The flash point is a term often used by fire professionals as the measuring stick of a fuel’s safety. Unfortunately, the flash point is deceptive in this duty and occasionally is counter-productive in preventing fires. Many people think that flash point is the temperature at which the fuel can be ignited, or the temp at which the fuel begins to produce ignitable materials. Neither is fully accurate.
When the term was coined, flash point was used to determine the temperature at which any substance would flash in the presence of a flame. This was applied to wood, piles of garbage, stuff that leaked out of the ground, anything. Later, it was discovered that the “flash” was actually the ignition of volatile gasses. In modern times, this property is still determined for a variety of substances, but for petrol fuels, this point is established by putting a fixed quantity of a petrol mix in a specifically sized cup, in a large sealed jar (or testing area). The mix is slowly heated until they get, you guessed it, a flash. So the flash point is the temperature at which a substance will produce enough volatile vapors, from a standing liquid, to ignite in a closed space, at fixed pressures.
So, where do things fall apart? Let’s take it step by step. The phrase “enough volatile vapors” is the first thing. Just because a fuel isn’t at its flash point temperature doesn’t mean it’s not producing volatile vapors. Most petrochemicals are producing vapors constantly. Low flash point fuels, like Naphtha, do indeed produce copious amounts of vapors, but the vapors generally disperse quickly enough in open air to keep from reaching a self-perpetuating density. When high flash point fuels manage to produce vapors in quantity (in the presence of cotton balls, fur, hair, or in that vapor cloud you just spit out), they usually cannot move through air quickly and will remain in concentrated levels so that “enough volatile vapors” exist in any area of the cloud.
The next phrase, “from standing liquid” is another issue. Liquids convert into vapors differently depending on the amount of surface area they have in relation to air pressure. Standing liquid is a very low surface area state. If you change the surface area of the fuel, however, the rate at which it can push vapors into the air changes as well. No difference is seen in fuels that are already producing vapors, however, high flash point fuels (like lamp oil) produce vapors much more quickly than usual in the presence of some form of wick, and can radically change how fast they get to “enough volatile vapors” to ignite. One of the highest surface area wicks you will ever find is a feather, just short of that is a fuzzy cotton cloth or a bit of lint.
The next issue is with “in a closed space”. Some fuels have to fill an area equally until the entire area acquires enough density to form an ignition, others produce clouds that are already dense enough to light. Low flash point fuels like [[Naphtha]] only have enough density of gas to ignite in the immediate area of the liquid until they’ve filled the entire room with gas. High flash point fuels produce clouds that tend to remain in sufficient density to ignite at great range. If your fuel is not locked in a room, but outside or near an open window, low flash point fuels will continue to disperse and will only ever have an ignitable density near the source. Higher flash point fuels might still have dense clouds even if outside.
Finally, like wicking, the localized pressure can greatly affect even standing liquids. Once again, this is only really noticed in high flash point fuels. Let’s say you have a cup of lamp oil in a closed room. Someone opens the door outwards, and the barometric pressure of the room temporarily changes pretty drastically, causing the fuel to release a small cloud of vapors. Once atomized, the low vapor pressure of the substance keeps the cloud from expanding. You now have an accident waiting for a cigarette, static spark, or electrical device (like a phone) to set it off. With low flash point fuels, you’re likely to smell the vapors long before they reach a density needed to ignite, high flash point fuels tend to hang in invisible clouds that can become problematic.
With all this as a given, the most dangerous fuels are the ones that have a flash point right around room temperature. Not only can they form vaporous clouds from unexpected circumstances, but the ‘flash’ from igniting the vapor cloud might be enough to get the fuel burning in its storage container. The fuels that have this level of flash point tend to be kerosene, low grade gasoline, lighter fluids, etc. Regardless, all fuels should be handled as though they readily produce vapors at ambient temperature, quickly fill an area with explosive fumes, and ignite readily with the capacity to flash-ignite a fuel source of standing liquid. If your fuel isn’t heavily purified and consistent in quality, this may be exactly what you have.