What Is an Explosion?

Ex|plo|si|on; -s <lat.> (explosio) means "escaping under pressure." What happens during an explosion, which factors are important and what protective measures are available? You will learn everything you need to know about explosion protection right here.

ISO 8421-1, EN 1127-1 defines an explosion as “a sudden oxidation or decomposition reaction with increase of temperature, pressure or both at the same time.” This refers to a chemical reaction that with a simultaneous convergence of oxygen (air), a flammable material and a source of ignition in a particular ratio, causes temperature and pressure to increase abruptly. If the heat that arises cannot be discharged quickly enough, there is a sudden volume expansion of the concurrent gases and a release of great heat energy accompanied by a pressure wave: the explosion.

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Principles of Explosion Protection

Everything worth knowing about Ex protection at a single glance.

Severity of the 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.

Prerequisites for an Explosive Atmosphere

The following factors must be present simultaneously for an explosion to occur:

  • A flammable material in the production process or surroundings
  • Oxygen (air)
  • Ignition source
  • A particular ratio between oxygen and flammable material

The knowledge that an explosion only arises as soon as an atmosphere subject to explosion and a source of ignition are available simultaneously is relevant for explosion protection.

Combustible Materials

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.

Flash Point and Explosion Limits

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.

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