It is the vapour of a flammable liquid that ignites and causes fire or explosions and not the liquid itself. The flammability of a liquid depends on its physical properties, which are:
Flammable materials or substances liable to self ignite when exposed to water or air (pyrophoric), or which emit flammable gas.
The Flash Point is the lowest temperature at which enough vapour escapes to form an ignitable mixture with the ambient air (EN 1127-1). ►Video
The Auto-Ignition Temperature is defined as the lowest temperature at which vapours, from a flammable liquid in contact with air or a hot object, self-ignites without external assistance. Many widely used chemicals (e.g. Petrol) have an auto-ignition point of 220 °C (428 °F) or greater.
Vapour Pressure is measured by how fast a liquid evaporates into the atmosphere.
The higher the vapour pressure, the faster the liquid will evaporate. It is very
important to understand that vapour pressure changes with the temperature of the
liquid. So the same chemical can have different rates of evaporation during day or
night, winter or summer.
Vapour Density is the measure of how heavy a vapour is in comparison to air.
Vapours with a density greater than 1.0 are heavier than air and will collect at the
lowest possible point. Consider that a vapour with a density above 1.0 will flow like
water and may travel until it reaches a possible source of ignition.
The Limits of Flammability (also referred to as the Explosion Limits) are defined as the range in which a mixture of air / fuel vapour is actually flammable.
Explosion limits
Explosion limits are the minimum or maximum concentration levels of vapour of a flammable or combustible material (expressed as percent by volume in air) at which an explosion will occur in a confined area if an ignition source is present. Mixtures can be too lean, with not enough vapour present, or too rich with too much fuel, to a point where it will not ignite or burn. ►Video
The basis for the handling of hazardous materials is knowledge of the fire triangle. This illustrates in a clear way that a fire or an explosion is only possible if three conditions are fulfilled:
To be 100% safe, at least two elements of the fire triangle should be removed. However, in some circumstances such as the laboratory environment, this may not be possible and therefore to be 98% safe, at least one element of the fire triangle
must be removed.
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