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Suppose for the moment a noteworthy author published a paper dealing with the chemical properties of a flammable substance (like ethanol) made the following statements:

  1. The flammability as measured by the LFL (Lower Flammable Limit) and flash point (FP) of ethanol vary with the amount of ethanol that has evaporated.
  2. The LFL and FP of ethanol are not constant values and may vary with time even at constant temperature and pressure.
  3. A container containing 2 ounces of ethanol is more flammable than one containing 12 ounces.

Even a novice scientist would disagree with the accuracy of the statements (1) and (2) because they contradict the fundamental principles of the properties of a substance, i.e., the LFL and FP of substance are intensive properties (like boiling point or density) and do not vary with the amount of material as the extensive properties (like mass or volume) vary and therefore must remain constant regardless of the amount of material present or the amount of time that has past. Statement (3) may require a bit more thought. We will get to this later.

The properties of LFL and FP are constant for a pure substance (provided certain criteria like LFL at a specific temperature and flash point type - closed or open cup are observed). But the key word is "pure" substance. Crude oil, however, is not a pure substance but a mixture of a variety of liquid petroleum fractions and dissolved volatile gases, each of which has their own characteristic LFL and FP values.

Therefore, from the moment the crude oil is loaded from the well head onto tanker cars or into a pipeline, the more volatile fractions start to degas from the bulk liquid changing the properties of the delivered crude oil depending on the amounts lost through evaporation.

Oil production is now at an all-time high in America and a significant proportion delivered by rail - over 100 tanker cars each trip carrying over two million gallons of crude oil travelling distances of over one thousand miles. These many rail shipments inevitably have led to derailments resulting in fiery explosions, destruction of many towns, and spills to the environment.

Crude oil is comprised of a blend of various types of hydrocarbons: lighter components of lower molecular weights, C1 to C10, which volatilize readily within hours, a middle fraction of medium molecular weights, C11 to C22, which volatilize over days, and the heavy components, which are separated into petroleum fractions at the refinery.

Bakken Crude is the name of the crude oil from the oil producing rock formation in North Dakota, Montana, and the adjoining Canadian provinces, which has become the second largest oil producer in the US, only behind Texas.

Bakken Crude is a light-weight petroleum crude, containing a higher percentage of the lighter end petroleum components - including flammable gases like methane, ethane, propane, and butane (called "condensates"). The main differentiating characteristic of Bakken crude is its vapor pressure - the ability of a liquid to vaporize at a specific temperature. These flammable gases that are in solution at loading but as the ambient temperature increases, the gases held in solution evaporate into the vapor space inside the tank cars. It is this mass of flammable vapor in the tank car that becomes a fire ball in the event of a train derailment. Tank cars do contain the useful safety feature: relief valves (set for both maximum temperature and pressure limits) which vent excess gas from the vapor space.

Vapor pressure measurements of tank cars transporting Bakken crude oil at loading are typically higher than average crudes (11.8 psi vs. 7.0 psi) which is attributed to the higher percentages of volatile condensates which average 3% in Bakken crude versus 1% in other average light crudes. These values are still within the North Dakota state limit of 13.7 psi for transport in rail cars.

Not only are the tank cars filled to capacity with the Bakken crude oil considered dangerous loads, but even empty cars present a hazard. According to an API publication ( "Classification and Loading of Crude Oil into Rail Tank Cars", September, 2004), the likely volume of crude oil "heel" remaining in an "empty" car is approximately 7%, or about 2100 gal for a 30,000 gal tank car after unloading.

Therefore, a 30,000 gal tank car filled to capacity is 4,010 cubic feet (cf) but when empty contains 2100 gal or 280 cf of heel and 3700 cf of space. Assuming an average density of 8 lb/gal for crude and a 3% content for the condensates (as propane),

Weight (propane) = (8 lb/gal) (2100 gal) (.03) = 504 lb condensates total

Using a simplified ideal gas law to estimate the quantity of condensates that could occupy the 3700 cf of space in the vapor headspace (provided the gas pressure remained below 13.7 psi),

. . .Continue to read rest of article (PDF).


Gerard A. Macri is a Forensic Chemist with 40+ years' experience and a comprehensive understanding of the physical and/or chemical properties and behavior of various substances to assist in the support and vindication of the client's claim. Dr. Macri has served as an expert witness for liability claims on behalf of plaintiff / defendant attorneys, as well as insurance companies.

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