Lower Flammable Limit (LFL)
The concentration of a vapor in air (or other oxidant) below which propagation of flame does not occur on contact with an ignition source. The lower flammable limit is usually expressed as a volume percentage of the vapor in air. Sometimes called Lower Explosive Limit (LEL).
API RP 2201, Safe Hot Tapping Practices in the Petroleum & Petrochemical Industries, Fifth Edition, July 2003 (Reaffirmed October 2010), Global Standards
API RP 2009, Safe Welding, Cutting, and Hot Work Practices in the Petroleum and Petrochemical Industries, Seventh Edition, February 2002 (Reaffirmed, March 2012), Global Standards