API 1184 – Pipeline Facility Construction Inspection Practice Test

The primary factor that determines whether a flammable gas or vapor is ignitable in a class location is the mixing with air. For a substance to ignite, it must be within a specific range of concentrations when mixed with air, referred to as the flammable range or explosive limits.

In general, each flammable gas has a lower and upper limit of concentration in the air within which it can ignite. If the concentration is too low, there isn’t enough fuel to support combustion, while if it’s too high, there isn’t enough oxygen for combustion to occur. Therefore, proper mixing of the gas with air is critical for the potential ignition of flammable vapors.

While the temperature, pressure, and concentration of the gas are relevant to the overall flammability and combustion characteristics of the substance, it is the interaction of the vapor or gas with air that ultimately governs its ignitability. Understanding the importance of mixing with air is crucial for safety in environments where flammable gases may exist.