Flash point is the lowest temperature at which a combustible substance gives off enough vapor to form an explosive or ignitable mixture with air. In plain English, it is the temperature at which a material’s vapors will ignite and keep burning
The lower a material’s flash point is, the greater the risk of a fire involving the material. People who ship and transport hazardous materials use flash point to measure the risk of an explosive or ignitable mixture forming when a liquid escapes from its container or packaging.
Here are examples of two liquids, one with a "low" flash point and one with a "high" flash point.
Low Flash point
100% isopropyl alcohol (isopropanol)
Flash point: 11.7°C (53°F)
Even when kept cool, isopropanol can ignite and burn steadily.
High Flash point
Flash point: Around 225°C (440°F)
Unless heated dramatically, mineral oil does not pose a fire hazard.
Under the United States Department of Transportation’s Hazardous Materials Regulations (HMR), a liquid with a flash point at or below 60 degrees Celsius (140°F) is a Class 3 flammable liquid. A liquid with a flash point above 60 degrees Celsius (140°F) but below 93°C (200°F) is a combustible liquid.
Flammable and combustible liquids are subject to requirements for classification, naming, packaging, marks and labels, placarding, reporting, shipping records, training, and more.
Employees involved in shipping or transporting flammable and combustible liquids must complete DOT hazmat training
to safely do their jobs and ensure compliance with US DOT (and international) regulations. US DOT’s Hazardous Materials Regulations are found in Title 49 of the Code of Federal Regulations (49 CFR), Parts 100 to 181, et al.
How to Test Flash Point
A flash point test works like this: A specified quantity of the liquid at a low temperature is placed in a receptacle. The material is heated slowly. Periodically, a small flame is brought near the surface of the liquid. The flash point is the lowest temperature at which a “flash” is observed.
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.
Packing Groups for Class 3 Flammable Liquids
Together with the material’s boiling point, flash point is also used to determine the degree of hazard posed by a flammable liquid, also known as the material’s Packing Group (PG) in hazmat-speak. The PG dictates the strength of package required to contain the material.
Like many DOT hazard classes, Class 3 (Flammable liquids) is divided into three Packing Groups—I, II, and III.
Among all flammable liquids, PG I materials pose the greatest danger in transportation. PG III materials pose the lowest degree of danger. Combustible liquids are assigned to Packing Group III.
A liquid's flash point is a crucial piece of information that influences how flammable liquids are classified, packaged, and transported safely. Class 3 flammable and combustible liquids are among the most frequently shipped types of hazardous materials every year, according to the US Census Commodity Flow Survey.
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