Will a gun make a a gas leak asplode?
May 7, 2008 12:06 PM   Subscribe

Would the act of firing a gun cause an explosion in a room filled with natural gas?

I'm writing a story and I'm trying to figure out if gunfire is enough to cause an explosion given an ongoing gas leak in a home. Yes? No? Physics?
posted by vraxoin to Media & Arts (26 answers total)
Well, everything I have ever read about a gas leak in your home says that anything that causes a spark can ignite the gas. Even turning a light switch on or off. If that's true, then I'm sure a spark from a gun could do the same.
posted by Fuzzy Skinner at 12:12 PM on May 7, 2008

Since all that is necessary for a natural gas explosion in a case like that is an ignition source, and a gun can certainly serve (one example only) as an ignition source, the answer is most definitely "Yes, possibly." That is, it would not necessarily cause an explosion, but it certainly could.
posted by cerebus19 at 12:13 PM on May 7, 2008

This utility company page says:

On your way out, don't use anything that could generate a spark, such as a light switch, telephone, cell phone or car ignition.
posted by Fuzzy Skinner at 12:14 PM on May 7, 2008

Best answer: It would depend on the concentration of the gas. My understanding of gas explosions is that it's not as easy as Hollywood makes it out to be to make a house explode. The gas only explodes if it's mixed with air at the right ratio throughout the house. (I'm trying to think of where I remember this from ... it might have been Mythbusters or something similar.)

It also might depend on the gun and the ammunition being fired. A relatively low-powered round fired from a long-barreled firearm won't produce much in the way of superheated gas at the muzzle (at least, not hot enough to incandesce; I've never worked out the actual muzzle-exit temperature, although it's probably a pretty easy problem). However, shorter-barreled guns with powerful loads can produce substantial gouts of flame. Snub-nose revolvers in .357 and .44 Magnum are notorious for it.

I suspect that the temperature of the gas exiting a revolver like that, being hot enough to glow brightly, would probably be more than hot enough to ignite gas, if it existed in the right fuel/air concentration in the vicinity of the muzzle.
posted by Kadin2048 at 12:19 PM on May 7, 2008 [1 favorite]

It's absolutely the case that gunfire could ignite natural gas. What's less clear to me is: how much gas has to be in the air to expect the burning gas to reach the gas leak's source, causing a notable explosion, and what effect that level of gas would have on people in the room (like, say, the person firing the gun). Would they be suffering from hypoxia? Would they have already passed out?
posted by Zed_Lopez at 12:19 PM on May 7, 2008

It's possible, but not likely unless your character is firing a flintlock musket.

Firing gun causes a miniature explosion inside of the bullet casing. While there maybe some muzzle flash (and you could certainly argue that this could ignite gas) it's highly unlikely that such an action could in fact set off an explosion, unless we're talking about a room that is so filled with gas that people would have trouble breathing.

My question would be how much gas is required to blow up a room or a house? When messing around with propane or natural gas I feel like the tendency is for these gases to go up in a *woomf* type fireball, with little or not explosive properties. That said homes do "explode" from gas leaks (google it.)
posted by wfrgms at 12:21 PM on May 7, 2008

I'll retract my 'absolutely' in light of Kadin2048's point about the possibility that some particular gunfire might not be hot enough.
posted by Zed_Lopez at 12:21 PM on May 7, 2008

I'm sure Adam Savage can provide more details, but Mythbusters pretty much said no. Granted this was shooting a bullet into a propane tank, but that seems pretty close to your scenario.

Although on preview I now realize you may be talking about the gun actually being IN the room with the gas leak, not firing a bullet into said room. In that case I suppose the flash may be sufficient to cause combustion, but I admit that I have no professional expertise in gas combustibility.
posted by JJtheJetPlane at 12:30 PM on May 7, 2008

I would think that the fire reaching the gas leak's source would not be the thing that causes the big explosions - the cloud of gas mixed with air would be the source of the massive explosion, if the ratios were correct. The pipe should just become a massive flamethrower. This is assuming the gas leak's source is a high pressure pipe and not a tank, which would be much worse.
posted by Jupiter Jones at 12:32 PM on May 7, 2008

Mythbusters did an episode on shooting a propane tank with a gun to make it explode. It didn't, except in very specific circumstances where the bullet sparked off of the metal -- the spark caused the explosion.

