Gases, Vapours and Mists
Flammable gases, vapours, and mists represent the most common explosive atmosphere hazards in industry. Understanding their distinct behaviours and properties is essential for effective zone classification and equipment selection.
Flammable Gases
Gases are substances that exist in gaseous state at normal temperature and pressure (NTP: 20°C, 1 atm). Common industrial flammable gases include:
- Methane (CH₄): LEL 5.0%, UEL 15%, relative density 0.55, AIT 595°C—primary component of natural gas, also generated by anaerobic decomposition
- Hydrogen (H₂): LEL 4.0%, UEL 75%, relative density 0.07, AIT 560°C—extremely wide flammable range, very low MIE (0.017 mJ), rises rapidly due to low density
- Propane (C₃H₈): LEL 2.1%, UEL 9.5%, relative density 1.56, AIT 470°C—heavier than air, accumulates in low points, common in LPG applications
- Acetylene (C₂H₂): LEL 2.5%, UEL 80%, AIT 305°C—uniquely unstable, can decompose explosively without air above 2 bar pressure
Gas behaviour depends significantly on relative density. Gases lighter than air (density below 1.0) disperse upward and dissipate rapidly in open areas. Heavier-than-air gases flow along ground level and collect in trenches, pits, and enclosed spaces.
Flammable Vapours
Vapours are the gaseous phase of substances that are liquid at NTP. Vapour production depends on the liquid's vapour pressure, which increases with temperature. Key parameters include flash point, which is the lowest temperature at which liquid produces sufficient vapour for ignition with an external source—petrol at -43°C, diesel at +52°C, ethanol at +13°C.
Common industrial flammable vapours include solvents (acetone: LEL 2.5%, flash point -18°C; toluene: LEL 1.2%, flash point +4°C), fuels (petrol vapour: LEL 1.4%, AIT 280°C; kerosene: LEL 0.7%, flash point +38°C), and chemical feedstocks (styrene: LEL 1.1%, ethylene oxide: LEL 3.0%). Vapour density relative to air determines dispersion behaviour—most organic solvent vapours are heavier than air, typically 2-4 times air density.
Mists
Mists are liquid droplets suspended in air, formed by mechanical action (spraying, splashing) or condensation. Unlike vapours, mist flammability depends on droplet size distribution. Droplets below 10 μm behave like vapours, while larger droplets (above 100 μm) may settle before ignition occurs.
Mist hazards commonly arise from high-pressure hydraulic leaks (oil mist at pressures above 30 bar), spray coating operations, and lubricating oil systems. Mineral oil mists with droplet sizes below 40 μm have been ignited at concentrations as low as 45 g/m³. Importantly, mists can form explosive atmospheres at temperatures well below the liquid's flash point—hydraulic oil with flash point of 200°C can form explosive mist at ambient temperature.
Gas Groups
For equipment selection, gases and vapours are classified into gas groups based on their ignition characteristics. Group I covers methane (mining). Group IIA includes propane, butane, and most common industrial gases (MESG above 0.9mm). Group IIB covers ethylene, hydrogen sulphide (MESG 0.5-0.9mm). Group IIC covers hydrogen and acetylene (MESG below 0.5mm). Equipment certified for a higher group (IIC) can be used with lower group substances (IIB, IIA), but not vice versa.
Hazard Assessment Considerations
When assessing gas and vapour hazards, identify all potential release sources including process equipment, sampling points, vents, and flanged connections. Consider abnormal conditions such as pump seal failures (typical release rate 0.1-1 kg/hour) and pressure relief events. Account for secondary substances including cleaning solvents, lubricants, and maintenance chemicals. Process changes may introduce new substances requiring reassessment of zone classification and temperature class requirements.