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Earth Sense: Physics of gasoline explain why it can be explosive
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There’s enough talk about explosives in the media to have a person worried. Not worried enough, it seems, are some people handling the liquid explosive we use every day: gasoline. Understanding the hazards of this substance requires knowing about its physics.

Gasoline isn’t intended to serve cleanup or insect-killing projects, for which you see it used sometimes. It’s a motor fuel. It does its job not in liquid but in gaseous form, because as a vapor it’s much more flammable than as a liquid.

In the combustion chamber of the engine, gasoline vapor mixed with air is ignited by a spark. It then detonates and drives a piston with great force.

This means that when gasoline vapors mix with air outside of an engine, they can also detonate and cause a fire in a fraction of a second. The spark, in this case, is often supplied by the person trying to fill the tank. Warning labels on the pump caution against several scenarios, but you can still observe them being ignored.

The most common mistake, on cold days, is to sit in the car while the pump is running. Static electricity builds up on one’s body because the inside of the car isn’t grounded. Step outside, touch that nozzle, and a small spark can fly.

Gasoline vapors from inside the tank have been pushed into the air around the hose, displaced by the liquid gas going into the tank. They can now blow up in a huge “whoosh!” accompanied by a big fireball. That’s right where the operator’s hand and upper body are located.

Another common mistake is to use a cellphone at the pump, for the same static electricity reasons. Just google “gas pump static fire” for facts and amazing surveillance video, showing how easy it is to set oneself on fire with these seemingly innocent mistakes.

Most people know not to smoke near a gas pump, whether it’s running or not, because vapors are around. But the other day I had to tell a motorist to put her cigarette away while she was filling up.

Wherever there are gasoline vapors you can smell, there’s an extreme flash fire hazard. Static sparks can ignite around jerry cans during filling, too. The can should always sit on solid ground, never on the pickup truck bed or held by hand while being filled.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University.


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