I think that this would apply to igniting any kind of gas - there needs to be a spark/blast.

Is the muzzle of the gun exposed to the gas? Then I'd say yeah - a spark/fire is triggered when the bullet is set off, so it'd probably ignite the gas. However, the gunman is outside, and shooting a bullet into this gas-filled area, I think it'd have to hit something and cause a spark...
posted by twiggy at 12:37 PM on May 7, 2008

Muzzle flash might do the trick, but the bullet's impact with metal in the room might give you the spark that you need too.
posted by allkindsoftime at 12:39 PM on May 7, 2008

One thing about the ratio of gas to air: Eventually the ratio is reached as the quantity of gas increases.
posted by Pants! at 12:43 PM on May 7, 2008

From the Merck Index:

American natural gas is about 85% methane.
the loudest explosions occur when one vol of methane is mixed with 10 vols of air (or two vols of oxygen). Air containing 5.53% methane no longer explodes. Air containing more than 14% methane burns without noise. Autoignition temp 650C.

posted by jamjam at 12:51 PM on May 7, 2008

This upper and lower limit table for concentrations might help you. As others have said, there's a point where there's insufficient air (there's a reason these are called fuel-air explosions) to allow combustion. I think stovetops use primarily methane, which that table claims tops out at 15%.

However, you also would need to keep in mind that at some point - past 15% I am fairly certain - there's not enough O2 to breathe, which is a whole other kind of problem...
posted by phearlez at 12:53 PM on May 7, 2008

Natural gas has a lower explosive limit of 5% in air and an upper explosive limit of 15% (Wikipedia). So provided the gas is mixed with air between those levels you have the potential for an explosion. I don't think it will take much to set it off.

The Mythbusters propane tank example above is not a good comparison as there the propane in the tank is not mixed with air beforehand.
posted by pombe at 12:56 PM on May 7, 2008

I'm writing a story and I'm trying to figure out if gunfire is enough to cause an explosion given an ongoing gas leak in a home. Yes? No? Physics?

Keep in mind that at the concentrations of gas/air required to cause an explosion, it's likely that anyone standing in the house at the time would be suffering from the ill effects of breathing the gas/air mixture itself (at the very least, they would notice the gas via the smell of the odorizing agents -- mercaptan or a similar sulfur-based compound). This is potentially useful for your descriptions of the scene.

Exposure to low levels of natural gas is not harmful to your health. However, if a gas leak is severe, the amount of oxygen available for breathing could be dramatically reduced, which can lead to asphyxia. Symptoms of asphyxia include:

* dizziness
* fatigue
* nausea
* headache
* irregular breathing

Exposure to extremely high levels of natural gas can cause loss of consciousness or even death.

posted by Cool Papa Bell at 1:09 PM on May 7, 2008

One thing about the ratio of gas to air: Eventually the ratio is reached as the quantity of gas increases.

Or exceeded. Too lean, no combustion. Too rich, also no combustion. But nobody remembers this now that engines all use programmed fuel injection.

Some gases are more sensitive than others to the fuel/air ratio. E.g. hydrogen. Everyone thinks OMIGAWD HYDROGEN, RUN, RUN, RUN AWAY but in fact hydrogen has such a narrow ignition range that 9 times out of 10 it'll float away (and very quickly at that given it's the least dense gas out of all) before it ignites.
posted by randomstriker at 1:12 PM on May 7, 2008

Maybe, if the gun was fired inside the home. Apparently the auto ignition temperature of methane is about 1076 deg. F * while the temperature of gunpowder burning is 3000-5000 deg F *. (Which sounds a little high to me but it was the only source I could find.)

