[Targan]

Originally Posted by kato13

The Mythbusters detonated one using incendiary bullets. The explosion was not that large but the flames were substantial. My initial thoughts would be equivalent to a frag grenade plus a double or triple radius Molotov cocktail.

You've hit the nail right on the head there Kato. As some of you will know I have some knowledge in the area of explosives and after reading this thread today I spent a while looking up the specific data relating to the properties of LPG.

If you crack an LPG bottle it will probably explode but that is due to the contents being under pressure and not due to the explosive force of the fuel inside. This is because LPG needs oxygen to combust and there is little or no free oxygen inside the pressure vessel. LPG's gas to liquid volume ratio is about 250:1 (!) so when it is released from a pressure vessel it expands tremendously but in the event of a catastrophic failure of a pressure vessel (as opposed to a leak) Wikipedia describes the effect thusly:

Originally Posted by Wikipedia

If someone were to burst the container by any means, the LPG would first spread out as a supercooled liquid. This would freeze anything within range. Then it would boil into the atmosphere and become an oxygen-displacing gas, which would asphyxiate any living creature in the affected radius. This gas would spread out to cover several hundred times more area than the liquid from which it comes. At some point this gas is diluted by the atmosphere. It will then reach a point of an ignitable mixture.

The 20lb LPG bottle typically used to fuel portable barbecues etc is probably the LPG container PCs are most likely to find in the US. They have a capacity of 4.73 gallons of LPG but are often only filled to around 4 gallons to allow for thermal expansion of the contained liquid. At 68 °F (20 °C) LPG only need to be under 2.2 bar of pressure to remain a liquid but that increases to 12.2 bar at 100 °F (37.8 °C) and 22 bar at 131 °F (55 °C) so obviously it isn't a good idea to let your gas bottles get too hot.

So to cut to the chase here is what I think the effects of a 20lb LPG bottle exploding would be. Initial concussion would be relatively modest (non-lethal) and similar to a concussion grenade. Fragments from the pressure vessel would be fewer in number than, say, a 120mm mortar round and would travel at much slower velocities (damage and penetration of a thrown knife or shuriken perhaps?) but would fly out to a similar distance as a mortar fragment because of their greater mass. Anyone adjacent to the explosion would be at risk of suffering burn-like injuries from being splashed with liquid propane. If the bottle ruptured in a confined space (and the fragments failed to punch sufficient ventilation holes) the displacement of air by propane gas may result in anoxia within a round or two.

In the event that the cause of the rupture included an incendiary effect, or there is an ignition source within the immediate area, the expanding gas fuel will ignite within a second or two to perhaps a few rounds resulting in a flash fire. The fire will be non-persistant and relatively low temperature (it might ignite easily flammable objects like paper or unsealed combustable liquids but probably not clothes, ordnance or ammo unless the fire is in a small, contained space) but it will cover a wide area and start close to the ground. The thermal shock will act as a low explosive blast if it is in a contained area (inside a vehicle or intact building), blowing out or cracking windows and popping canopies etc but the concussion wouldn't be particularly debilitating for people. The flash fire will lower the oxygen levels in the air, temporarily blind those in the affected area and exposed skin will suffer at least first degree burns.

Edit: The Mythbusters blew up a 100 lb gas bottle by the looks of it. Naturally that is a substantially more fearsome event than what I described because it involved five times more gas. I suspect it would have an even more catastrophic effect than what it looked like in the video if it was detonated inside a vehicle or building, but less available oxygen may slightly retard the flash fire effect.

Edit #2: I'm thinking that puncturing a gas cylinder with API rounds would have a different result compared to a cyclinder catastrophically failing due to corrosion, adjacent explosion or exposure to a fire. In the event that any of the latter events occur what I wrote above should hold true. In the case of the Mythbusters video the gunner walked his fire down onto the top of the cylinder (the top is the weakest point because that is where the valve is screwed into an otherwise contiguous structure). I think the cylinder in the video was either punctured had the valve shot off, ruptured across the top or all three and some of the escaping gas ignited while there was still liquid and gas inside the cylinder. In a case like that there would only be fragments heading skywards and the remainder of the cylinder would act like a giant blow torch. If it was inside a building or vehicle I'd say the roof/ceiling and probably anyone in the immediate vicinity would be toast but at least anyone further away wouldn't be hit by shrapnel or the blast.