Flammability limit

Flammability limit refers to the range of concentrations of a gas or vapor in air that will combust or explode if an ignition source is introduced. This concept is critical in understanding the safety measures necessary to prevent fires and explosions in environments where flammable gases or vapors are present. The flammability limit is divided into two main categories: the Lower Flammability Limit (LFL) and the Upper Flammability Limit (UFL).

Lower Flammability Limit (LFL)

The Lower Flammability Limit (LFL), also known as the lower explosive limit (LEL), is the lowest concentration of a gas or vapor in air that can propagate a flame when exposed to an ignition source. Below this concentration, the mixture is too "lean" to burn. The LFL is expressed in percentage by volume. Maintaining concentrations below the LFL is a key safety practice in industries dealing with flammable substances.

Upper Flammability Limit (UFL)

Conversely, the Upper Flammability Limit (UFL), or upper explosive limit (UEL), is the highest concentration of a gas or vapor in air that can sustain a flame. Above this level, the mixture is too "rich" to burn. Like the LFL, the UFL is expressed in percentage by volume. Ensuring that gas or vapor concentrations remain below the UFL is crucial in preventing fire and explosion hazards.

Factors Affecting Flammability Limits

Several factors can influence the flammability limits of a substance, including temperature, pressure, and the presence of other gases. For instance, an increase in temperature generally increases the flammability range, as it lowers the LFL and raises the UFL. Similarly, changes in pressure and the addition of inert gases can also affect the flammability limits.

Safety and Prevention

Understanding the flammability limits of substances is essential for designing safe chemical processes, storage, and handling procedures. Safety measures may include controlling the concentration of hazardous gases, ensuring adequate ventilation, and using inert gases to dilute flammable vapors to concentrations outside the flammable range.

See Also

• Explosion limit
• Flash point
• Autoignition temperature
• Chemical safety