Eli5: how come when you light a barbecue the spark doesn’t ignite all the gas in the pipes?

Question

Eli5: how come when you light a barbecue the spark doesn’t ignite all the gas in the pipes?

In: Physics

Answer 1

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.