Firefighting

 
Fire at sea is frightening and dangerous. You cannot escape from the fire, and you usually cannot call in outside help. Usually, flammable and explosive liquids will be present.

For these reasons, all members of the crew need to know how to PREVENT fires, and how to take IMMEDIATE action if a fire does occur.

The Elements

It is important to understand what starts, and what sustains a fire. Once fire starts, it will continue to burn as long as it has something to consume. It is dependent on four elements:
  1. Flammable Material such as wood, oil, paper and wood.
  2. heat
  3. Oxygen which together with heat ignites the fuel.
  4. Chain Reaction


When a solid or liquid is heated, its molecules move about rapidly. If there is enough heat applied then the molecules break away from the surface to form a vapour just above the surface. This vapour can now mix with oxygen. If there is enough heat to raise the vapour to its ignition temperature, and if there is enough oxygen present then the vapour will burn. This is clearly seen in a gas flame or even in a match burning. It is not the solid but the vapour itself that is burning.

The triangle with its three sides of heat, oxygen and fuel is the usual way of simplifying fire theory, that is, remove one side and the fire extinguishes.

The triangle does not explain completely the nature of the fire. In particular, it does not include the chain reaction that results from chemical reactions among fuel, oxygen and heat. The burning vapour releases heat and ignites more vapour. The additional vapour burns and produces more heat and in so doing produces more vapour, and so the process continues with increasing flame.

The fire tetrahedron is a better representation of the combustion process. (A tetrahedron is a solid figure with four triangular faces.) It is useful for illustrating and remembering the combustion process because it has room for the chain reaction as each face touches the other three faces.) The tetrahedron illustrates how flaming combustion is supported and sustained through the chain reaction. In a sense, the chain reaction face keeps the other three faces from falling apart. This is an important point, because of the extinguishing agents used in many modern portable fire extinguishers, automatic extinguishing systems and explosion suppression systems.

Take for example fuel oil dripping on to a very hot engine manifold in a well-ventilated engine room. The heat vaporises the fuel oil and combines with the oxygen. A fire will occur if it reaches ignition temperature, that is the lowest temperature at which a substance will ignite. All the elements are there for a fire to begin, by taking away just one element ignition is impossible. Take away the fuel vapour by stopping the leak, eliminate the heat by stopping the engine, close up the engine space and prevent the entry of oxygen or displace the oxygen with an inert gas.

In other words a fire can be extinguished by destroying the fire tetrahedron. By removing one of the elements the chain reaction is broken, resulting in vapour and heat production. (However, additional cooling with water might be necessary where smouldering or re-flash is a possibility.)