Is your organisation powered by diesel fuel? From agriculture to mining, a broad range of industries rely on diesel to fuel their operations. However, due to the potentially dangerous properties associated with diesel fuel — including its flammability — it’s crucial that businesses understand how they can implement controls and mitigate potential hazards. One property of diesel fuel that’s often in question is whether it’s a flammable or combustible liquid. To answer this question, we must explore the difference between flammable liquids and combustible liquids. We’ll also be looking at the importance of determining the fuel’s flash point and how to interpret the flash point of diesel in your own organisation.

What are Flammable and Combustible Liquids?

Put simply, flammable liquids and combustible liquids are substances that emit vapours which can burn in air. Substances are categorised as either a flammable or combustible liquid by testing their flash points.

Truck refuelling-2The flash point of your diesel fuel will determine whether it's flammable or combustible.

The Importance of Understanding the Flash Point of Diesel

Flash points are used as a general guide to the flammability or combustibility of a substance. The flash point is the minimum temperature that a liquid emits sufficient vapours to be ignited at the liquid’s surface. This flash point is what determines whether the diesel will burn or whether it will explode. 

Flash points are measured by heating a substance to a specific temperature under controlled conditions. To measure a substance's flash point, it's necessary to introduce an ignition source, as this allows the volatile substance to reach a particular temperature before it "flashes" or ignites.

There are two methods to determine flash points: the closed-cup test or open-cup test. The open-cup method measures flash points in a vessel that is exposed to outside air, whereas the closed-cup method takes place in a closed vessel that is not affected by the external atmosphere. These different flash point testing methods are designed to reflect the workplace environment and the conditions of storage for the substances.Open cupAn example of an open cup tester


Closed Cup

Example of a closed cup tester

Flash Point Of Flammable Liquids

 The Australian Dangerous Goods Code outlines the definition for flammable liquids. 

This code states: 

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:

(a) liquids offered for transport at temperatures at or above their flash point; and

(b) 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. 

In summary, a flammable liquid is defined as a liquid that has a flash point below 60 °C.

Flash Point Of Combustible Liquids

 The Australian Standard (AS1940-2017) that outlines the requirements for the storing and handling of flammable and combustible liquids provides us with a definition of combustible liquids. 

The standard states: 

A combustible liquid is any liquid, other than a flammable liquid, that has a flash point, and has a fire point that is less than its boiling point. 

There are two different classes of combustible liquids: C1 & C2: 

  • Class C1—A combustible liquid that has a closed cup flash point of greater than 60°C and no greater than 93°C.
  • Class C2—A combustible liquid that has a flashpoint greater than 93°C. 

To summarise the Australian Standard (AS1940-2017), a combustible liquid has a flash point above 60 °C, but below its boiling point.

Is Diesel Flammable?

 So, is diesel explosive? Can diesel burn? There are real risks associated with the use of this fuel, so it’s important to get the facts straight. 

Diesel fuel is defined as any kind of liquid fuel that can be utilised in diesel engines. A diesel engine uses the heat produced from the compression of air to ignite the fuel that is injected into its cylinders.

However, as there are many types of diesel fuel, there is no straightforward answer as to whether they are classed as flammable or combustible. To find out if a diesel fuel is classed as a flammable liquid or combustible liquid, you must know the fuel’s flash point. You can check the safety data sheet for your particular type of diesel fuel to find the substance’s flash point. 

Diesel fuels generally have a flashpoint between 52 °C and 93 °C. Therefore, diesel fuels with a flash point below 60 °C are classified as flammable liquids and those that have a flash point above 60°C are classified as combustible liquids. 

While combustible liquids have similar properties to flammable liquids, they are regarded as less volatile substances because they have a higher flash point. Both flammable and combustible liquids can burn and explode, so sufficient controls must be implemented when handling and storing any diesel fuel. 

IMPORTANT: By studying the safety data sheet (SDS) of your diesel fuel, you will be able to determine if it’s flammable or combustible.

Safety Data Sheet Examples

Checking the fuel’s safety data sheet is essential in determining the appropriate measures to store diesel fuel. Here, we highlight some examples of flammable and combustible liquids with information that can be sourced from the fuel’s safety data sheet. These examples are used only to illustrate how you can find the flash point of the diesel fuel and shouldn’t be interpreted as a guide to the fuel’s flammability or combustibility. You should always check the current safety data sheet of your particular type of diesel fuel to find the correct data.

