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In a quickly upvoted comment to this answer regarding propane tanks, community member crip659 wrote:

I would change "any level" to ever contained. Empty containers can be as bad(deadly) as containers with some left in. Also know what was in it and if it can be explosive. Who knew an empty peppermint oil could cause life changing injury/death.

Why can empty tanks or containers be so dangerous? What precautions are best to take when working on or with them?

Answers covering as many types of tanks/containers are encouraged, including propane, natural gas, and septic.

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3 Answers 3

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The basic one common to all is that they can accumulate gasses you can't breathe, or smell. Without active ventilation (a blower, run for some time before entering and all the time a person is inside) they can accumulate a bunch of dead people, as one goes in to save the one before and they all die (rarely one lives) before someone has the sense to stop going in.

Some of those odorless gasses are also explosive, given some oxygen (recall that odorants are added to piped methane in gas distribution - that's the way piped gas smells, not the way methane doesn't.)

As for things like cutting oil drums from the outside, the local welding supply had a gallery of "what happens when people do that without taking the proper precautions" and the fumes ignite. The combination of air, fuel, and a contained space turns them into low-grade bombs.

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    See also diy.stackexchange.com/a/223513/18078 for more detail on old septic tanks.
    – Ecnerwal
    Apr 7 at 23:13
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    + one important thing: no matter what the tank is (supposedly) used for, deadly and/or explosive gases can emanate and build up in the tank from pretty much innocent substances like the tank material, well or utility water, food stuff (milk, wine, grain, whatever) and so on. Known bad things like sewage, general waste, etc... are of course out of question.
    – fraxinus
    Apr 8 at 7:21
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Filling any container completely/over fill that might have held explosive/flammable contains with water or non explosive gas(CO2) before cutting/doing something that might cause sparks, should be the first thing.

It is the fumes left in the container that are dangerous, so you make sure all fumes that might be in the container is removed.

One link to the dangers, https://projects.jsonline.com/news/2017/12/13/dangerous-drums-peddled-to-unwitting-public.html

Something relativity safe as an empty plastic soda/pop bottle(no deposits,need to take to the dump) with the cap on, can act like a rocket if placed/toss into a fire. If you have a hold my beer moment and watch it burning, you might end up with burning plastic on you.

This is for mainly sealed containers that tend to go boom in your face. A container that you can remove the full top, might only singe you instead of blowing the top off towards your head(and taking it off). The movie Jaws has a good scene showing what might happen at the end. Might be over the top, but close enough to not say hold my beer.

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  • Well, pressure release and flammability are different beasts, although both can hurt you.
    – Huesmann
    Apr 8 at 12:53
  • I know a man who lost his eyes processing scrap metal. An unpunctured aerosol can in the scrap exploded and sprayed him with liquid metal.
    – MackM
    Apr 9 at 13:13
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For a true explosion to happen, you need the right mixture of flammable gas and air inside a container. When the air/gas mixture is ignited, it expands quickly, breaking the container and thus the explosion. If you just shoot a full propane tank, the tank will empty through the bullet hole. Don't believe me? Look at this video at 6:37: https://youtu.be/sqFr_M9E808?si=6MS2qmG4cm-scEQR&t=397 . Notice that the gas does not ignite until it leaves the tank AND other bullets create a spark outside of the tank. If you watch the whole thing, notice that there's a lot of fire, but no true explosion.

As a second example, read about this Darwin Award genie who decided to cut into a propane tank. Although the article speaks of a "blast", what happened was that the tank flew up propelled by the hole he made to the tank, but there was no true explosion.

This is still dangerous, of course, as now you have propane in the air, which can ignite given a source. But if it is not ignited, it the air/gas mixture will go below the critical level and stop being dangerous.

An empty tank with some flammable liquid residue inside, however, is another matter. There you have an air/gas mixture inside a container. If you were to penetrate the tank with a grinder, you would be introducing sparks inside a closed container, ignite the mixture, the mixture then would expand rapidly and the container explode.

In my neighborhood, a guy (it's always a guy) tried to repair an old truck-mounted gasoline tank by soldering. He thought that because the tank was empty, he was in the clear, but it was the gas fumes he should have been worried about. I don't know if they found all the pieces for the burial.

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    Most fuel tanks which aren't essentially empty will have an equilibrium fuel-air mixture within the tank that will be too rich to sustain combustion until mixed with outside air. If the tank is almost empty, however, there may be enough air inside to burn all of the fuel without wasting energy on fuel that isn't going to have any oxygen immediately available for combustion.
    – supercat
    Apr 9 at 3:12

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