Theory of Fire and Backdraft fire

Fire Triangle

Fuel, heat, and oxygen are the three components of fire.

All three components are require to start a fire and absence of any one component will stop the spread of fire

Tetrahedron fire Triangle

The base of a fire tetrahedron represents the fourth component in a fire triangle. It is the ‘chain reaction’ due to the molecular combination of fuel, oxygen and heat energy. 

When you break the chain reaction, fire does not spread.

The four phases of fire

1. Ignition
2. Developing
3. Absolute Fire
4. Burning Out

• Ignition phase

When there is sufficient air-vapour mixture and the temperature of the substance exceeds its ignition point, the fire starts.

This fire can be due to an external heat source, auto ignition or spontaneous ignition. Spontaneous ignition is the one in which certain materials become warm during oxidation producing more heat and in turn accelerate the oxidation process.

In the ignition or ‘incipient’ phase, there is a heat flow back to the material due to radiation, which results in a chain reaction and the components will have the ability to sustain fire.

• Developing phase

During developing phase, the flame spreads on the surface of the material. After sufficient heating, it gives off vapours, which act as a source of heat. Solids and liquids convert to vapour state before combustion. In case of flammable gases, there is a high risk of explosion, because the combustion process takes place at an extremely fast rate.

The burning speed depends on the rate at which the heat source gives off the vapours and its combustibility. Petrol has a very high surface burning speed, while wood has a low surface burning speed.

• Absolute Fire phase

In this phase, the heat spread on the surface of the material penetrates into the depth of the solid or liquid. Intense flammable vapour will be generated at this stage.

• Burning Out phase

The fire continues to burn, until it consumes most of the vapour. In the burning out phase,most of the fuel gradually burns out and the fire ‘dies’ thereafter.

Backdraft Fire on Ships

A backdraft is a dangerous explosive event that can occur during a fire when a compartment or enclosed space has a fire smouldering in a low-oxygen environment, and oxygen is suddenly introduced, often when a door or hatch is opened.

How a Backdraft Happens

• Fire starts in a closed compartment (e.g., engine room, store, cabin).
• The fire consumes most of the available oxygen, slowing down but continuing to produce heat and flammable gases like CO and unburned hydrocarbons.
• The space becomes filled with superheated, oxygen-depleted air and dense smoke.
• When the compartment is suddenly ventilated, such as by opening a hatch or door, fresh air rushes in.
• The reintroduction of oxygen causes the flammable gases to ignite explosively, often violently.