Do Class 3 Flammable Liquids Not Actually Catch Fire?

To some, flammable liquids might be considered a bit of a misnomer. The liquid itself does not actually ignite.

Why call it flammable liquid? Because—the material poses a hazard in transportation when in a state it could reasonably reach during transportation. 

For example, Gasoline (UN 1203) is a Class 3 flammable liquid and hazardous material. 

The “flash point” (the temperature at which a material’s vapors will ignite and keep burning) of gasoline is 45°F below zero or -45°F. Very few places on earth reach temperatures that cold, so temperatures are typically above gasoline’s flash point.

Under the Hazardous Materials Regulations (HMR), a liquid with a flash point at or below 140°F is a Class 3 flammable liquid. The lower a material’s flash point is, the greater the risk of a fire involving the material.

Given that gasoline has a flash point of -45°F—approximately 185 degrees lower than the threshold for a material to be classified as a flammable liquid—it becomes clear how significant a hazard it can pose during transportation, even though the liquid itself does not catch fire.

How to Test Flash Point

A flash point test works like this: The tester places a specified quantity of the liquid at a low temperature in a receptacle. Then they heat the material slowly. Periodically, a small flame is brought near the surface of the liquid. The tester notes the lowest temperature at which they observe a ”flash” or ignition of vapor. That is the material’s flash point.

A variety of different test procedures can be used to determine a material’s flash point. US DOT provides a list of authorized flash point test procedures for different liquids in 49 CFR 173.120(c)(i) and (ii).

Professionals use one of two methods to conduct the test—open-cup or closed-cup. Open-cup tests yield flash points a few degrees higher than a closed-cup test. The tests conducted with a closed-cup apparatus produce the more accurate and reproducible results.

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Tags: Class 3, flammable liquids, hazardous materials, hazmat