I teach about a dozen flammable liquid courses annually, and it is one of my favorite courses to teach, as I love the technical side of hazardous materials. And my course always starts with a discussion around the 1st step in making a flammable liquid a hazardous material… FLASH POINT.
And over the 30+ years, I have been teaching people about flammable liquids when I ask the class to explain what a flash point is I always get the same answer…
“the temperature it ignites”
For 30+ years, I have always asked for more details in those answers, as that is NOT the correct answer. In dealing with flammable liquids, as is the case with all hazardous materials, DETAILS MATTER. The temperature “it” ignites is actually the “autoignition temperature,” but even then, we have to DEFINE what “it” is. As “it” is NOT the liquid, but rather the vapor that comes off the surface of the liquid.
When we discuss flash points, there are several critical data points that have to be taken into consideration:
- Temperature of the liquid
- Achieve the flammable’s “flammable range”
- Having an ignition source capable of igniting the flammable VAPOR
The first thing that has to happen is the liquid mass must be at its flash point or warmer. This allows enough vapor to be liberated from the surface of the liquid at a rate that allows the vapors to exceed their Lower Explosive/Flammable Limit (LEL/LFL). And when this occurs in the presence of a proper ignition source, the vapor will “flash.” So, when we look at NFPA’s official definition of flash point, we have to consider all three (3) data points:
The minimum temperature of a liquid at which sufficient vapor is given off to form an ignitible mixture with the air, near the surface of the liquid or within the vessel used, as determined by the appropriate test procedure and apparatus specified in Section 4.4.