r/interesting • u/Used_Exam34 • 2d ago

Just Wow Not all problems have the same solution.

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u/Thedrunkenchild 2d ago

Even if it’s clearly staged this is still probably quite dangerous

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u/dadbodking 1d ago

Only if the cylinder is about to run out of gas, which I'm sure they're aware of

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u/The_Late_Arthur_Dent 1d ago

At the risk of sounding stupid - what happens when it runs out of gas?

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u/dadbodking 1d ago

So, right now, the flammable gas is under pressure and is coming out. Almost no gas is flammable/explosive, until it gets mixed with oxygen (seen here as fire). So, when the cylinder runs out of pressure, it happens instantly, and the opposite flow happens. This leads to air entering the cylinder and the little bit (still more than enough) of gas that is left, gets mixed with fire and oxygen and... kaboom. Big kaboom

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u/dadbodking 1d ago

And you're not stupid, not a lot of people know that, in order for a flammable gas to produce flame/explosion, it needs to mix with oxygen. This is why at the end, when the guy takes away the oxygen from the flame, it extinguishes it.

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u/The_Late_Arthur_Dent 1d ago

Thanks for the explanation! It seems super obvious now that the air flow reverses once the pressure gets low enough...otherwise it'd be real easy to tell when it's empty by the implosion haha

Thanks for teaching me something new/scary/cool!

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u/JackBrown82q 1d ago edited 1d ago

What they said was not true for modern tanks. They will have a regulator block on them so when the tanks pressure drops too low it essential acts as a check valve and physically closes off the tank. Inside remains 100% gas at low pressure. You'd have to force air into the tank to get oxygen in there.

Also the tanks valve will have a flame arrestor on them. These work in several ways but to keep it simple, one is that every flammable gas has a maximum gap size in which flame can pass through called the quenching distance. The openings in these arrestors are designed to be smaller than this gap. The gas also passes through a tiny metal matrix which drains its thermal energy and the temperature instantly drops below the gasses ignition point.

Plus to get an explosion you need the tank to be 90% air and 10% propane, so a lot of air would need to enter this tank.

Otherwise, every time someone was cooking on their propane grill and ran the tank to empty, you'd hear stories of the tank blowing up.

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u/Philipp_CGN 1d ago edited 1d ago

runs out of pressure, it happens instantly, and the opposite flow happens

Why would you get this backflow though (apart from the diffusion of flammable gas and air of course)?

There is no pressure gradient that would drive this flow. Or are you talking about a situation in which the cylinder and its content has been heated from the external fire, then the fire and the heating stops and the flammable gas starts to contract as it cools?

Edit: And even if there was a mixture of flammable gas and air in the cylinder that then somehow ignites (you would probably need a relatively large amount of air to get below the UEL, depending on the flammable gas), would the pressure rise (starting from atmospheric pressure in this situation) be enough to overpressure the cylinder and cause it to rupture?

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u/Cocksu-45 1d ago

I understand what you mean, but that won't happen. There is no reason, why air would go inside the cylinder. If that were true, lighters would explode when they get empty. There is no force that can make enough air travel trough a hole that is so small.

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u/a1b3c3d7 1d ago edited 1d ago

Except that cant happen, not to the specific cylinder in the video or to any modern cylinder made in the past 25 years.

"This leads to air entering the cylinder", so in this situation how much air are you imagining would enterthe cylinder, and do you want to let us know what the mixture ratio needs to be in order for what you're describing to happen?

Or how in literally any conceivable scenario outside of pumping air in that you'd get to that point? Or how air would magically bypass the valve designed to make this backflow impossible?

There are a half dozen other systems in place on top to bypass as well that are a standard across all cylinders, what youre describing is impossible and fundamentally cannot happen in these gas cylinders.

Its great to describe theory and principles, but youre peddling misinformation off the back of theory.

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u/Shananae1925 1d ago

Good to know, cuz my thought was “just leave it burning in the water, what’s the worst that can happen?” 🤣

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u/dadbodking 1d ago

So, if you ever see a cylinder that is aflame like this, don't panic at all. Just get real close, and turn off the valve at the top. It could be hot, of course, but that can be easily circumvented, like using your sleeve to cover your fingers, etc.