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Control measure

Cordon control: Unignited flammable vapours

Control measure knowledge

An unignited vapour cloud at concentrations above its ‘lower explosive limit’ (LEL) creates a dangerous atmosphere and the area should be evacuated. As flammable vapours are generally invisible, their exact presence can only be determined by monitoring equipment.

The presence of flammable vapour clouds can be predicted where there is a release of flammable gases or liquids. As flammable gases are invariably stored under pressure, if gas is escaping from its container, the temperature will drop. Using a thermal imaging camera can therefore assist in identifying leaking or open cylinders.

The extent of this hazard area will depend on:

  • The volume of release
  • Weather conditions, such as wind speed, direction, etc.
  • Local topography, proximity of storage tanks, buildings, pipelines, etc.
  • The explosive limits of the gas (N.B. For practical purposes explosive and flammable limits are the same)
  • Vapour density

Flammable vapour clouds can extend significant distances from the point of release. An effective cordon will need to be extended beyond the flammable vapour cloud to include the area to which radiant heat would spread if ignition occurred. Therefore, for large leaks of flammable liquids that produce significant vapour clouds, the initial cordons may be reduced or extended according to explosimeter monitoring, using a safe percentage of the lower explosive limit (LEL).

See National Operational Guidance: Hazardous materials – Contaminated members of the public

Strategic actions

Fire and rescue services should:
  • Provide procedures and support arrangements for the hazards that may be encountered and actions to take in recognising unignited flammable vapours

  • Provide suitable means to identify the presence of flammable vapours

  • Have arrangements to make flammable vapour detection equipment available at incidents

Tactical actions

Incident commanders should:
  • Establish exclusion zones, inner and outer cordons based on the level of risk from flammable vapours

  • Ensure all non-essential personnel and members of the public are removed from areas where flammable vapours could ignite

  • Use atmospheric monitoring equipment to identify concentrations and extent of hazard area

  • Use gas monitoring to identify the extent of the hazard area

  • Consider the potential for irrespirable vapours to be inhaled

  • Consider the effect of buoyancy, weather and environment on the spread of the vapour cloud

  • Consider the potential for the vapour cloud to move (e.g. due to wind, low-lying areas, buoyancy)

  • Consider the risk of a vapour cloud explosion (See explosives and cylinder scenario)