Was Vapor Pressure Really the Culprit in the North Dakota Rail Accident?

The Wall Street Journal recently covered an incident in which a train carrying crude oil out of North Dakota derailed and exploded.

Fortunately, there were no injuries as a result of the accident. But the article—which misses the mark on a couple of key points—has raised questions about the safety and regulations governing the crude-by-rail method of transportation.

“So much combustible gas”?

The Wall Street Journal article leads with this sentence:

The crude oil aboard the train that derailed and exploded two weeks ago in West Virginia contained so much combustible gas that it would have been barred from rail transport under safety regulations set to go into effect next month.” (emphasis added)

Although the statement is definitely an effective attention-grabber, a little bit of analysis reveals that it falls well within the category of hyperbole.

In fact, the same WSJ article reveals that vapor pressure of crude oil (VPCRx) was reported at 13.9 psi by a third party . While it is true that North Dakota will implement new regulations in April that will require trains to transport crude with a VPCRx no greater than 13.7 psi, the 0.2 difference in psi absolute amounts to 1.2 percent. It’s hard to believe that such a difference merits framing the accident as one where vapor pressure made a difference.

Even more so once we consider that, when the North Dakota regulations were being drafted, the regulators recognized that a margin of error of 1 psi due to “sampling procedures” and “measurement equipment” had to be accounted and adjusted for. In other words, a margin of error of as much as 7.3 percent in such measurements is not uncommon in crude vapor measurements—although they vary depending on the methodology that is being used. In any case, the 1.2 percent difference falls well within that margin.

It is also important not to lose sight of the fact the definition of “stabilized crude oil” according to ANSI/API RP 3000, which is considered the national standard, requires a VPCRx no greater than 14.7 psi. This means that the tankers in the accident reported by the WSJ were around 5 percent more stable than what national standards have deemed to be stable for transportation.

Crude vapor pressure does not equal risk

Although vapor pressure is related to volatility, it’s not a dependable measure of the risk profile of a crude product in transport by itself.

Consider, for example, the Lac-Megantic tragedy. The crude being sent through it had a Reid vapor pressure of 9-9.5 psi, which is significantly lower than what any standard requires. As this terrible case shows, the notion that high crude vapor pressure by itself equals greater likelihood of crude explosions is just flat wrong.

As a recent report from the U.S. Congressional Research Service notes, viscosity, corrosivity, density, flash point, degree of sulphur, and distillation characteristics, among others are all relevant in determining how hazardous a substance is.

But, wait, what about the actual cause of the accident?

All the details about vapor pressure and volatility mask a simple fact: it’s nearly impossible to attribute the derailment and crash to vapor pressure. And, had the trail not derailed, its tankers would not have exploded.

In fact, even the WSJ article acknowledges—after spending several paragraphs on crude volatility—that “the cause of the derailment remains under investigation.” But, still, the way the media in general and the WSJ article in particular has framed the issue makes it seem as if the vapor pressure in a tanker car was the sole culprit.

A much better alternative would be to portray the facts in the appropriate context—even if it’s not a sensationalistic one and does not advance an advocacy agenda.


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