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)

LFL and UFL define the range of vapor concentration in air that can ignite. They’re expressed as a percentage by volume of the fuel in the air at a given temperature and pressure. If the mixture is leaner than the LFL, ignition won’t propagate; if it’s richer than the UFL, there isn’t enough oxygen to sustain a flame. Within this window, ignition is possible with an ignition source.

The idea that these limits refer to pressure ranges in a closed system isn’t correct. Flammable limits describe fuel-to-air ratios, not a pressure window, and while they depend on temperature and pressure, they are fundamentally about concentration in the airflow. For example, methane has an LFL around 5% and a UFL around 15% by volume at room conditions; mixtures outside that range won’t ignite.