I was just reading a news report of a collision between a light rail train and a city bus in Los Angeles this morning. One sentence in the story mentioned that even an hour later, the smell of natural gas still lingered in the air.

That got me to thinking--more and more cities are switching their bus fleets to greener, compressed natural gas (CNG). How dangerous are they when they leak after some accident? We all know the danger of sparking an explosion in some gas leak at home. I'm just wondering if the physics of a vehicle crash change the danger any? For example, unless the gas is leaking into the interior of the bus, I don't think you'll be building up any appreciable quantity of natural gas, like you might inside your home.

I'll be taking a first aid refresher course in a couple weeks and so I was just thinking about one of the first things to do--surveying the scene. If I came upon an accident scene like I mentioned above and could smell the natural gas leaking, is that scene too dangerous to approach?

We've discussed the issue of electric hybrid vehicles after a crash before. I don't think we've ever covered CNG-powered vehicles after a crash, though.

I don't consider CNG or propane to be any more dangerous under ambient conditions than regular gasoline. Ounce for ounce, their explosive combustibility is roughly similar. CNG and LPG are both usually stored under pressure, and with small volumes this may accent the explosive force some, but I've seen what happens to a 5 gallon container of gasoline when it is ignited, and I don't think it is gonna be any safer an explosion to be around. In greater volumes, my experience is the disparity is even less.

As to the spread of gases compared to vapors, I would reckon that ambient conditions can make a big difference there. Gasoline vapors are more dense than methane or propane, so will tend to settle and stay put in low places longer. However, they don't tend to concentrate like gases do, so it may be a tradeoff.

I wish there were a way to sequester CNG and propane like what can be done with hydrogen. I watched an experiment where a cylinder of propane, a can of gas, and a cylinder of hydrogen were each shot with a high powered rifle bullet. They all exploded in a predictable and violent way. Then a cylinder full of anhydrous pyrite saturated with hydrogen roughly equivalent to what was in the regular cylinder was shot, and it didn't explode, it had a small jet of flame at the hole, but did not burst into a fireball.

Someday, I expect we will go to hydrogen combustion vehicles, and the safety factor will go up.

CNG, LPG, Propane and other gasses, which are compressed to liquid state can cause a BLEVE (boiling liquid expending vapor explosing) when the container fails or overheats.

When the tank is puctured, the gasses will rush out and cause a instand vapor cloud, which will expand and mix with the air. One spark and you get a big fireball and explosion. (unless the leak is really small).

The tank can also be overheated by fire, causing the contents to expend, which will cause the tank to fail. Other causes are overfilling and corrotion.

The chance that the tank will fail during a crash is very limited. There are many people driving on LPG, but very little is recorded of tank failures during crashes. If the tank does fail and causes a BLEVE, it will either be instandly or when the tank is on fire. Exceptions for pinhole leaks, which will cause a more controlled leak.

I was helping a DVM one summer. This family had a pet cow? And Bossie suffered that bovine illness known as bloat, when to much methane gas builds up in the stomach ( I forget which of the four at the moment.)

Standard veterinary procedure is to use a TROCAR CANULA, a rather large, stainless steel instrument not unlike a big old bleeder valve that is inserted via a large needle through the animals side.

I looked out of the corner of my eye ( how can round eyes have corners?)and to my horror realised the family dad was leaning over SMOKING A CIGARETTE.

Gas ignited and the DVM and I lost much of our eyebrows.

I tell you, in California if it isn't one thing it's another.

Puts my tin hat on and loads my SMLE .

Chris,
ROFL
Mike

Most (all?) LPG tanks in vehicles have "pop" valves. In the case of a fire, when pressure builds enough, the valve releases some of the gas. Results in a nice blowtorch effect in whatever direction the valve is pointed 'til the pressure drops, then the valve shuts off, and the blowtorch is out. For a minute or three, depending on the heat involved, how full the tank is, etc. But a BLEVE in a vehicle tank is very rare...

Explain THAT to your insurance company.....

Well you see we were 'venting' this cow of excess gas and it ignited.....

ROFL too.

You know, people need to be more careful about what they write on this forum. Certian people could get bad ideas from it.

-Blast, looking over his selection of syringe needles...

Oops....sorry (to Blast's family).....I didn't mean to cause problems.........I take no responsibility for whatever insane (though probably fun) ideas that come from my comments.

Okay, I've heard about burning down a barn that way, but I have a few questions...

How do you get a cow into a car, esp a little one? And how do you keep it aimed in the right direction? Do you have to feed it anything special to get 'high octane'? Does it come with a 'lifetime guarantee'?



Sue

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.