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 Technical bulletins
August 2010

Gazoline: an explosive subject!

By Clément Caron P. Eng., IAAI-CFI, Regional Director, Montreal,

Gasoline is used as a combustible liquid on a regular basis and is so well known that the dangers involved in its handling are often forgotten. This is also the case for several other solvents and combustible liquids used in our daily lives.

Gasoline vapors are heavier than air and can travel along the ground and be ignited several meters away from the area where the vapors are being produced. That is why gasoline must always be stored within closed containers and away from ignition sources.

Figure 1: Molotov cocktail

The flammability limits of gasoline are 1.4% for its lower flammability limit and 7.6% for its upper flammability limit. These percentages refer to the volume of gasoline vapors with respect to the total volume of air present within this fuel/air mixture. Outside of these percentages, the vapor mixture is either too lean (below 1.4%) or too rich (above 7.6%) and will not ignite.

As either the temperature or the pressure rises, the lower flammability limit is reduced and the upper flammability limit is moved upward thus widening the flammability window. The possibility of an explosion thus increases as the temperature or pressure rises.

Gasoline is a poor conductor of electricity. Consequently, liquid gasoline can accumulate static charges when it flows through a pipe or a hose. This static charge will dissipate slowly and can accumulate on the surface of the tank when it is being filled with gasoline. If conditions exist where an electric discharge occurs, producing a spark, an explosion can happen. However, the area where the spark is produced must be where the flammability limits are within the prescribed values and this is most likely to occur near the opening of the tank.

Gasoline fueling stations also have as a warning not to enter and leave a vehicle while filling operations are going on. Indeed, the sliding in and out of the vehicle can produce some electrostatic charges on the clothing and the body. If this accumulated static charge produces a spark, it could ignite the vapors.

It is occasionally mentioned that the energy contained within a liter of gasoline represents approximately the equivalent explosive force of several sticks of dynamite.

This above statement is not entirely true. In terms of energy, one kilogram of dynamite is approximately equivalent to two liters of gasoline. However, in terms of power, which is energy divided by time, the ratios are quite different. Indeed, unconfined gasoline vapors have a tendency to explode rather than detonate, that is the speed of the wave front in the case of an explosion of gasoline vapors will be below the speed of sound. In the case of dynamite, we have detonation meaning that the wave front surpasses the speed of sound. This produces, for a given quantity of energy, a more rapid and more powerful reaction.

When comparing gasoline and dynamite from a power point of view, one stick of dynamite will be the equivalent of 30 liters of gasoline, which roughly corresponds to a half of a gas tank of an automobile. In any case, the power available is considerable and should always invite a high degree of caution when the possibility of an explosion involving gasoline presents itself.

Numerous cases have been investigated by CEP, in which gasoline had been spilled on the floor in great quantities either accidentally or intentionally. When this occurs in an enclosed space, the explosion can move a building from its foundations or project windows and wall materials such as cinder blocks several meters away.

Gasoline is a complex product having numerous ingredients, the recipe of which can vary, depending upon the seasons of the year and the manufacturer. Although the sampling and detection equipment is highly sophisticated and can identify the presence of gasoline within post-fire debris, the identification of a particular supplier for this gasoline is improbable.

This results from the fact that the heat of a fire and the numerous other combustibles involved in a fire as well as the evaporation of the gasoline modify the signature of the gasoline and it then becomes extremely difficult to identify the source at which the gasoline was produced.

Here are a few safety recommendations regarding the use and handling of gasoline:

  • Never use or store gasoline near sources of heat or equipment susceptible of producing sparks;
  • Store gasoline outside of your residence within a close and airtight container;
  • Never store gasoline within glass containers or within plastic containers such as those used for windshield washer fluid;
  • Never use gasoline as a solvent for cleaning purposes;
  • Never use gasoline instead of barbecue lighting fluid;
  • During the refueling of your vehicle, do not enter and exit the vehicle as there is a low but non-negligible risk that electrostatic charge can accumulate on your clothing or your body, which could produce the ignition of the vapors which are being produced during the filling of the tank;
  • When filling gas containers, have them rest on the ground and not within a vehicle since there could be a risk of an accumulation of vapors within the vehicle and also the presence of an accumulated static charge on the vehicle itself.