Flammable Limits 
    of Gases and Vapor

Lower Flammable Limit (LFL)

Upper Flammable Limit (UFL)

The flammable limits in air of a gas or vapor are important safety benchmarks in processes where flammable gases may be present.  Tests to determine flammable limits are carried out in a closed test vessel fitted with a pressure transducer and a centrally positioned igniter. Mixtures of fuel and air are prepared to precisely known compositions. A low-energy ignition source is activated. If a flame is propagated the pressure in the vessel will rise. The limit of flammability is determined by specific pressure rise criteria of the test standard. Tests may be performed at ambient temperatures for volatile fuels or at elevated temperatures for fuels having low vapor pressure.

Liquid sample size: 250 ml

Flammability Diagram

A flammability diagram of a fuel-air-inert gas system is a general representation of the flammable composition space where fuel, air and an inert gas are mixed. The LFL and UFL values are two points on a flammability diagram where the concentration of added inert gas is zero. For each concentration of inert gas there is a corresponding lower and upper flammable limit. The flammability diagram is defined by the set of all such flammable limits.