The elements of fire and explosion
Governing: STCW Table A-VI/1-2, "the elements of fire and explosion (the fire triangle)". Builds on: nothing, start here.
Every technique on the practical course follows from one idea: fire needs certain things, and taking one away puts it out.
The fire triangle
| Side | What it is | Aboard a yacht |
|---|---|---|
| Fuel | Anything that burns | Furnishings, linen, teak, cooking oil, diesel, petrol for tenders and toys, paint stores |
| Heat | Enough energy to reach ignition temperature | Galley, laundry, engine exhausts, electrical faults, hot work |
| Oxygen | About 21% of air | Always present; forced ventilation adds more |
The tetrahedron
Once burning, fuel breaks down and reacts in a chain reaction that keeps generating heat and flammable gases. The fire tetrahedron adds that chain reaction as a fourth face. Some agents, notably dry powder and some clean agents, work mainly by interrupting it.
Four ways to put a fire out
| Method | Removes | Example |
|---|---|---|
| Starving | Fuel | Shut the fuel quick-closing valve; isolate gas; move combustibles away |
| Smothering | Oxygen | Fire blanket on a pan; foam over a fuel spill; shut down ventilation; CO2 flooding |
| Cooling | Heat | Water jet or spray; boundary cooling |
| Breaking the chain reaction | Chain reaction | Dry powder; some gaseous agents |
Most real firefighting combines them: shut off the fuel, close the space, cool the boundaries, then attack.
Flammable liquids
Liquids do not burn; their vapour does.
- Flashpoint: the lowest temperature at which a liquid gives off enough vapour to form an ignitable mixture with air.
- Auto-ignition temperature: the temperature at which the mixture ignites without an external spark or flame.
- Flammable limits: the vapour must be within a range of concentration in air. Too lean or too rich will not burn, but a rich mixture becomes flammable as air leaks in.
Petrol has a very low flashpoint, which is why tender and jet-ski fuel is stowed under the special rules of REG Yacht Code Chapter 14.1. Diesel has a much higher one but, as a fine spray from a leaking high-pressure pipe onto a hot exhaust, it ignites readily.
How fire spreads
| Mechanism | How | Shipboard effect |
|---|---|---|
| Conduction | Through solids | Steel decks and bulkheads carry heat into the next space |
| Convection | Hot gases rise and travel | Smoke and heat move up stairwells, lift shafts and trunking |
| Radiation | Heat travels in straight lines | Ignites nearby materials across an open space |
| Direct contact | Flame touches fuel | Spread along furnishings and linings |
This is why firefighting includes boundary cooling and checking the spaces above, below and beside the fire.
Products of combustion
Most fire deaths are caused by smoke, not flames. Smoke is hot, blinding and toxic:
- carbon monoxide: odourless, causes confusion and unconsciousness;
- carbon dioxide and lack of oxygen;
- irritant and toxic gases from plastics, foam and synthetic fabrics, which yachts are full of;
- hydrogen fluoride and heavy-metal particles from lithium-ion batteries (L4.3).
WARNING — Smoke kills first Never enter smoke without breathing apparatus. A few breaths can incapacitate you before you see a flame.
TIP — Backdraught and flashover A closed, oxygen-starved space full of hot gases can ignite violently when a door is opened. Feel doors with the back of your hand before opening, and follow the fire party leader.