What Is an Explosion?

The severity of an explosion depends on the properties of the flammable materials and the mixture of these materials with oxygen: the oxygen present in the air burns only with a particular quantity of the flammable material (oxidation). Depending on the severity of an explosion and the associated spread speed of the pressure wave, a distinction is made between low-speed detonation (cm/s), deflagration (m/s) and the strongest type of explosion, the detonation (km/s). The most severe of the explosions spreads at a rate faster than the speed of sound, thus developing immense destructive power.

Flammable materials include vapors, fog, gases and dust. These can arise accidentally during the manufacturing and production process or during transport or storage. Dust from materials that are broken up for further processing is particularly common in industrial areas. Dust explosions can have effects just as devastating as gas explosions: the gas-air mixture spreads quickly during an explosion, thus reducing the concentration of flammable material (lean mixture). Further combustion, then, is no longer possible. By contrast, the dust-air mixture raises additional dust layers during an explosion that can ignite. However, the explosion of a gas-air mixture can also stir up dust, which then transforms the gas explosion into a dust explosion.

Flammable materials mixed with oxygen are only flammable in a particular ratio and combust as soon as a source of ignition is present. Here, the flash point of a material and its explosion limit play the decisive role.

Flash Point

“Flash point” describes the lowest temperature of flammable liquids at which a flammable vapor-air mixture forms. The flash point in this hybrid mixture may be lower than that of the individual components. With these vapor-air mixtures, the concentration ratio decides whether an atmosphere capable of explosion can form or not. This describes the explosion limits of individual materials: Each flammable material has a particular range as mixture with oxygen in which there can be an explosion. With concentrations that are too high (rich mixture) and with concentrations that are too low (lean mixture), an explosion does not take place but a stationary reaction or no combustive reaction at all. The mixture only reacts in an explosive manner when ignited in the range between the upper and lower explosion limit.

Explosion Limits

Explosion limits, however, are dependent on pressure, temperature and oxygen concentration. Further, there are also chemically unstable substances such as cesium, rubidium or even white phosphorus, which ignite just from contact with oxygen or air; these substances are referred to as “pyrophoric.” Especial caution is called for when handling these substances. This also applies to dust accumulations. In this case, the danger of self-combustion increases with the thickness of the accumulation. The insulating effect of the dust can cause heat to build up, leading to self-combustion. Information about the precise flash points and explosion limits of a material is described in the appropriate safety data sheet. If the formation of an atmosphere subject to explosion is possible, this is pointed out in the document.