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