Information on Ex Protection

When dealing with substances which can react with oxygen the risk of an explosion has always to be expected if the combustible is present within a room volume at a certain partiacal pressure or as fine-grained dust in the air. An explosive mixture of gas and air exists when the proportion of the inflammable gas or of a vaporized liquid is between the lower (LEL) and the upper explosion limit (UEL). Dusts must have a sufficiently small grain size and a minimum density to cause the appearance of an explosive atmosphere.
An explosion is defined as a sudden oxidation or decay reaction generating an increase in temperature or pressure or both simultaneously (ISO 8421-1, EN 1127-1). An explosion is only possible at a certain mixing ratio of combustible substances (gas, dust) and air. During dust explosions the grain size constitutes another important parameter. The smaller the grains, the larger the surface – and thereby the speed of reaction increases.

The maximum explosion pressure is attained at a stoichiometric ratio of combustible gas and air. The maximum explosion pressures of  hydrocarbons and air lie between 8 and 10 bar. The more the composition of a combustible gas and air mixture deviates from the stoichiometric ratio, or a gas which does not take part in the reaction (inert gas) is mixed in, the less is the temperature and pressure increase in case of an ignition. When the temperature is not high enough anymore to form radicals for the reaction, an explosion cannot continue to spread.

Since 1st July, 2003, equipment and protective systems intended for use in potentially explosive atmospheres have to comply with the Directive 94/9/EC (ATEX 95). The European Community created with this directive a basis for binding product, installation and maintenance requirements in regard to explosion protection of systems, equipment and components. Due to differing international conditions, for manufacturers of machines and installations as well as for their customers a number of changes become effective.