What are flammable substances?
Apr 20, 2018

What are flammable substances?

Walter Ingles Walter Ingles

Flammable substances are substances that will ignite and continue to burn when they are brought into contact with an ignition source. Flammable substances can exist in a solid, liquid or gaseous state. Most flammable liquids are volatile, and they give off vapours that mix with air to form a flammable mixture that will ignite in the presence of an ignition source.

To determine the flammability of a substance, there are a number of factors that must be considered. These factors include:

  • Flash point
  • Auto-ignition temperature
  • Flammability limits

These factors will be discussed below:

Flash point

Flammable liquids are volatile, which means that they easily evaporate and give of flammable vapours at lower temperatures. This is because the forces of attraction between flammable liquid molecules are relatively weak and very little heat energy is required to break these forces and cause the molecules to escape as gases.

The flash point of a liquid is the temperature at which a flammable liquid will give off enough flammable vapours to ignite in the presence of an ignition source. This point will depend on the volatility of the substance. Petrol, which is very volatile, has a flash point of -43 °C, while the kerosene, which is less volatile, has a flash point between 37 - 65 °C.

Auto-ignition temperature  

Any combustible substance will burn if it is subjected to enough heat. The lowest temperature at which a flammable or combustible liquid will ignite and continue to combust without the presence of a spark or flame, is defined as the auto-ignition temperature.

The difference between flash point and auto-ignition temperature is that the flash point is the point in which a flammable liquid will ignite with an unlimited temperature. On the other hand, auto-ignition temperature is defined as the lowest temperature at which a substance will ignite and continue to combust. To determine the flash point of a flammable liquid they are often exposed to a spark. A spark can have a temperature anywhere between 1000 - 1600 °C.

Generally, the auto-ignition temperature of a hydrocarbon will decrease with an increase in molecular mass. This is different to the flash point of a substance which decreases with decrease in molecular mass.

Flammability limits

Flammable liquids will only ignite and burn when the concentration of the flammable vapours in air are at certain levels. These levels are referred to as the lower flammability limit (LFL) and the upper flammability limit (UFL).

The lower flammability limit is the lowest concentration of flammable vapours that are required for the mixture to ignite in the presence of an ignition source. Concentrations of flammable vapours below the LFL will not be strong enough and therefore they will not ignite.

The upper flammability limit is the highest concentration of flammable vapours that will ignite in the presence of an ignition source. If the concentration of flammable vapours exceed the UFL, their will not be enough oxygen present for the mixture to ignite and continue to combust.

Flammable gases, liquids and solids

The Australian Dangerous Goods Code classifies flammable gases, liquids and solids in to 3 different dangerous goods classes. These dangerous goods classes include:

  • Class 2.1 - Flammable Gases
  • Class 3 - Flammable Liquids 
  • Class 4 Flammable Solids

The Australian Dangerous Goods Code provides different definitions for each of these classes and their subdivisions. These different classes will be described in more detail below.

Class 2.1 - Flammable gases

The Australian Dangerous Goods Code defines flammable gases as:

Gases which at 20 °C and a standard pressure of 101.3 kPa are ignitable when in a mixture of 13 per cent or less by volume with air; or have a flammable range with air of at least 12 percentage points regardless of the lower flammable limit. Flammability should be determined by tests or by calculation in accordance with methods adopted by ISO (see ISO 10156: 1996). Where insufficient data are available to use these methods, tests by a comparable method recognized by the competent authority may be used;

Some examples of flammable gases include:

  • Acetylene
  • Liquefied Petroleum Gas
  • Hydrogen
  • Methane
  • Propane

Class 3 - Flammable Liquids

The Australian Dangerous Goods Code defines flammable liquids as:

Flammable liquids are liquids, or mixtures of liquids, or liquids containing solids in solution or suspension (for example, paints, varnishes, lacquers, etc., but not including substances otherwise classified on account of their dangerous characteristics) which give off a flammable vapour at temperatures of not more than 60 °C, closed-cup test, or not more than 65.6 °C, open-cup test, normally referred to as the flash point. This class also includes:

  •  liquids offered for transport at temperatures at or above their flash point; and
  • substances that are transported or offered for transport at elevated temperatures in a liquid state and which give off a flammable vapour at a temperature at or below the maximum transport temperature.

Some examples of flammable liquids include:

Class 4 - Flammable Solids

The Australian Dangerous Goods Code divides flammable solids into 3 subdivisions. These subdivisions are outlined below.

Class 4.1 - Flammable solids

Solids which, under conditions encountered in transport, are readily combustible or may cause or contribute to fire through friction; self-reactive substances which are liable to undergo a strongly exothermic reaction; solid desensitized explosives which may explode if not diluted sufficiently.



Class 4.2 - Spontaneous Combustibles

Substances which are liable to spontaneous heating under normal conditions encountered in transport, or to heating up in contact with air, and being then liable to catch fire.

Class 4.3 - Dangerous When Wet

Substances which, by interaction with water, are liable to become spontaneously flammable or to give off flammable gases in dangerous quantities.

Some examples of flammable solids include:

  • Metal powders
  • Alkali metals
  • Activated carbon
  • Aluminium phosphide
  • Sodium batteries

Next Steps

Flammable substances come in many different forms. Whether it be solids, liquids or gases, all flammable substances pose risks upon the people, property and the environment. To reduce the risks that flammable substances have upon your organisation, it is very important that you are familiar with their dangerous properties so that you can implement controls to reduce the risks that they have upon your organisation. If you would like more information on how to reduce the risk of flammable liquids, download our FREE eBook by clicking on the image below.

How to reduce the risk of flammable liquids in the workplace

Walter is STOREMASTA’s Dangerous Goods Adviser. He loves helping business reduce the risk that Dangerous Goods pose upon their employees, property and the environment through safe and compliant dangerous goods storage solutions.

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