As do CNG tank valves. It's actually pretty hard to get a ruptured CNG tank to ignite or to even rupture. Most cylinders are carbon fiber wrapped aluminum and are resistant to most non-AP small arms fire.

Other things to consider is that likely the concentration of NG adjacent to a tank rupture will be too rich to ignite. NG is lighter than air so it will tend to float up rather than settle or accumulate near the ground.

I would think that an accident with enough force to rupture a composite CNG bottle would kill the car occupants long before you'd have to worry about an explosion.

I've seen a demonstration where they dropped a car equipped with a CNG tank from 100 feet, and it did not rupture the tank. The one case I've heard of a ruptured tank was on a bus where the battery box was located above the tank and leaking acid from a battery ate away at the bottle resulting in a rupture.

All in all, it's a pretty safe technology. We have passenger ferries which go between Norfolk and Porsmouth, VA which are CNG powered.

Regards,

Tom

As a first responder, I will be interested in the potential for explosion after the crash.

Because everyone likes videos of explosions, for your viewing pleasure I give you BLEVEs:
Seoul BLEVE

Train BLEVE

Note in the second one, a BLEVE throws a railroad car three quarters of a mile. And the reporter can feel the heat wave over half a mile away. Mind your eyebrows.

Very little danger of a BLEVE when you have arrived, but remember your own safety first, a fire usually means it's unsafe. Not just because of the fuel tanks, but also the dangerous gasses/fumes, exploding tires, exploding hydraulics, etc.

As somebody else mentioned, most (if not all) tanks are equipped with a overpressure valve, which however does not mean it's safe to be around. When the valve blows, you will get a big fire ball coming out the valve, which then turns in to the blowtorch effect. Or it will just shoot occasional fireballs, depending on the valve and pressure.

And ofcorse with any safety system, the valve can also fail to operate. HIGHLY unlikely, though.

in the end, it's all about judgement and experians. Always check your own safety. When you spot a possible danger, than retreat and let the right people do there job. It's not just fuel tanks which can be dangerous, there are plenty of other possible dangerous things somebody can be carring in there trunk.

Thanks for everyone's replies, ehem, including Chris' interesting recollections...

Tom, I think that maybe you've hit the nail on the head with these comments. I was Googling a bit and ran across the website, The Properties and Benefits of Natural Gas . Two things you said resonate with stuff from this webpage--that a lighter-than-air gas will tend to dissipate rather quickly (unless, I suppose, it is leaking into an enclosed space, like the passenger compartment), and that natural gas actually has a rather narrow combustion range. According to the website, concentrations below 5% and above 15% will not ignite. Relatively speaking, I think a CNG leak would be less dangerous than a gasoline spill at an accident scene, like the train/bus collision I was mentioning in my OP.

I don't think I would ever worry about BLEVE. Hanging around near/inside a CNG-powered bus (CNG-powered cars are exceedingly rare around my neck of the woods) that was completely engulfed in flames would be the last place I would want to be.

Speaking of BLEVE, for some reason I caught two examples of it on TV this weekend on those TV shows that compile old news footage. One was a tanker rail car full of propane that had derailed and caught on fire. The explosion looked like a small atomic bomb, with the mushroom cloud rising from the site. Or maybe what a "daisy cutter" fuel-air bomb detonation looks like. The other one was a fire at a welding supply company in Dallas. The storage yard had dozens of acetylene tanks. When they started to "cook off," they literally turned into little rockets and could see the flaming trail of the tanks as they rocketed hundreds of yards away, narrowly missing vehicles on a nearby freeway. Scary stuff.

I'm waiting to hear what happened to the cow.

I think I read that there are over 1,000,000 autos in The USA with natural gas tanks on them. If it were a big explosion hazard, you would think there would be a tank exploding every so often and the news media telling us about it. But I can’t ever remember seeing a story on it.

Natural Gas has is within explosive range from about 5% to about 15%. If the air is made of of less than 5% of natural gas or more than fifteen percent of natural gas, then it does not really pose a risk as a fire hazard.

Gasoline is within the explosive range from 1.5% - to about 7.5%. It's range is narrowewr, but I think both of these would be considered to be narrow explosive ranges.

One thing that is better about natural gas is that it is lighter than air. Once it is released, if the natural gas is not burned quickly, then it is more likely to dissipate into the upper atmosphere.