Where flammable vapours are released under pressure, such as when escaping from a ruptured pressurised container, their temperature will drop rapidly. Because of this, even vapours that are lighter than air will sink to low-lying areas initially and therefore the highest concentrations of vapour will be found low to the ground and close to the release. As the temperature falls, the rate of release will reduce and in the case of some flammable vapours, ice will form at the point of release. This property may be exploited in some instances by using a spray or damp piece of material to cover the rupture (usually only on small bore, low pressure pipelines and vessels). As the water freezes a seal will be formed. (Straps may be used to hold material in place.)
Once a vapour cloud is formed there are limited tactical options to contain it. It is therefore a priority to stop any escape at source if possible, for example, by closing a valve.
Liquefied pressurised releases from pipelines may take considerable time to depressurise following an emergency shutdown.
When a vapour cloud is formed, particularly where the release is indoors, one option would be to prevent ventilation. This is not generally recommended as it will increase the concentration of flammable vapours and therefore increase likelihood of ignition. However, if the flammable vapours pose additional health effects such as being toxic (such as ammonia and hydrogen sulphide), ventilation may reduce the flammable hazard only to produce a much wider toxic hazard. In such instances, factors such as the total quantities of vapours, location of the incident, wind speed and direction will all contribute to determine the tactic that will take precedence.
Flammable vapours will only ignite when they encounter a source of ignition at concentrations within their flammable range. The precise range is specific to each substance; consequently, flammable vapours with a wide flammable range are more likely to create large ignitable vapour clouds than those with a narrow flammable range.
At operational incidents, a vapour cloud at concentrations above its upper explosive limit (UEL) should still be regarded as a dangerous environment. A key control measure for flammable vapours is to reduce the concentration in air below the lower explosive limit (LEL) and prevent ignition. Vapours in an uncontrolled state will naturally spread and in doing so, dilute.
Ventilation – Good ventilation can assist in dispersing flammable vapours to minimize the size of any ignitable plumes. This approach will need to be weighed against the generation of a larger plume and the potential to find ignition sources. This depends on the LEL of the vapour.
Water sprays – Adding water in the form of fine spray or mist will create convection currents that will assist in dispersing flammable vapours. Water mist will also act as a good absorber of heat if ignition occurs.
Most hydrocarbon fuel vapours have little or no solubility in water. Where vapours are water soluble, such as ammonia and hydrogen sulphide, water sprays may be used to dissolve the vapour cloud out of the air. This technique is called atmospheric scrubbing.
Where water sprays are beneficial, the hazards may be increased if water run-off enters any liquid pool. If the liquid spillage is significantly colder than the run-off and has a very low boiling point, this will cause a more rapid boil-off of the substance and increase the size of the vapour cloud. Even where the liquid is at an ambient temperature, with a relatively high boiling point, adding water if the liquid is immiscible will spread the spillage over a wider area, increasing the risk and increasing the surface area for vapours to be produced. Additionally, any water run-off may present a risk of pollution to the environment.
Weather – Strong winds can disperse flammable vapours and gases, rain can dissolve soluble gases (atmospheric scrubbing) and promote the mixing/dilution of any vapour plume.
Inerting gas – By replacing the air with a gas that does not support combustion, the risk of fire can be greatly reduced or eliminated. This will not only reduce the concentration of flammable vapours but also reduce the concentration of oxygen present.
Portable monitoring equipment known as LEL meters or combustible gas detectors can be used to detect the presence of flammable vapours. These meters are not substance-specific and can detect any vapour that is flammable.
Using LEL meters will enable cordons to be refined and allow monitoring for changes in the incident profile.
Once a flammable vapour cloud has formed, the options available to reduce this hazard are limited, until vapours have dissipated to a safe concentration. As a priority, responders should isolate any sources of ignition (electrical sparks, static electricity, friction sparks or naked flame) from the hazard area and manage cordons to ensure none are introduced to the area. For further information on ignition sources refer to Fires and Firefighting.
Entering such a dangerous atmosphere is therefore inherently dangerous and should only be considered in circumstances where the benefits outweigh the risk, for example, to perform rescues and prevent catastrophic escalation.
Structural firefighting kit and breathing apparatus (BA) will provide the most appropriate level of personal protective equipment (PPE) for the flammable risk. This will need to be weighed against any possible health effects (such as corrosivity) from the vapour. Chemical protective clothing (CPC) is generally not suitable where heat, fire or flammable risks are present. Where the balance of risks means that personnel need to enter a vapour cloud in chemical protective clothing (CPC), additional controls should be considered such as ventilation and water sprays. Personal protective equipment (PPE) for emergency teams may need to be sufficient to protect wearers in the event of ignition of a vapour cloud that leads to a flash fire or uncontrolled vapour cloud explosion. For further information on emergency arrangements see A foundation for Breathing Apparatus. Where the risks present outweigh the benefits, defensive tactics should be implemented.
For further information refer to Incident command – Ineffective safety management.