Firefighters frequently encounter leaks and spills of gasoline at motor vehicle accidents, gas stations, and scrap yards. Fortunately, at most of these incidents, gasoline does not ignite. This can, however, breed a sense of complacency, causing us to not take the fire hazard of gasoline as seriously as it really is.
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It is common knowledge that gasoline as a liquid is very hard to ignite; it is gasoline vapors that burn, and what makes gasoline such a fire hazard is the temperature at which it will produce vapors. Firefighters are familiar with the term flash point, the minimum temperature at which a liquid will produce vapors that mix with air to sustain combustion. The lower the flash point of a liquid the more dangerous it is. Gasoline has a flash point of –45°F—that means that unless you are in the farthest reaches of the Artic, gasoline will produce flammable vapors at any temperature that we would encounter. By comparison, the flash point of diesel fuel is around 100°F, which is why diesel fuel, when spilled, is less dangerous than gasoline. This is not to imply that diesel is not dangerous; it’s just less dangerous than gasoline. When diesel is spilled on hot pavement it is dangerous because it is heated above its flash point. Gasoline has a relatively narrow explosive range. With vapor concentrations below 1.2% in air, its lower explosive limit (LEL) are too “lean” to burn; whereas concentrations above 7.1%, its upper explosive limit (UEL), are too “rich” to burn. What contributes to the danger of gasoline vapor is that it is invisible and heavier than air. See the video above. Depending on its blend, gasoline has an ignition temperature of roughly 500°, hence, any open flame such as a lighter can be a source of ignition.
The attached gas station security video begins with a gentleman pushing a shopping cart and then sitting on a curb next to the coin-operated vacuum. When the gasoline tanker truck arrives, the driver and his helper attach elbows to the inlet and vapor return connections of underground tanks. Gasoline tankers fill underground tanks strictly by gravity; no pump is involved in the filling process.
Cutting Corners
The Miami-Dade (FL) Fire Department Training Division asked a fire marshal and a fire protection engineer in the petroleum industry to view the security video. Both authorities concur that the tanker driver and his helper “cut corners” by not connecting vapor return hoselines. As an underground tank is gravity filled, liquid gasoline displaces gasoline vapor, pushing it out of the tank. Tanker truck operators are required to connect vapor return hoses for three reasons: First, environmental regulations require that gasoline vapors are captured to protect air quality. Second, highly flammable vapors that are heavier than air are an extreme fire hazard. following the ground in search of an ignition source. Third, for economic reasons, since gasoline vapors that are returned to the tanker will be condensed back into liquid at the fuel terminal.
Lighting Up Between the LEL and UEL
The gentleman sitting on the curb had the misfortune of being within the narrow explosive range of the gasoline. When he lights his cigarette, he is seriously burned and a flame front travels back to the underground tank connections. The result is a classic three-dimensional gasoline vapor fire that subsides when the flow of gasoline is stopped and the vapor is consumed; only a small fire involving residual gasoline is burning at one of the inlet hose connections At this point, the driver and helper connect hoses to the vapor return elbows before the arrival of firefighters. Fearing that the fire could follow the product hose back to the tanker, a firefighter attempts to disconnect an elbow, which causes the remaining fire to intensify. Fortunately a fellow crew member rapidly extinguished the fire with a 20-pound ABC dry chemical fire extinguisher.
Why Dry Chemical?
Dry chemical suppresses fires in burning liquids by interrupting the chemical chain reaction of combustion. Dry chemical is the most effective extinguishing agent for three-dimensional fires such as burning gasoline vapors or flowing spill fires, but it does have limitations. Since dry chemical does not remove the heat from the fire tetrahedron, a fire involving burning liquid will still be well above its flash point and could re-flash at any moment. When you knock down a fire with dry chemical, don’t turn your back on it. You could be standing in a sea of flammable vapors that could suddenly become a sea of flame. At vehicle fires, firefighters often encounter three-dimensional fires of burning gasoline vapors. When possible, cool hot metal surfaces before applying dry chemical to reduce the chances of re-flash. This incident begs the question: Why not C02? C02 is excellent for fires that are confined, such as an oven and small electrical fires. C02 suppresses fires in liquids by excluding the oxygen but CO2 will rapidly dissipate on outdoor fires
Skill Acquired Through Training Increases Extinguisher Capacity
Underwriters Laboratories assigns a B rating for class B fires in terms of what could be extinguished by an untrained, unskilled person, which is significantly less than if the same extinguisher was in the hands of a skilled operator. As a firefighter, do you consider yourself a skilled operator? How much training did you receive on extinguishing class B fires with fire extinguishers?
Lessons Learned/Reinforced
- Off-loading gasoline tankers is an inherently dangerous operation made much more dangerous when petroleum industry best practices are not followed.
- Dry chemical is an excellent class B fire extinguishing agent but hot vapors can re-flash.
- A dry chemical in the hands of a trained firefighter, skilled in extinguishing techniques, will extinguish a fire involving a much greater surface of a burning liquid than an untrained person.
- Gasoline vapors are invisible and heavier than air. They will hug the ground in search of a source of ignition.
- Firefighters should avoid handling equipment they are not familiar with.
BILL GUSTIN is a 49-year veteran of the fire service and a captain with Miami-Dade (FL) Fire Rescue. He began his fire service career in the Chicago area and is a lead instructor in his department’s Officer Development Program. He teaches tactics and company officer training programs throughout North America. He is a technical editor and an advisory board member of Fire Engineering and FDIC International.
Correction: An earlier version of this article contained some inaccuracies regarding the UL rating system for extinguishers.
