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Flammability Range

What Does Flammability Range Mean?

The flammability range describes the minimum and maximum concentrations at which a given vaporous substance will ignite or combust when mixed with air. The minimum concentration is referred to as either the lower flammable limit or lower explosive limit (LFL/LEL), while the maximum concentration is referred to as an upper limit (UFL/UEL).

When the concentration of a vapor in the air is below the LFL, the amount of vapor in the air is too “lean” to burn, while concentrations of vapor above the UFL are too "rich” to burn. The UFL and LFL are dependent on the temperature, pressure, and amount of oxygen in the air. Increases in any of these factors lower the LFL and raise the UFL, while the opposite is true if any of these factors decrease. The flammability range of a substance is typically defined in terms of environments at room temperature (20oC–25oC) and atmospheric pressure.

Safeopedia Explains Flammability Range

The flammability range provides a tool for mitigating the dangers posed by flammable vapors and the various substances that emit them, including many chemicals (such as gasoline) commonly used in occupational settings. It is usually illegal to conduct work that takes place in environments in which a flammable vapor exists in concentrations that fall within the flammability range.

Existing standards for the prevention of fire and explosion are primarily focused on preventing concentrations of flammable vapors from breaching the lower explosive limit of the flammability range. In the United States, occupational standards that reference the LEL typically require employers to prevent concentrations of a substance from exceeding 10% of the LEL concentration. For example, gasoline has an LEL of 1.2%, which means the OSHA maximum-allowed concentration of gasoline vapors in the air is 0.12%.

Per OSHA, workplace environments that contain flammable vapors must be equipped with ventilation capable of reducing the concentration of the substance to less than 10% of the LEL. Ten percent of the LEL is also the concentration at which a given substance is recognized as being “Immediately Dangerous to Life or Health” (IDLH) due to its flammability. If worker exposure to a substance causes IDLH effects at a concentration lower than 10% of the LEL, the substance's actual IDLH value will be set to reflect this lower concentration.

The upper explosive limit of the flammability range is sometimes used to ensure safe storage of vapor-emitting substances. By storing a substance so that its vapor concentration remains above the UEL, it is possible to ensure that the space within the container is inflammable as long as the container itself remains intact.


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