Dealing with Flammable Liquids

If you ask the majority of firefighters out there, they will tell you without a doubt that they did not join the fire service to deal with hazardous materials. Even if you didn’t sign on for responding to hazardous materials, there is one hazard class that you absolutely cannot get away from. That hazard class is DOT Hazard Class 3, Flammable Liquids.


Flammable liquids account for approximately 85 percent of all hazardous materials shipments. The cars and trucks that we drive are fueled by flammable liquids in the majority of cases. Many of our homes may be heated by flammable liquids. In order to understand these materials and how to respond to them, there are a few terms that we must come to know.

DOT Hazard Class 3 is divided into two groups, flammable liquids and combustible liquids. All flammable liquids are placarded the same with a red background, flame icon at the top and the number “3” denoting the hazard class at the bottom. Product specific placards will have the four-digit United Nations Identification Number (UNID) in the middle. Generic placards will have the terms “Flammable” or “Combustible” in the middle in place of the UNID.

The flash point is what separates the flammable liquids from the combustible liquids. The flash point is defined at the temperature at which a liquid will give off enough vapor to ignite if subjected to an ignition source. Flammable liquids have a flash point below 100° F and are the most dangerous. Gasoline is the most common example of a flammable liquid. Combustible liquids are those that have a flash point above 100° F. Diesel is the most common example of a combustible liquid.

The flash point can be a very important term to know when responding to a flammable liquid spill. Some flammable liquids, like gasoline for example, will always produce flammable vapors due to their low flash points. (The flash point of gasoline is -43° F.) In other cases, extreme care must be taken in order to avoid a catastrophic mistake. If the flash point is 115° F for example, would you assume that you are safe because the ambient air temperature is lower than 115° F? Do not make this mistake! If this spill occurs in the middle of a hot North Carolina summer where the ambient air temperature is approaching 100°F, the ground temperature — assuming the leak is on concrete or asphalt — could easily exceed the 115° F flash point. The hot ground could easily heat the liquid to the point where flammable vapors could be produced. If an ignition source is nearby, a flash fire could easily occur.

Another term that we must understand as it relates to flammable liquids is the boiling point. The boiling point is defined as the temperature at which a liquid will continually give off vapors in sustained amounts. The most common boiling point that you may be familiar with is that of water, which is 212°F. When water is heated to its boiling point, it changes from water to steam (or vapor). Do not be lulled into a false sense of security due to the relatively high boiling point of water. Flammable liquids with low boiling points can produce large volumes of flammable vapor at relatively low temperatures.

If you are faced with a flammable liquids spill that is entering a water way such as a stream or pond, there are two more terms that we must understand. Those terms are specific gravity and miscibility.

The specific gravity is the ratio of a substance’s density to that of water. (It is very similar to the term Vapor Density that we discussed in a previous article regarding gases.) The specific gravity of water is 1.0. The flammable liquid will sink if its specific gravity is greater than 1.0. The flammable liquid will float if its specific gravity is less than 1.0. This term becomes very important if a dam is to be built. The specific gravity will determine what kind of dam is built in order to restrict product flow downstream.

The other term that comes into play is miscibility. Miscibility is a measure of whether a chemical will mix with water or not. Not all chemicals mix well with water. Flammable liquids such as hydrocarbons — gasoline, kerosene, etc. — will float on the water’s surface and not mix at all. Other flammable liquids such as polar solvents (alcohols) will mix with water and dilute themselves to the point where they are no longer dangerous or flammable. Some substances will react violently with water.

All of these terms can be found in the material’s Safety Data Sheet or SDS. In a fixed facility, the SDSs should be on file somewhere in the structure unless the firm is subscribing to an online service. In a transportation emergency, the SDS may be found with the shipping papers. If you have a smart phone, the SDS can easily be found by using an online search engine or any of the hazardous materials related applications available through the App Store. Many of the apps are free or relatively inexpensive.

When dealing with any flammable liquid, remember that foam is your best tool for fire attack and vapor suppression. For a review of foam operations, please see the article “There’s No Place Like Foam” from the Winter 2018 issue.

Until next time, stay safe out there.

Mark Schmitt is a Captain/HazMat Specialist for the Greensboro Fire Department assigned to the Foam/ARFF Task Force and a veteran of 25 years in the fire service. The majority of his career has been spent in Special Operations. He holds a Master of Public Administration in Emergency Management and is a graduate of the National Fire Academy’s Executive Fire Officer Program. He has taught numerous hazardous materials courses for the Greensboro Fire Department, local community colleges and the North Carolina Office of the State Fire Marshal in addition to serving as a contract instructor with the National Fire Academy.

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