What are LFL and UFL?
The lower flammable limit (LFL) and upper flammable limit (UFL) are the concentrations, in volume percent in air, between which a fuel-air mixture will propagate a flame. Below the LFL the mixture is too lean; above the UFL it is too rich. These two numbers underpin flammable hazard distances, gas detector set points and inerting practice.
Typical values
- Methane: about 5 to 15 vol% in air.
- Propane: about 2.1 to 9.5 vol%.
- Hydrogen: about 4 to 75 vol% — an exceptionally wide range and a very low ignition energy.
- Hydrogen sulphide: about 4 to 44 vol%, though the toxic hazard is reached at concentrations thousands of times lower.
How conditions change the limits
Flammability limits are usually quoted at ambient temperature and atmospheric pressure. Raising temperature widens the range, mainly by lowering the LFL. Raising pressure typically raises the UFL substantially while barely moving the LFL. Enriching the atmosphere with oxygen dramatically widens the range, which is why oxygen enrichment is treated as a hazard in its own right.
For mixtures, Le Chatelier's rule gives a reasonable estimate of the mixture limit from the component limits and mole fractions.
Where LFL is used in practice
- Flammable hazard distance — the LFL or half-LFL contour from a dispersion calculation.
- Gas detector alarm settings, commonly a low alarm around 20% LFL and a high alarm around 40 to 60% LFL.
- Confined space entry and hot work permits, where a fixed fraction of LFL is required before work can start.
- Inerting and purging targets, usually referenced to the limiting oxygen concentration rather than LFL alone.
Frequently asked questions
Is a rich mixture safe because it is above the UFL?
No. A mixture above the UFL will dilute as it disperses and will pass through the flammable range somewhere. The flammable envelope simply moves outward from the source.
What is the difference between LFL and LEL?
None in practice. Lower flammable limit and lower explosive limit are used interchangeably in industry and refer to the same concentration.
Why do detectors alarm well below LFL?
To give time for executive action before a flammable atmosphere forms. Alarms at 20% and 40 to 60% LFL provide margin for detection delay, cloud fluctuation and response time.
Calculators for this topic
- Gas Dispersion — Atmospheric gas dispersion calculator
- Gas Detection Mapping — Gas detection coverage mapping
- Chemical Properties — Chemical and mixture property data for safety calculations