Lower Flammability Limit (LFL), Upper Flammability Limit (UFL), and the Limiting Oxygen Concentration (LOC) are used frequently in fire and explosion hazard evaluation and protection designs. Unfortunately, different countries use different test standards which may result in different flammability limit values. Global companies are often faced with the difficult decision of selecting the most appropriate values for their applications. The problem has recently been compounded with the discovery that some of the old US Bureau of Mines LOC data published in reference books can be non-conservative. Consequently, the 2008 edition of NFPA 69, “Explosion Prevention Systems,” requires either using new LOC data or subtracting 2 volume % from the old LOC values.

This paper presents a critical comparative review of the modern flammability standards currently used in Europe and in United States, and the data obtained in accordance with them. There is significant room for improving these standards. Visual test methods employing small tube or flask tend to make the flammable range appear wider than it is. They can also conceal flammability. The pressure rise criterion methods, on the other hand, are much less subjective than the visual methods and are becoming more popular. This paper presents a simplified analysis which links the visual criteria to the pressure rise criteria. The analysis also identifies a technical flaw inherent in the fixed pressure rise criteria. This paper also proposes a new pressure rise criterion which is volume dependent for smaller test enclosures.

A comparison of how US and European safety professionals use the flammability data, and a fundamental philosophical difference between the US and European methods for flammability determination are also provided.