Recent accidents involving trains carrying flammable liquids (crude oil, ethanol, etc.) and consequent release of these flammable liquids have resulted in the formation of large fires. These fires have caused significant property damage and, in some cases, fatalities. The focus of reducing such accidents has been on implementing train operational controls, improving tank car puncture resistance, and providing thermal protection systems on tank cars to reduce the rate of heat input from an external fire to the liquid in the tank. In addition, one of the current regulatory approaches for reducing the post-accident fire and explosion risk is to require the reduction in the product vapor pressure at the time of loading of the product into tank cars. This is based on the assumption vapor pressure is the sole metric of volatility and flammability. This paper demonstrates that vapor pressure alone cannot be a metric to evaluate the hazard potential of a flammable liquid. Other vapor properties, including the flammability range concentrations in air and the minimum ignition energy, must be considered. A Flammability Index (FI) is developed and applied to example flammable liquids. FI for a specific Bakken crude oil sample is 1.25 and for ethanol 11.3, making ethanol a more "flammable risk" material than crude oil, at normal temperatures. This result is completely opposite to what one would conclude based purely on vapor pressure (ethanol vapor pressure at 77 °F is 1.2 psia vs. 8.7 psia for crude oil at the same temperature).
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