Officials consider science of oil fires at accident scene
MONTREAL — The fire from a train derailment that lit up the night sky in Lac-Mégantic, Que., on Saturday and destroyed much of the small town’s core has now been extinguished, but it didn’t go down without a fight.
It took four days and 150 firefighters working around the clock to bring the blaze, which claimed 15 lives and left another 35 missing, under control. Even now, the ashes are smouldering and volatile. As residents grieve for the dead and wait anxiously for news of those still missing, the threat of another explosion hangs over the town like a dark cloud.
To understand why this fire burned so stubbornly and still poses a threat, says professor Raj Mehta of the University of Calgary’s Schulich School of Engineering, it’s important to look at what started it — and what kept it alive.
“It’s like when you make tea,” Mehta explained. “You heat the water, steam comes out. Crude oil has hundreds of components. If you start to heat it up, the lighter elements will vaporize first.”
It’s those vapours that are highly combustible, he said. They begin mixing with the surrounding air, and then all that’s required is a small spark to ignite them. The resulting fire would have begun heating the other derailed tank cars following Saturday’s derailment. As this happened, the crude oil inside those tanks would have begun to boil and the internal pressure of the tanks would have increased so quickly that they exploded. Mehta said the containers probably fell victim to what is known as a BLEVE — pronounced “blev-ee” and short for “boiling liquid expanding vapour explosion.”
It’s for this reason that fire crews have tried to keep the remaining tanks cool. If they are allowed to heat up too much, more explosions will occur.
Once a crude oil fire has started, dousing it with water can even make things worse, Mehta explained.
The only way to put out these types of fires is to smother them with a foam or similar substance. Aerial photos of the Lac-Mégantic site show large areas covered in what appears to be white foam used for this purpose.
Even when light crude oil runs into a body of water, Mehta said, it will float to the surface and can continue to burn.
As they work to extinguish the fires, officials must take all of this into consideration. They must also look for smouldering areas that could reignite.
“We’re working with metal machinery, and we could maybe start another fire, and that’s why we’re still there with firefighters in full dress,” chief firefighter Denis Lauzon told reporters on Tuesday morning.