What Does Explosion Limit Mean for Gas Detectors
In many workplaces with combustible gases, massive leakage of combustible gas reaching a certain concentration may trigger explosion upon exposure to open flame or other ignition sources. Gas detectors are often purchased to measure explosion‑limit values. Many users are unfamiliar with the definition and related concepts. Knowledge of explosion limits for combustible gases, vapors and dust is critical for fire prevention and explosion‑protection work. The meaning of explosion limits for gas detectors is explained below.
First, definition of explosion limit: Combustible substances (combustible gases, vapors and dust) must mix uniformly with air (or oxygen) within a certain concentration range to form premixed gas, which will explode when encountering ignition sources. This concentration range is
defined as explosion limit, or explosion‑concentration limit. The minimum and maximum concentrations enabling explosion are respectively
Lower Explosion Limit (LEL) and Upper Explosion Limit (UEL), also referred to as lower and upper ignition limits. No explosion or ignition occurs below LEL; no explosion but combustion may take place above UEL. Below LEL, insufficient combustible content and cooling effect of excess air inhibit flame propagation. Above UEL, lack of air prevents flame spread. Maximum explosive power occurs when combustible concentration approximates stoichiometric concentration calculated from complete‑combustion reaction equations.
Second, representation of explosion limits. Explosion can only occur within the range between LEL and UEL. UEL stands for Upper Explosion Limit, and LEL stands for Lower Explosion Limit. For gases and vapors, explosion limits are expressed in volume percentage (%) of combustible substance in mixture. For example, hydrogen‑air mixture has an explosion limit of 4%‑75%. For combustible dust, explosion limits are expressed in mass‑per‑volume unit g/m³, e.g. aluminum‑dust explosion limit is 40 g/m³. These two units are frequently displayed on gas‑detector screens.
Finally, hazards of explosion limits. Wider explosion‑limit range, lower LEL and higher UEL mean higher explosion risk. A wider range increases probability of reaching explosive conditions; a low LEL means minor leakage may form explosive atmosphere; a high UEL means small air ingress into vessels can create explosive mixture with internal combustibles. It should be noted that mixtures above UEL will not ignite or explode inside confined vessels, but may burn when escaping and mixing with fresh air, so fire hazards still exist.
