Theory of Fire – Fire Fighting on Ships
Fire Tetrahedron
To start a fire, requires fuel, oxygen and a source of ignition (heat). These requirements are often represented by the sides of the familiar fire triangle. This is the theory of fire.
Conversely, removal of any side of the triangle or breaking the chain reaction between the transient chemical species formed following ignition will extinguish a fire. It is there fore more appropriate to use a fire tetrahedron to represent the ways a fire can be extinguished.
In the tetrahedron shown, the chain reaction is represented by its base. The effect of removing each of the four faces of this fire tetrahedron is discussed below.
Removal of Oxygen
Except in those substances that contain their own oxygen, the removal of sufficient oxygen will extinguish a fire.
Small fires can be smothered with sand from a fire bucket, and a rug or blanket can be used to smother flames from a person’s clothes. It is imperative to ensure the door is properly closed when leaving a fire to burn in a compartment or room.
Fires in cargo holds can be starved of oxygen by closing hatches and blanking off ventilators. In all spaces affected by fire, ventilation fans should be shut down and doors and other openings closed.
In fire extinguishing operations, oxygen is extinguished by smothering the fire with a layer of foam. Oxygen is also cut off during the operation of portable and semi portable carbon dioxide extinguishers and to some extent, during he operation of dry powder extinguishers. But for CO2 and dry powder, the smothering action is temporary and there is a possibility for re ignition.
In total flooding by fixed fire extinguishing systems on board ships, carbon dioxide displaces the air inside the compartment and fire is extinguished due to insufficient oxygen.
Removal of Heat
A reduction in temperature is achieved by the use of a suitable cooling medium, normally water, at a sufficient rate. The rate at which heat is removed by the cooling medium must be greater than that produced by the fire. Cooling of boundary bulkheads will reduce the possibility of igniting material outside the affected compartment.
For a given quantity of water, about six times more heat will be removed if the water droplet size is small enough for it to be vaporised into steam. Coincidently, a degree of smothering can also be achieved from the steam generated. Heat can also be absorbed by decomposition of dry powder. The source of power should be cut off in electrical insulation and galley fires.
Removal of Fuel
The removal of fuel is not always possible. However, in the case of liquid fuel fires caused by leaking pipes or fittings, the fuel supply should be closed. It may also be possible to drain the fuel from a burning tank.
It is particularly important to shut off the supply in a gas fire. However, gas could also be left burning in a controlled manner to exhaust itself. In accommodation spaces, combustible materials should be removed from the vicinity of fire, including any adjacant compartment affected by the heat. On some occasions, it may be prudent to dump burning or potentially dangerous materials overboard.
Breaking the Chain Reaction
A fire may be extinguished by breaking the chain reaction between transient chemical species produced on ignition (these species are described as ‘transient’ since they are not present prior to ignition or in the final products of combustion). For instance, Halogenated hydrocarbons (Halons) and dry powders attack the structure of the species and prevent their reaction by killing the flame, sometimes in less than one hundredth of a second.
The extinction takes place without any appreciable removal of heat, fuel or oxygen. However, the remaining three sides of the tetrahedron will still be present and, unless the heat is removed, there is a danger of re-ignition of the concentration of extinguishing agent is not maintained.