In order for an atmosphere to be capable of burning explosively, four conditions must be met.
- Adequate oxygen
- Adequate fuel
- A source of ignition
- Sufficient molecular energy to sustain the fire chain reaction.
These four conditions are frequently diagrammed as the fire tetrahedron. If any side of the tetrahedron is missing, incomplete or insubstantial, combustion will not occur.
The minimum concentration of gas or vapor in air that will ignite and explosively burn if a source of ignition is present is the lower flammability limit (LFL). Although OSHA uses the term “LFL,” most gas detection instrument manufacturers provide readings in percent lower explosion limit (LEL) increments. In standard atmospheric and temperature conditions the two terms mean essentially the same thing, and can be used interchangeably.
Different gases and vapors have different LEL concentrations. Below the LEL, the ratio of combustible gas molecules to oxygen is too low for combustion to occur. In other words, the mixture is “too lean” to burn.
Most (but not all) combustible gases and vapors also have an upper limit of concentration beyond which ignition will not occur.The upper explosion limit (UEL) is the maximum concentration of combustible gas or vapor in air that will support combustion. Above the UEL, t
he ratio of gas to oxygen is too high for the fire reaction to propagate. In other words, the mixture is “too rich” to burn. The difference in concentration between the LEL and UEL is commonly referred to as the flammability range. Combustible gas concentrations within the flammability range will burn or explode provided that the other conditions required in the fire tetrahedron are met.
Because the flammability range varies widely between individual gases and vapors, most regulatory standards express hazardous condition thresholds for combustible gas in air in percent LEL or percent LFL concentrations. Instruments used to measure combustible gas in confined spaces typically display readings in percent LEL increments, with a full range of 0 – 100% LEL. The most commonly cited hazardous condition threshold concentrations are 5% LEL or 10% LEL. OSHA 1910.146 defines 10% LEL as the hazardous condition threshold for flammable gas.
Ten percent LEL is the default alarm setting for many instruments used to measure percent LEL combustible gas. This is the least conservative (or highest acceptable) alarm setting for instruments used for monitoring combustible gases and vapors in confined spaces. Many circumstances warrant a more conservative, lower alarm set point. The presence of any detectable concentration of flammable/ combustible gas in the confined space indicates the existence of an abnormal condition. The only completely safe concentration of combustible gas in a confined space is 0% LEL. In addition, specific procedures or activities may require taking action at a lower concentration.