What is the difference between flammable and combustible liquids?
Jun 6, 2018

What is the difference between flammable and combustible liquids?

Rachel Chesterfield Rachel Chesterfield

Put simply, flammable and combustible liquids are liquids that can burn.  The real difference lies within their flash point range, which is the lowest temperature at which vapours of the material will ignite when supplied with an ignition source.  Flammable and combustible liquids are utilised in most workplaces, from manufacturing to retail and service entities. To ensure the safety of people, property, and the environment, flammable and combustible liquids need to be handled and stored correctly at all times.

What are Flammable Liquids?

Besides just being ‘liquids that burn,’ The Australian Standard AS1940 defines flammable liquids as being:

“Liquids, 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.”

Flammable liquids will have flash points around or below normal working temperatures, making them susceptible to ignition.  The lower the flashpoint, the more volatile a liquid is.

A Flash point is not the same as autoignition.  Flash point is the temperature at which a substance releases a vapour capable of ignition.  On the other hand, autoignition is the point at which the substance will spontaneously ignite without an ignition source.

For a flammable vapour to catch fire, a source of ignition must be present.  Static electricity, open flames, hot surfaces and sparks are all possible ignition sources.  A common example of a flammable liquid that we all can relate to is petrol. The flash point of petrol ranges between approximately -40°C to -45°C.  This is well below standard working temperature.

It is not the flammable liquids that burn, but rather the mixture of the flammable vapour and air that burns. Therefore, care must be taken to keep any possible ignition sources away from flammable fuel vapours.


Common Examples of Flammable Liquids

  • Ethyl chloride
  • Pentane
  • Propylene oxide
  • Acetone
  • Benzene
  • Ethyl acetate
  • Gasoline
  • Hexane
  • Methyl alcohol
  • Turpentine

For more information on flammable liquids, read our article; Examples of Flammable Liquids and their Flash Points

What are Combustible Liquids?

According to Australian standards, Combustible liquids are defined as:

“Any liquid, other than a flammable liquid, that has a flash point, and has a fire point that is less than its boiling point.”

This definition highlights just how similar combustible liquids are to flammable liquids.  The simplest explanation is that while flammable liquids will ignite and burn easily at normal working temperature, combustible liquids are less volatile and usually require a higher range of temperature before releasing a vapour capable of ignition.

Common examples of combustible liquids

  • Cyclohexane
  • Hydrazine
  • Mineral spirits
  • Kerosene
  • Benzaldehyde
  • Carbolic acid
  • Dichlorobenzene
  • Formic acid
  • Naphthalene
  • Oils

The Risks of Flammable and Combustible Liquids

If using, transporting or storing flammable or combustible liquids above their flash point temperature, extreme caution must be exercised to prevent ignition.

The vapour of flammable material, particularly in poorly ventilated areas, is prone to ignition at standard working temperatures.  Similarly, combustible liquids release a flammable vapour when in environments above their flashpoint temperature. Vapours of both flammable and combustible liquids are virtually invisible. Don’t be fooled into thinking that combustible liquids are less dangerous than flammable liquids.

Being a gas, flammable vapours can escape from any open liquid source.  Flammable liquid can be easily absorbed into other materials, releasing flammable vapours into the air even after the flammable liquids have been cleaned up. Flammable and combustible vapours are often more dense than air. They can quickly spread long distances and build up in low areas. If ignited, a flashback can occur, where the fire travels along the vapour fumes to the liquid source.

If an ignition source is present spray mists of both flammable and combustible material will ignite.  Both combustible and flammable liquid fires tend to burn extremely fast and hot, releasing dense clouds of toxic smoke.

Flammable and combustible liquids present significant fire dangers and in turn present health hazards such as burns and intoxication. If the vapours or the liquid of either flammable or combustible substances combine with other incompatible substances, they will react dangerously.

How to store Flammable and combustible liquids

As flammable liquids pose significant risks upon the people, property and environment of your organisation, its very important that you store and manage them in a safe and compliant manner. The Australian Standard that outlines the requirements for the storage and handling of flammable and combustible liquids is AS1940-2017.

A couple of key points that must be considered when storing flammable and combustible liquids include;

  • Adequate ventilation,
  • Complete separation from ignition sources
  • Correct segregation from incompatible chemicals
  • Spill containment

If flammable liquids spill, there is the risk of cross contamination with other incompatible classes of dangerous goods. When flammable liquids mix with other incompatible substances, it can result in dangerous chemical reactions including oxidation and instantaneous combustion. This makes segregation of incompatible classes of dangerous goods mandatory. The require separation distances for incompatible chemicals can be determined with a dangerous goods segregation chart. 

Download a FREE Dangerous Goods Segregation Chart

Next steps

As flammable and combustible liquids pose a number of risks upon the people and property of your organisation, it's very important that you take proactive measures to reduce the risks that these dangerous goods have upon your organisation. One important factor that must be considered when storing flammable and combustible liquids is ensuring that they are segregated from other incompatible classes of dangerous goods. If you could like a free dangerous goods segregation chart, go ahead and download our free chart by clicking on the image below. 

Dangerous Goods Segregation Chart

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