Pockets of flame have been observed to occur when flammable gas concentration is at what level?

The occurrence of pockets of flame is closely associated with the concentration of flammable gases in the air. The Lower Flammable Limit (LFL) is a critical threshold that indicates the minimum concentration of a gas or vapor in air necessary for ignition and combustion to occur. When the concentration of a flammable gas is at 0.6 times the LFL, it is often in a range where there is sufficient fuel for combustion available, and given certain conditions (like ignition sources), it becomes very likely that pockets of flame may form.

At this concentration level, the mixture is not only above the threshold that would typically allow for a flame to sustain itself but may also be concentrated enough in localized areas to create hazards such as pockets of flame. Concentrations below this range (like at 0.3 LFL) generally do not support combustion effectively, while concentrations significantly above (1.0 LFL and 1.5 LFL) may present overly rich conditions that can lead to different combustion behaviors, often resulting in reduced ignition potential.

Thus, the observation of pockets of flame indicating flammable conditions is most associated with concentrations at around 0.6 LFL, making this the correct choice.