Flash - instant combustion of a mixture of air and vapors of flammable substances , accompanied by a bright short-term glow. There is no sustainable burning. Flash point - the minimum temperature of condensed substances at which vapors are formed above their surface, flashing up when a spark, flame or incandescent body appears.
Liquids belonging to the category of flammable possess the ability to flare up at relatively low temperatures. The maximum flash point of such substances in closed crucibles is + 61 ° C, in open - + 66 ° C. Some substances are able to ignite spontaneously, reaching the ignition temperature characteristic of them.
The determination of saturated vapor pressure is possible for any flammable liquid. It increases in proportion to the increase in the temperature of the substance. As soon as the flash point reaches a critical (maximum) value, it is possible to maintain combustion.
However, the onset of vapor-liquid equilibrium will require some time, which is proportional to the rate of vapor formation. Sustainable combustion can be achieved by reaching a certain (for each individual substance) ignition temperature, since the combustion temperature is always higher than the flash point.
A direct change in temperature at which substances flare up has certain difficulties. Therefore, the flash point is considered to be the temperature of the walls of the reaction vessels in which this flash is observed. The temperature depends directly on the conditions of the ongoing heat transfer inside the vessel itself, on its catalytic activity, on the environment, on the volume of liquid in the vessel.
The indicators of the experiments allow us to calculate the temperature of flammable substances acceptable for transportation and storage. The flash point in closed crucibles is measured, as a rule, according to Pensky-Martens. In open crucibles, it is refined experimentally.
Especially dangerous are liquids that can flare up at temperatures below –18 ° C in closed crucibles, lower than 13 ° C in open crucibles. Liquids are considered to be permanently hazardous, the flash of which is possible at a temperature of + 23 ° C in closed crucibles and up to + 27 ° C in open. Temperature indicators of hazardous liquids are up to + 60 ° C inclusive with closed crucibles, up to + 66 ° C inclusive - with open.
The difference in flash, ignition and combustion temperatures varies significantly, and it is individual for each substance. The flash point of diesel fuel, for example, is no more than + 70 ° C. Its burning temperature is + 1100 ° . Ignition temperature - from + 100 ° to + 119 ° . But the flash point of gasoline, due to its very high volatility, is + 40 ° C, and sometimes even less. Its ignition temperature is + 300 ° C. The figures for gasoline are somewhat generalized. They should be considered average, since there are various types of gasoline (automobile (summer, winter), aviation) with significantly different characteristics and, accordingly, different flash points, ignition, and combustion.
Combustion is a process accompanied by the release of a large amount of heat with a characteristic light emission (glow), which is possible when a temperature is reached for each substance and when oxygen or other substances (sulfur, bromine vapor, etc.) are accessible to it.
Explosions are considered the most dangerous, characterized by an instantaneous chemical reaction with the release of huge energy and carrying mechanical work. Explosion fire can spread to 3,000 meters in one second. Burning the mixture at this speed is called detonation. Shock waves resulting from detonation often cause significant damage and accidents.