By Heather Brown

MINNEAPOLIS (WCCO) — Investigators say the blast at Minnehaha Academy happened when someone was moving a gas meter.

The explosion was so strong it could be felt blocks away.

So, why is natural gas so explosive?

“An explosion is a very rapid fire by definition. But this is a very, very rapid fire,” said Steve Zaccard, St. Paul’s fire marshal.

Zaccard spoke generally about gas leaks and explosions, rather than discuss specifics about Minnehaha Academy.

natural gas Why Is Natural Gas So Explosive?

(credit: CBS)

Natural gas is 90 percent methane. It has to interact with the air for it to be combustible. The gas is only flammable when the air is filled with between 5-percent to 15-percent natural gas.

“If it’s below that 5 percent, it’s too lean to burn. If it’s above that 15 percent, it’s too rich to burn,” Zaccard said. “That’s why it’s not dangerous in the pipe because there’s no air mixture with it. It’s all methane.”

When the mix is in the 5- to 15-percent range, virtually anything can ignite it — including a cellphone call, a static spark, a lit cigarette or flipping a light switch.

Zaccard says to take out a brick building or home would require a large amount of gas. How much would depend on the size of the room or building.

Firefighters have a device that can detect gas levels when they arrive at a leak. Natural gas detectors, similar to carbon monoxide or smoke detectors, are also available for people’s home. Zaccard recommends people get them.

Natural gas is odorless, so that rotting egg smell — called mercaptan — is added for safety.

“If you smell the mercaptan, you can smell natural gas, get out and call 911,” he said.

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