Flash Point and Flammable Range

Liquids do not burn. The vapour above them burns, which is why the properties that matter describe vapour behaviour.

Key terms

• Flash point — the lowest temperature at which a liquid gives off enough vapour to form an ignitable mixture with air. Below it, the liquid cannot be ignited by a spark; above it, it can.
• Lower explosive limit — below this vapour concentration the mixture is too lean to burn.
• Upper explosive limit — above this the mixture is too rich. The space between the two limits is the flammable range.
• Auto-ignition temperature — the temperature at which vapour ignites without any spark or flame, from a hot surface alone.
• Vapour density — most hydrocarbon vapours are heavier than air and flow downhill, collecting in pits, drains, basements and bunds, sometimes travelling a long way to an ignition source.

Why flash point matters practically

Petrol has a flash point around −40 °C, so it is always above it in normal conditions and always capable of ignition. Diesel has a flash point around 55 to 65 °C, so at ambient temperature it will not flash — but heat it, spray it as a mist, or soak it into a rag or lagging with a large surface area, and it becomes readily ignitable. Mists and soaked absorbent materials ignore flash point entirely.

"It has a high flash point" is not a control. It is one condition that may or may not hold in the situation you are actually in.