The basics of explosion protection - STAHL - #6

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2. The Basic Physic Principles and Definitions of Explosion Protection

An explosion is the sudden chemical reaction of a flammable substance with oxygen with the simultaneous release of high energy. Flammable substances may be present in the form of gases, vapours, mists or dusts. Explosion can only occur, when three factors come together (fig. 1):

1. Flammable material (in ignitable quantities)

2. Oxygen (in the air)

3. Ignition source

Certain characteristic properties of these materials are required for safety considerations. The flash point of a flammable liquid is the minimum temperature at which a liquid gives off vapour in sufficient concentration to form an ignitable mixture with air near the surface of the liquid (at normal air pressure). If the flash point of a flam­mable liquid is well above the maximum tempera­tures that arise, an explosive atmosphere can not be formed. The flash point of a mixture of various liquids may be lower than that of the individual components. In addition to the boiling point, the flash point of a liquid serves to classify liquids as highly flammable, easily flammable, and flammable liquids in the Council Directive 98/24/EC "risks related to chemical agents".

Fig. 1: An explosion can only occur, when these three factors come together

To form an explosive atmosphere, the flammable substance must be present in a certain concentra­tion (fig. 2).

If the concentration is too high (rich mixture) or too low (lean mixture), no explosion occurs. Instead, there is just a steady-state combustion reaction or none at all. It is only in the range between the lower and upper explosion limit that the mixture reacts explosively when ignited. The explosion limits depend on the ambient pressure and the proportion of oxygen in the air (table 2).

Table 1: Classification of flammable liquids

R.STAHL explosion protection