Sunday, November 19, 2006

A Potentially Lethal Electrostatics Lesson

I pulled into a Chevron station a few years back to gas up my ride, when a rather graphic flier taped to the pump caught my eye. It was a laser-printed page with a picture of a charred car complete with melted tires, glass and all, sporting a headline that said "BEWARE OF GAS TANK FIRES AT FILLING STATIONS! THIS COULD HAPPEN TO YOU!"

There followed a couple of dense paragraphs that I simply couldn't resist reading at that point, outlining a horrific story of the author's wife who inadvertently killed herself by starting a fire at the pump after topping-off her car.

The flier went on to explain that while this hazard was quite common, it was generally underreported. "Please pay attention and change any dangerous habits so that forewarned, you don't loose your wife like I did," it said.

I was completely hooked on the curious phrasing at that point. Why were women in particular at risk? The flier went on to claim an "extensive" level of post-accident research that determined the filling-station fire hazard beset a population heavily skewed towards women because they tended to get back in the car while waiting for the tank to fill, whereas men tended to stand around outside the car while waiting.

This seemingly innocuous relaxation is indeed a very real risk because the very act of getting into or out of your car can transfer or induce a rather large electrostatic charge to your body. I suspect most people have at one time or another felt a tiny shock of static discharge getting in or out of their car. Just imagine if that spark arises at the wrong time and place. And when people wear think-soled (insulating) shoes, and particularly when the air is dry, there are no fast mechanisms to bleed off the accumulated charge, which will then be retained until some connection to a ground is established.

The gas tank pumping risk is managed automatically for those who stay out of the car for the duration, because the pumps are grounded, and when the fueling operation is initiated, the driver is grounded when he/she first touches the pump before any pumping begins. The danger begins when someone releases their ground connection, and gets charged up electrostatically-speaking by getting back in the car. If the first contact to ground after re-emerging from the car happens near the pump nozzle, the resulting spark can ignite any fumes that had been released during fueling. (This, incidentally, is one of the principle reasons why all modern gas pumps have sleeves to trap fumes in the tank, and it is important to be sure the sleeve seal is fully engaged and mated to the gas tank pipe.) If enough fumes have been released over a protracted or poorly-sealed fill-er-up type operation, the resulting explosion could set fire to anything flammable nearby, like the poor late woman mourned in the flier.

So the first thing I did upon returning home, was to ask my wife what she tended to do while waiting for the gas tank to fill up at the self-serve gas station. To my surprise, she said, "I just sit in the car and wait." I personally NEVER get back into the car. I then went on to relate the story, and physical explanation. My hope at this point is that she will keep herself grounded forever more whenever near any gasoline pumping operations.

For those of you in any doubt as to the serious nature of this warning, here is a YouTube video that I found this morning for your review entitled "Always Ground Yourself at the Pump."

So what should you do, using your newfound electrostatics knowledge to stay safe at the gas station? Well, there are a few options.

1.) If you can afford the price premium, you might consider staying inside the car and having someone else pump the gas who is always outside and stays grounded.

2.) If you do get out to pump your own gas as I generally do, stay outside of the car until the filling operations is complete, and the gas tank is capped off.

3.) If for some reason you HAVE to go back inside the car, make sure to ground yourself to the outside of the car, and the pump FAR from the junction between the pump and the car before you approach the nozzle.

Remember, Physics is your friend!


Anonymous said...

Excellent precaution -- the tv clip is amazingly clear. Good it did not blow up. Phil

Anonymous said...

Amazing, my wife was telling me exactly the same the other day when charging gasoline. The message is to touch the pump body with your hand before touching the hose valve. Phil

Anonymous said...

On second thought, my wife has the best solution: she sends me to load the gasoline. Phil

Anonymous said...

On second thought, my wife has the best solution: she sends me to load the gasoline. Phil

Web said...

Just a minor FYI -- regarding the part where you say "all modern gas pumps have sleeves to trap fumes in the tank, and it is important to be sure the sleeve seal is fully engaged and mated to the gas tank pipe" -- this is only true in localities that require the sleeve. Here in "middle America" we have never had the vapor recovery sleeves even on brand-new pumps. I'm sure the sleeve would serve to reduce the risk of explosion so it's a bit surprising the insurance companies don't insist they be added even where people aren't concerned about VOC pollution.

I'm also surprised there haven't been more stories of explosions -- so either it's rarer than you would think or not reported much. The new pumps do include more warnings about various things that might produce sparks (including cell phones and filling metal gas cans in a pickup truck bed). So the warnings are there for those of us who hang around the pump with nothing else to read. ;-)