Minimize Exposure and Protect From Adverse ConditionsLocate Detectors With Respect to Grade, Floor, or Operating Level - According to Building Design, HVAC System, Characteristics of Potential LeakWhen Monitoring Specific Equipment, Place Detectors Near (12 Inches) Pump, Seal, Tank, Valve, etc.Sensitivity of Detector Depends on Proximity to Leak. Adjust Alarms if Earlier Annunciation is RequiredMount Detectors Securely, Independent of Vibration, With Weather Shield Facing DownwardConduct Smoke Trace Behavior Studies If in Doubt


Detector Spacing IndoorsThere are Few Published Guidelines and No Standards Indicating Area or Volume Effectively Protected By a Diffusion Sensor. There is a Corollary in Fire ProtectionUL Suggests a 900 Ft2 Ceiling Space Per Smoke Detector, Which is a 30 Ft. Square or 15 Ft. RadiusUsing This Base Guideline, the Total Number of Detectors Must be Based on Gas Dispersion Characteristics and Air MovementPotential Leak Source Locations and Characteristics, Sources of Ignition LocationsInterior Space Division by Walls or BarriersEconomics of the Procurement


Detector Spacing Indoors, Contd.UL Suggests a 900 Ft2 Ceiling Space Per Smoke DetectorReferencesNFPA 72 E, Standard On Automatic Fire DetectorsSchaeffer, M.J., The Use of Combustible Detectors in Protecting Facilities from Flammable Hazards, ISA Transactions, Volume 20, No. 2, Instrument Society of America 1981DetectorDetector


IndoorsWindowDoorLN2 Cl2Door40 feet20 ftCeiling VentilationNatural GasLab BenchSTORAGE


Outdoor Detector Location GuidelinesUse same considerations outdoors as indoors.Consider Angle and Direction of Prevailing WindThe Orientation of Structures and Surrounding Terrain with Regard to Shielding AffectsThe Proximity of Large Quantities of Toxics to Personnel and Equipment, Which May Require Added Detectors to Isolate the Two


Outdoor Detector Location GuidelinesPossible Entrapment of Leaking Gases and Vapors Within Columns, Low Lying Areas or Confined Spaces

Sources of Ignition and Processes With Fugitive Leak Potential Are Considered for Detector Placement


Outdoor Detector Location, Contd.Heavier Than Air Gases or Vapors: Vapor Density >1The Preferred Location for Detectors is ~18 Inches Above Grade. For Liquid Spills, As Close to the Vapor/Liquid Interface as Possible, and Still Allow for Detector CalibrationLighter Than Air Gases and Vapors: Vapor Density

designing a gas detection system (1)

Documents

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