Eli5: how come when you light a barbecue the spark doesn’t ignite all the gas in the pipes?
In: Physics
Because of LEL and UEL. Lower Explosive Limit and Upper Explosive Limit. The maximum or minimum amounts that will support combustion. If a gas is too thin, there’s not enough to support combustion, and if there is too much gas, there’s not enough oxygen to support combustion. Propane has a lower limit of 2.1% and an upper limit of 9.5%. The propane in the pressurized lines is approximately 94-100% propane, so it’s way too rich to burn. Flammable materials of any kind are only flammable if they have just the right amount of material to oxygen ratios.
Imagine trying to dye the water in a squirting hose. All the dye you put in would just be the water on the end, the dye would never flow up the moving water. Fire is the same way as that.
Pressure and oxygen. The cylinder is pressurised and pushing the gas out, and the oxygen is needed in the right ratio with the gas for good ignition.
Because you need oxygen for fire and there isn’t any oxygen in the pipes, it’s just pure gas. Even if there was a little oxygen, the ratio needs to be just right or nothing would happen.
For gas to burn it needs to be mixed with the right amount of oxygen (which just comes from the air).
Inside the cylinder and pipes there is no air – only the gas being released – so it cannot burn. When it reaches the nozzle at the end it will start to mix, and can be ignited.
You need three things for a fire to burn: fuel, air, and spark. The only place that all three of these things are present are outside the burners.
That gas isn’t just sitting there, there’s always a flow which is critical. The point where you see a flame is where Oxygen has entered into the equation.
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