Shipboard Fire Theory: the Fire Triangle/Tetrahedron, Ignition Sources and Fire Chemistry
This domain tests whether you can apply fire chemistry rather than recite it. Expect a fuel, an environment and an extinguisher, and a question about what happens next.
The tetrahedron
Fuel, heat, oxygen and the chemical chain reaction make up the fire tetrahedron. The older triangle stops at three sides and cannot explain why flaming combustion continues after the ignition source is removed: flames burn in the vapour phase through a free-radical chain reaction. Liquids and solids do not burn; their vapours do.
- Starvation – remove the fuel: shut the tank valve, stop the transfer pump, drain the line.
- Cooling – remove heat: water has the highest latent heat of vaporisation of the common media.
- Smothering – cut oxygen: foam blanket, CO2, closing hatches. Flaming normally stops below about 15% oxygen, but smouldering survives much lower.
- Chemical inhibition – dry powder breaks the radical chain.
Numbers that earn marks
Flash point is the lowest temperature at which a liquid gives off enough vapour to flash when an ignition source is applied - burning is not sustained. Fire point is a few degrees higher. Auto-ignition temperature needs no ignition source. The flammable range runs from the lower to the upper flammable limit; petrol is roughly 1.4% to 7.6% by volume. Hydrocarbon vapours are heavier than air (petrol vapour about three to four times), so they sink into bilges and floor plates.
Worked example
A 200-litre drum of petrol (flash point about -43 °C) ruptures in a store at 35 °C. Is the space dangerous? Yes: the store is far above flash point, so vapour is produced continuously and the air above the spill is inside the flammable range. Cooling the drum does not stop the vapour. Blanket the spill with foam or dry powder, or shut the space and remove ignition sources; a water jet would spread the fuel.
Traps
- Quoting flash point as the temperature at which a liquid ignites.
- Saying water works because it removes the fuel.
- Ignoring the chain reaction when asked why dry powder works.
- Treating a too-rich atmosphere as safe - it becomes explosive as it ventilates.