Which statement correctly defines the lower and upper flammable limits (LFL and UFL)?

Which statement correctly defines the lower and upper flammable limits (LFL and UFL)?

• A. LFL is the minimum vapor concentration for ignition; UFL is the maximum vapor concentration for ignition; outside these, ignition cannot occur.
• B. LFL and UFL refer to pressure ranges in a closed system.
• C. LFL is the maximum vapor concentration; UFL is the minimum.
• D. Ignition can occur outside LFL and UFL.

Answer: (B)

Flammable limits define the range of fuel vapor in air that can sustain ignition with an energy source. The lower limit is the minimum vapor concentration in air needed for flame propagation; below this, the mixture is too lean to burn. The upper limit is the maximum vapor concentration in air at which a flame can still be sustained; above this, the mixture is too rich to burn. Between these two concentrations, ignition is possible if an ignition source is present. These limits are expressed as a percentage of vapor in air (volume percent) under given conditions, not as a pressure range in a closed system.