Which statement about the upper flammable limit is true?

Which statement about the upper flammable limit is true?

• A. It is the maximum oxygen level in air
• B. It is the maximum fuel concentration in air at which ignition is possible
• C. It is the minimum fuel concentration in air for ignition
• D. It is unrelated to ignition

Answer: (B)

The upper flammable limit is about the highest amount of fuel vapor that can still burn in air if an ignition source is present. Fuels have a flammable range in air, bounded on the low end by the lower flammable limit and on the high end by the upper flammable limit. Within this range, ignition is possible; outside it, the mixture is either too dilute (not enough fuel) or too rich (not enough oxygen) to sustain a flame. The upper limit isn’t about oxygen alone—it reflects the fuel-to-oxygen balance: too much fuel relative to oxygen means there isn’t enough oxygen to support combustion. That’s why the upper limit is the maximum fuel concentration in air at which ignition can occur.

The upper flammable limit is about the highest amount of fuel vapor that can still burn in air if an ignition source is present. Fuels have a flammable range in air, bounded on the low end by the lower flammable limit and on the high end by the upper flammable limit. Within this range, ignition is possible; outside it, the mixture is either too dilute (not enough fuel) or too rich (not enough oxygen) to sustain a flame. The upper limit isn’t about oxygen alone—it reflects the fuel-to-oxygen balance: too much fuel relative to oxygen means there isn’t enough oxygen to support combustion. That’s why the upper limit is the maximum fuel concentration in air at which ignition can occur.