Easiest flammability diagram in the world.! EXPLAINED!

Описание: Flammability Diagram

The diagram given above (Flammability Composition Diagram) can be considered the most important diagram to understand the concept of flammability.

Hydrocarbon gas normally encountered in petroleum tankers cannot burn in an atmosphere containing less than approximately 11% oxygen by volume. Accordingly, one way to provide protection against fire or explosion in the vapour space of cargo tanks is to keep the oxygen level below that figure.

The flammable limits vary for different pure hydrocarbon gases and for mixtures derived from different liquids. For practical purposes, the lower and upper flammable limits of crude oil vapours are taken to be 1% and 10% respectively by volume. These values are indicated by points C and D on the line AB in the figure.

Any point in the diagram represents mixtures of hydrocarbon gas, air and inert gas, specified in terms of hydrocarbon gas and oxygen contents. As inert gas is added to the hydrocarbon gas/air mixture, the flammable range decreases until a point, represented by E, is reached where LFL and UFL coincide. This point corresponds to an oxygen content of approximately 11%. For practical purposes and to allow a safety margin, 8% is taken as the level of oxygen at which no hydrocarbon gas/air mixture can burn under any circumstances.

To prevent fire or explosion in the tank containing hydrocarbon gas/air mixture, it is, therefore, necessary to produce and supply inert gas having an oxygen content not normally exceeding 5% and to displace the existing air in the tank until the resultant oxygen level throughout the tank does not exceed by 8% by volume.

Flammability Diagram Detailed Explanation Diagram
7 Commentson Flammability Diagram Detailed Explanation DiagramPosted in LifeAtSea, Marine, SafetyBy Amit AbhishekPosted on March 19, 2018
Last Updated on June 30, 2020 by Amit Abhishek


Flammability diagram or flammability composition diagram is one of the important documents used across the world to understand and prevent fire and explosion. They are mainly used for the hydrocarbon mixture and so a key to the cargo and ship’s safety on tankers.

It is essential on-board to follow certain procedures based on the studies from the flammability graph of the hydrocarbon mixture.

Now according to the diagram, When we go left to the line AB ( Add inert gas to the tank’s ) there is a sudden change in tank atmosphere.

Similarly, the upper and lower flammability limits changes with more and more mixing of inert gas. Finally, the upper flammability limit ( The maximum amount of hydrocarbon gas to air mixture which can sustain fire) and lower flammability limit (The minimum amount of hydrocarbon to air gas mixture which can sustain fire ) meet at a point, represented on the diagram as Point E.




Flammability diagram explains to us how a gas or mixture of two gases will behave and catch fire with dilution other gas or air.

They are also sometimes called a ternary diagram as it represents flammability range in the mixture of inert gas, oxygen (air), fuel, and nitrogen represented on a triangular diagram.

Fire ( Basics )
Fire safety is the key to the successful operation of industries including shipping. A detailed discussion and planning are required on tankers ( A type of ship ) to avoid any mishap. Proper tank cleaning, loading, and discharging produce to be followed to avoid fire and explosion.

It needs only air, heat ( source ) and fuel to start a fire. These three components are always available onboard ship, and risks are high with tankers.


On tankers fuel is always present in large quantities in different holds, the air is present everywhere and the heat source can be from hot work to a spark due to static discharge. removing any side of this fire triangle ( Air, Source, Fuel ) we can prevent fire on-board.

Inert gas is used on-board to remove the one side of the fire triangle i.e air; by pumping in inert gas in tanker holds to keep oxygen level at a minimum. Flammability diagrams are used to understand the tank’s atmosphere and help predicts; how much oxygen to be maintained at all times.

Flammability Diagram
Flammability Diagram
The flammability diagram is the key element for planning safe cargo operation onboard ship. the diagram itself varies for different hydrocarbon gases; so we must understand how to read and understand it.

For example, we get the lower and upper flammability limit of 1% to 10% based on the Flammability diagram for crude oil.


Here, line AB represents the mixture of oxygen ( Air ) and particular hydrocarbon gas. Anything that is left to the line AB represents the condition where inert gas is also mixed with the hydrocarbon-air gas mixture.

The gas mixture represented by AC on line AB is too lean to catch fire while DB is too rich mixture to get fire.

Now according to the diagram, When we go left to the line AB