Flammable Diesel Examples


HollyFrontier Diesel 

Flash Point : >37.8°C 



Trafigura Diesel 

Flash Point: 55 - 73°C 


Combustible Diesel Examples


BP Automotive Diesel Fuel 

Flash Point - Closed-cup: > 61.5°C 

Boiling Point: 180°C - 380°C 


Shell V

Shell V Power Diesel 

Flash Point: Typical 63°C 

Boiling Point: 170 - 390°C 

Different Types Of Diesel Fuels

There are many types of diesel fuels, derived from various sources. The different types of diesel fuel include: 

  • Petrodiesel - produced from crude oil
  • Synthetic diesel - produced from carbonaceous materials such as natural gas, biogas or coal
  • Biodiesel - produced from vegetable oils or animal fats
  • Hydrogenated oils and fats -produced by transforming the triglycerides in vegetable oils and animal fats into alkanes through refining and hydrogenation 
  • DME (dimethyl ether) -synthetically produced gaseous diesel fuel that provides clean combustion  

 Petrodiesel is the most widely used type of diesel. This fuel is common in the automotive industry, powering a wide range of diesel engines across cars, commercial vehicles, trucks, buses, trains and marine craft.

Incidences of Diesel Disasters 

When diesel is not handled and stored correctly, there can be devastating consequences. Diesel fires are notoriously difficult to put out and there is the imminent danger of a diesel explosion. 

Outbuilding Diesel Fire, Essex, 2021 

An outbuilding which contained 1,500 litres of diesel and a large amount of bitumen became the scene of a diesel fire in Little Canfield, England. 

When firefighters arrived at the site, they found the building destroyed. Two lorries, which were parked nearby, succumbed to the flames and vehicles had to be moved to prevent the fire spreading.  

ITV News reported that fire crews worked through the night to secure the area. 

Diesel Tanker Fire, Sydney, 2018 

With 32,000 litres of diesel onboard, a fuel tanker caught on fire while en route in Cataract, Sydney. 

Channel 9 reported that the front carriage of the rig was destroyed by the blaze, but fire crews worked quickly to prevent an explosion and safely remove the diesel from the tanker. 

Industrial Accident, Eromanga, Queensland 

A man in his 60s was flown to a Brisbane hospital after suffering significant burns after a diesel fire in south-west Queensland.  

ABC News reported that the fire was thought to have broken out at an exploration drilling rig in Eromanga.  

Safe Storage and Handling Of Diesel Fuels 

Diesel fuels must be stored in a safe and compliant manner, regardless of whether they are classed as flammable or combustible. 

To protect workplaces from the serious hazards associated with diesel fuels, it’s crucial that flammable liquids are safely stored in full conformance to the Australian Standards AS1940-2017 - the storage and handling of flammable and combustible liquids. 

There are specific requirements that relate to the storage of diesel and other Class 3 liquids, with specific requirements for minor storage, package storage, storage in tanks and bulk storage. 

The storage requirements within the Standard also relate to where the substances are stored. While fuel must be stored in facilities that are compliant with AS 1940, we recommend storing diesel drums or containers in: 

Outdoor locationsbunded Class 3 liquids stores

Indoor locationsClass 3 indoor safety cabinets

Both the indoor safety cabinet and outdoor bunded combustible liquids store have compliant features to minimise the risks that diesel fuels pose upon people in your workplace. These features include spill containment sumps, ventilation provisions and safety signage.


Class 3 cabinets are suitable for the indoor storage of diesel fuel.

Does Diesel Burn?

As some diesel fuels are classified as flammable liquid and others as combustible liquids, it’s vital that you check your fuel’s safety data sheet to determine its flash point. This will then help determine the handling and storage practices for your fuel. 

However, keep in mind that all diesel fuel must be stored to conform with the Australian Standards AS 1940:2017. To effectively reduce the risk associated with flammable or combustible diesel fuel, organisations must use, transport and store diesel fuels in the appropriate manner. If you’re interested in learning how to better manage the risk associated with flammable liquids, such as diesel fuel, why not access our free eBook? Our free guide will lead you through our 4-step risk management methodology, so you can reduce the risk of ignition and explosion. 

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