Interesting link found along the way, here.
posted by 517 at 1:21 PM on May 7, 2008

Remember that natural gas has stinky mercaptan added to it; anyone in the room with enough of a concentration for it to detonate is likely to be well aware of the gas leak- it's really really smelly.
posted by jenkinsEar at 1:25 PM on May 7, 2008

One thing about the ratio of gas to air: Eventually the ratio is reached as the quantity of gas increases.

Or exceeded. Too lean, no combustion. Too rich, also no combustion.

I don't think MythBusters has done anything exactly like this, but they've done a lot of similar experiments with exploding gases. The fuel/air mixture is always key, and the ignition source is usually important too.

When they tried to blow up a house with bug bombs, it didn't work until they purposely got the correct number of bug bombs and used matches instead of a stove igniter.
posted by burnmp3s at 1:33 PM on May 7, 2008

There was a house explosion in my town about a week ago. The admittedly unofficial version of the story I heard from a neighbor is that a guy had tied up his wife and daughter, turned on the gas, and left them in the house for several hours until he had second thoughts and went back to untie them. The wife and daughter got out of the house fine, but when he went to turn off the gas, the house exploded and fell on him. He did not fire a gun.

Just based on that I would think that two things are true.

a) people can survive for a while inside a house full of an explosive amount of natural gas
b) it doesn't take much to set off an explosion in that situation.

The police / fire department are still investigating, so the exact cause of the explosion is uncertain.

Story is here and here.
posted by dosterm at 2:47 PM on May 7, 2008

By the way, some anecdotal evidence for the scale of the explosion in your story:

We felt a 5.4 earthquake and its aftershocks several weeks ago. People I've talked to who live up to around ten miles from the house explosion said it was much more noticeable than the earthquake. The explosion also managed to damage 12 surrounding houses and completely level the house itself. It were a pretty big deal in this here little town, by golly.
posted by dosterm at 2:59 PM on May 7, 2008

It depends.

With natural gas, concentrations would have to be significant. (I had some friends who ignited leaky gas in a trailer and lived to tell about it... the trailer walls bulged out! They did theirs with a torch lighter.)

Propane gas is heavier than air and pools. If you get the muzzle flash in a pool, at the interface between it and the ambient atmosphere, why would it not ignite?

Muzzle flash is substantial. Large bore, short barrel pistols emit huge amounts. If a tiny spark from a non-intrinsically safe switch can ignite it, I'd bet a dollar to a donut that you could get a pistol to do it, too.

Glock muzzle flash
posted by FauxScot at 4:49 PM on May 7, 2008

Propane gas is heavier than air and pools. If you get the muzzle flash in a pool, at the interface between it and the ambient atmosphere, why would it not ignite?

Pretty certain it would ignite at that point. Bullets through a propane canister just don't really create a spark. *ahem* In the bad old days, we used to put a lit road flare next to out targets.
posted by oneirodynia at 7:02 PM on May 7, 2008

Gunpowder (modern smokeless) in a gun doesn't explode; it simply burns. Ideally the combustion finishes just as the bullet exits the barrel, but usually with handguns the barrel is too short and the gunpowder is still burning when the bullet exits. That flame can ignite surrounding fuel sources.
posted by ikkyu2 at 10:00 AM on May 8, 2008

Several SWAT teams in the US issue suppressors for the longarms when expecting to be in locations where high concentrations of airborne flammables are expected*. They also would use neither smoke nor flash bangs in these situations. Any firearm expels gas from the barrel (or the bullet wouldn't exit in the first instance). As to the temperature of that gas - it will depend on the ammunition, barrel length etc. It is also worth noting that with some exceptionally short barrelled weapons that the powder in the cartridge will not have fully burned through and may be expelled from the weapon as it is still burning.

*particularly an issue in meth labs apparently.
posted by longbaugh at 9:54 AM on July 17, 2008

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