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Page history last edited by Tedward 14 years, 3 months ago

Section II: "explosive" vapors.


In the last section we said that low flash point fuels burn directly from liquid. Well, let's clarify that statement first. No liquid fuel burns. None. In order to burn, any fuel must be converted into a vapor, mixed with oxygen, and ignited. This principal forms what fire safety folk call the fire triangle. Eliminate just one of those three things, and your fire goes out.


Okay, so just the vapors burn. But the fuel comes as a liquid. So how does the liquid become gas? Good Question. The answer is: it's ALWAYS trying to convert to a gas. In fact, most liquids are trying to convert, it's just dangerous when volatile chemicals manage to do so. Of course, the word volatile is also a bit subjective, even water vapor is capable of burning under the right conditions, it's just that it requires conditions similar to the surface of the Sun. The ones we're talking about work at room temp in earth conditions.


But if liquids are always trying to convert to gas, why are there any liquids at all? The simple answer is air pressure. If you have a glass of water and you leave it standing around it will (very) slowly evaporate. The reason it doesn't happen faster is that the pressure of the air pushes back the water molecules and condenses them back into water. The water is just a little bit more effective at getting stray molecules to begin the migration out into the atmosphere.


Petrol fuels are also trying to get out into the atmosphere, and their success is largely based on their size. Low flash point fuels have generally smaller molecules and provide more force against the air. High flash point fuels have longer carbon chains and aren't able to provide as much pressure against the air.


Naphtha is an extremely low flash point fuel. It's very good at slipping molecules out into the air and once they're out, they keep going. It doesn't need any help vaporizing so it lights quickly, burns on any surface (even running water since it floats), and it doesn't hang around once it's free, it keeps expanding out... rapidly.


Lamp oil is on the other side of the spectrum, it generally has slow-moving large molecules that have a very hard time getting vaporized. Once vaporized, they don't move around much. You can see this effect when you blow out a candle: the remaining heat carries off a visible stream of paraffin molecules that can even be re-lit in mid air.


Okay, now we're ready to get to the explosive part of our various vapors. "Explosive," in the way that pyrotechnicians use it, does not apply to petrol fuels. The chemical term, Explosive, refers to a reaction capable of converting chemicals at a certain speed which is much greater than our fuels are capable of producing. On the other hand, there's a more common use of the term "explosive" meaning that they can produce a whole lot of flame at a very rapid rate. In order to do so, however, they have to produce enough vapors.


Since White Gas is so good at moving through air, it's vapors will continue to expand until it fills its environment. If it's outside or has an open window, White Gas will passively return to nature. In a closed room however, it will fill the area with vapors until it runs out of liquid, or the density forces it to compress back into liquid. This is more than sufficient density to ignite into a self-propagating flame. Once you have this much white gas in an area, any spark, flame or intense heat source can ignite fuel and cause an explosion.


Higher flash point fuels aren't as efficient at moving molecules through the air, however, subtle changes in the environment can cause them to create a bubble of vapors that could flash back to the source. If a feather, fur, or some lint were to fall into a fuel source it could start producing vapors at a substantially higher rate than normal. Similarly, exposure to sunlight, shifts in pressure, general heat sources, etc, can change their character. The lower the flash point of the fuel, the more likely this can happen. With a large enough 'bubble' not only could it burn back to the source, but it could create the same kind of explosion as white gas.


Thus ANY fuel in an open container could easily become an "explosion" danger in a closed environment. In open environments, the low flash point fuels will still ignite if a flame source gets within a few inches. High flash point fuels, however, always tend to stay in clumps or bubbles once they get vaporized. These clumps could flash back to the source from great distances.



FuelBusters III: FlashPoint


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