The Science of a 'Perfect' Gas Explosion

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Wednesday's deadly and building-shattering explosion in New York City was an unusual display of the power of natural gas, but par for the course when it comes to what the gas can do when everything comes together and the chemistry is just right -- or wrong, depending on the outcome. And bad as the explosion was, it was tiny compared to some historic and far more terrible natural gas blasts.

Natural gas, which is mostly methane, is wonderfully useful when kept on the leash. Off the leash it has fueled some spectacular disasters on everything from oil rigs to coal mines to your average American home.

Natural gas started an average of 7,900 home fires each year from 2007 to 2011, causing each year $75 million in property damage and 35 deaths, according to a February 2014 National Fire Protection Association (NFPA) report by John R. Hall, Jr. More than half of those home fires (57 percent) started in the kitchen.

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That's just homes. In those same years there was an average of 5,830 non-home natural gas fires -- in commercial buildings, vehicles or outdoors -- causing a whopping $228 million in property damages and 4 deaths each year, according to Hall's report Fires Starting With Flammable Gas Or Flammable Or Combustible Liquid.

LP-gas hasn't fared any better, with an average of 7,620 homes scorched, 34 deaths and $62 million property damage each year. The non-home fires averaged 5,720 per year, killed 12 and cost $38 million in property damage, according to Hall.

This all sounds bad, but there's a silver lining. The number of home natural gas fires and deaths has almost halved dramatically since it peaked in 1981 with 11,750 fires and 77 deaths, Hall reports. The number of natural gas fires has remained fairly level since about 2002. LP-gas fires have also seemed to have leveled out.

One of the interesting differences in these two common fuels is that methane is lighter than air and LP-gas is heavier. So when the gases leak and manage to ignite, natural gas tends to blow the roof off, while LP-gas blasts a house off its foundation, explained Kevin Zumwalt, associate director of the University of Missouri Fire and Rescue Training Institute, which trains and certifies firefighters and other emergency responders.

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Just how the explosion happens is not as simple as filling a room with gas and igniting it. Like any chemical reaction -- and an explosion is just that, on steroids -- there have to be the right amounts to make it work.

If you fill a room with more than 17 percent methane, then try to ignite it, nothing will happen. The gas mix is too rich. It's like "flooding" an engine. If the methane volume is less than about 4.4 percent, that won't ignite either, since the gas mix is too lean.

The perfect mix, on the other hand, can cook dinner or trigger a ball of expanding flames and gases that can rip a building to shreds. Perhaps the most graphic examples in recent years was the fiery demise of the Deepwater Horizon oil rig in the Gulf of Mexico. On April 20, 2010, pressurized methane rose up through the drill rig, ignited and engulfed the entire platform in flames. Eleven workers were lost in the explosion and never found.

But even the infamous Deepwater Horizon pales in comparison to the natural gas explosion that devoured the Piper Alpha in the North Sea in 1988. That oil rig was not built to handle natural gas and when it blew, there was no place to hide for the workers and 167 men perished. The platform literally melted into the sea.

Bottom line: If you smell gas, don't fool around. Clear out and call 911 immediately.

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