Fire safety – the fire triangle and explosive limits

What it shows

Combustion needs fuel, oxygen and heat at the same time. Remove any one side of this fire triangle and the fire goes out, which is the principle behind every extinguishing method. A flammable gas mixed with air can only ignite between its lower and upper explosive limits: below the lower limit the mixture is too lean, above the upper limit it is too rich. Hydrogen has a far wider explosive range than methane or LPG and keeps burning at much lower oxygen levels, which makes hydrogen leaks especially dangerous.

How to use

Choose a Fuel gas and, with the spark on, slowly raise the Gas concentration in air from zero, noting where ignition starts and where it stops; compare the range for each gas. Then lower the Oxygen in air, or switch off Spark / heat source present, to show that all three sides are needed. Finally try the remedy buttons, such as Close gas valve, Spray CO₂ / foam and Spray cooling water, and ask which side each one removes.

Parameters you can change

• Fuel gas Methane (natural gas), LPG (bottled gas), Hydrogen
• Gas concentration in air 0–80 %
• Oxygen in air 5–21 %
• Spark / heat source present

Questions to explore

1. Why must you never flip a light switch when you smell a gas leak in a kitchen?
2. Why does a mixture containing too much fuel gas fail to explode, and why is it still dangerous?
3. Which side of the fire triangle does a CO₂ extinguisher remove, and which side does cooling water remove?