Flammable vapors in metal tubes: a dramatic demonstration

[Mr Happy]

When there are stories of aluminum flashlights exploding I am reminded of the following rather interesting and eye opening demonstration. Do you think your lithium ion cells containing combustible organic solvents and sealed inside a metal flashlight tube might lead to an explosion as powerful as gunpowder? Well no, not quite. Have a look:

http://www.youtube.com/watch?v=DO7R7hneQDc#t=2m50

It is true that demonstration was done with gasoline, but in practical terms the difference between gasoline vapors and the solvent vapors escaping from an overheating lithium ion cell is academic. Both of them can burn with the release of significant energy.

One should think really carefully about putting lithium ion cells and air inside a metal tube and screwing the end on tight. How many commercial lithium ion battery systems are constructed that way?

 

[Conan]

I remember reading an Eneloop warning about not using it in a sealed tube saying that it was not meant for that use. So in effect making the flashlight "sealed" is actually the source of all flashlight related "incidents"?

 

[Mr Happy]

Not all incidents. But making a flashlight sealed and then using cells that can vent gases (like even NiMH) is not a good idea. When using cells like lithium ion that contain combustible organic solvents it is a doubly bad idea.

Most lights should contain small vent holes to equalize any pressure buildup inside. This would avoid the "tailcap bubble" phenomenon sometimes seen in NiMH lights. Lights for lithium ion, especially monster custom lights, should contain large and open vents so that any escaping gases can disperse rapidly.

 

[Apollo Cree]

I think the comparison is not very valid. You need volume for the oxygen as well as the correct amount of fuel. If you dropped the teaspoon of gasoline in a flashlight sized tube and tried to make a mortar out of it, it wouldn't be nearly so dramatic as the gasoline in the big tube in the video.

Throw in the fact that most flashlight bodies are not only much smaller in volume than the tube in the video, most of the volume inside the flashlight is taken up by batteries and other things other than air. Imagine if you filled the interior space of the flashlight up with water and then poured it out. How much volume of water would you get.

Take the volume of air packed into the flashlight, make a perfect stoichiometric ratio with gasoline to get the maximum amount of boom, and you really don't have a lot of boom available.

However, that doesn't say a flashlight won't turn into a virtual pipe bomb. I'm saying that combustion of the air in the flashlight probably doesn't contribute significantly to the explosion. The problem is the chemical reaction of the chemicals within the battery itself. It's more akin to a gunpowder bomb than a piston in an engine. A gunpowder bomb does NOT require any atmospheric oxygen.

I'm not sure I believe a sealed flashlight body is going to make a much bigger boom than an unsealed flashlight body unless it's sealed in a way that will hold a lot of pressure and volume for a short period of time. I think a potentially explosive battery event is going to release a lot of gas over a short period of time. In order to turn the flashlight into a pipe bomb, the flashlight body has to build up enough pressure to cause pieces of the body to break off at high speed.

A small vent hole in a flashlight body will probably not vent enough gas fast enough to make a difference. You'd need something like the entire head or front glass blowing off before pressure gets too high to make a difference.

There's also a secondary "pipe bomb" concern. Some chemical reactions, such as gunpowder, proceed much faster and completely when they're confined. Gunpowder in a gun cartridge may burn over the course of a second or so if you take out the bullet. Put the bullet in and it all burns in a millisecond or so. A sealed flashlight might make the exploding battery release more energy than the battery sitting in the open. I'm not sure whether the chemistry in a lithium battery works that way, but it's possible.

 

[Mr Happy]

You points are true. Nevertheless, in some cases like a "monster" size body where the inherent volume is larger and cells are packed side by side, there is more air available for combustion and this might be significant.

My feeling in that case is that the body should more more like an open cage and less like a sealed pipe.

The demonstration in the video is not exactly the same as a flashlight, but it should serve as a cautionary warning about the power of vapor/air explosions. Many people may not even think of the possibilities, let alone the probabilities.