Vapor Space Flammability of Automobile Tanks Containing Low RVP Gasolines

As a means of reducing evaporative emissions from gasoline sources, the Environmental Protection Agency (EPA) and other legislative and regulatory bodies have required reductions in gasoline volatility. Further reductions are being proposed. This paper reports the results of an investigation into the resulting flammability potential of low volatility gasolines contained in automobile tanks when low ambient temperatures are experienced.

The hydrocarbon composition and the flammability tendency of the vapor in fuel tanks of passenger cars were determined via a matrix of 45 different gasoline volatility, temperature, and tank level combinations. A flammability device was developed and used to ignite vapors drawn from the tanks of in-use vehicles at the test conditions. The Reid Vapor Pressure (RVP) of the test gasolines ranged from 6.5 to 9.4 psi in the near-full tank level and 6.4 to 7.8 psi at a low tank level. Ambient test temperatures were selected to represent probable winter/spring and fall/winter transition period low temperatures. Results demonstrate that at moderately low temperatures and low gasoline volatility conditions, the vapor space of a vehicle's fuel tank will be flammable and no longer “protected” by the volatile components in the fuel. Such a condition will result in a flame if an ignition source is available. It was demonstrated that an initial fill of 9.4 psi RVP gasoline does not result in a flammable vapor space at temperatures above 0°F. However, upon drive-down to a low tank level, the same gasoline produced a fully flammable vapor space at 0°F and even a marginally flammable vapor space at 10°F. A simple model was developed to predict fuel tank hydrocarbon vapor concentrations as a function of the gasoline RVP and operating temperature. The model can be used to predict gasoline fuel tank conditions that will result in a potentially flammable and unsafe condition.

Marketing fuels under past American Society of Testing and Material (ASTM) recommended volatility classes and transitional periods helped assure that commercial gasolines produce fuel-rich (non-flammable) conditions in vapor spaces of storage tanks. The ASTM recommended geographical and seasonal adjustments helped to maintain this extra margin of safety regardless of location or time of year. Gasolines marketed today under the EPA Phase I regulations on gasoline volatility may continue to largely avoid this concern with their high volatility components producing a gasoline vapor concentration sufficient for the gaseous phase above the fuel to be richer than the fuel-rich flammability limit. This means that insufficient air or oxygen is present to sustain a flame. Thus, the tank is protected against ignition or explosion from extraneous sparks or arcing within the tank's vapor space.