What Does Explosive (Flammable) Limits Mean?
The minimum concentration of a certain combustible gas or vapor necessary for it to combust in air is defined as the Lower Explosive Limit (LEL). Whereas, the maximum concentration of a gas or vapor that will allow it to burn in air is the Upper Explosive Limit (UEL).
Every combustible gas or vapor has a specific range of fuel to oxygen mixture during which it will ignite. The flammable range for every gas or vapor is the range between the LEL and UEL.
Safeopedia Explains Explosive (Flammable) Limits
The possibility of explosions is a main risk associated with combustible gases and vapors. The LEL and UEL of every combustible gas or vapor vary with temperature and pressure. These limits are normally expressed as a percentage of the total volume of the gas or vapor at 25°C and at atmospheric pressure.
If the concentration of the gas or vapor is below the LEL, it will not burn as it’s too lean whereas if the concentration is above the UEL, the mixture will also not burn as the concentration is considered too rich.
For example, hydrogen has an LEL of 4%. This means if the atmosphere has less than 4.0% hydrogen, an explosion cannot occur even if a source of ignition is present.