FLASH POINT BASIC INFORMATION AND TUTORIALS

What is flash point?

The flash point represents the minimum temperature at which an ignitable mixture exists above a liquid surface. By definition, flash points are inapplicable to gases. Some solids, e.g. naphthalene and camphor, are easily volatilized on heating so that flammable mixtures develop above the solid surface and hence flash points can be determined. (However, although these substances can be ignited, they generally need to be heated above their flash points in order for combustion to be sustained: this is the ‘fire point’.)

Flash point determinations may be made in ‘closed’ or ‘open’ containers, giving different values; these are non-equilibrium methods. Alternatively equilibrium methods are available.

In general, the lower the flash point the greater the potential for fire: materials with flash points at or below ambient temperature are highly flammable and can inflame at ambient temperature on contact with ignition sources.

Flash point is used to classify liquids under many legislative systems: in the UK liquids with flash points <32°C (and which, when heated under specific test conditions and exposed to an external source of flame applied in a standard manner, supports combustion) are defined as ‘highly flammable’ under the Highly Flammable Liquid and Liquefied Petroleum Gas Regulations.

Chemicals may ignite below their flash points if the substance:

• Is in the form of a mist (or froth).
• Covers a large surface area (e.g. when absorbed on porous media).
• Contains a small amount of a more volatile flammable liquid, e.g. due to deliberate or accidental contamination.

In addition
• Flash points are reduced by increases in ambient pressure. Thus the flash point of toluene at sea level (101.3 kPa) is 4.5°C whereas at 83.3 kPa, e.g. in the mountains at 1685 m, the value is 1°C.

• Materials with high flash points such as heavy oils and resins can produce flammable vapours due to thermal degradation on heating. Dangers therefore arise when welding, flame cutting empty drums/vessels once used to contain such materials due to the presence of residues.

Substances may be heated to their flash points by other substances with lower flash points burning in close proximity. Storage of flammable chemicals, therefore, needs careful consideration.