arpadveress As a result of friction, impact or abrasion processes such as grinding, particles can become separated from solid materials and become hot owing to the energy used in the separation process. If these particles consist of oxidizable substances, for example iron or steel, they can undergo an oxidation process, thus reaching even higher temperatures. These particles (sparks) can ignite combustible gases and vapours and certain dust/air-mixtures (especially metal dust/air mixtures). In deposited dust, smouldering can be caused by the sparks and this can be an ignition source for an explosive atmosphere.
The ingress of foreign materials to equipment, protective systems and components, e.g. stones or tramp metals, as a cause of sparking shall be considered.
Rubbing friction, even between similar ferrous metals and between certain ceramics, can generate hot spots and sparks similar to grinding sparks. These can cause ignition of explosive atmospheres.
When stainless steel is involved in impact, friction or grinding it will easily produce hot surfaces which may become an effective ignition source. Applying high contact pressure in case of friction or grinding a burst of sparks will be created additionally.
Impacts involving rust and light metals (e.g. aluminium and magnesium) and their alloys can initiate a thermite reaction which can cause ignition of explosive atmospheres.
The light metals titanium and zirconium can also form incendive sparks under impact or friction against any sufficiently hard material, even in the absence of rust.
If hazards due to mechanically generated impact, friction and grinding have been identified, dependent on the type of explosive atmosphere (gas/vapour/mist or dust as the flammable substance) and from the category the following specific requirements for equipment, protective systems and components shall be complied with:
— Category 1: Equipment, protective systems and components which, even in the case of rare malfunctions, can give rise to incendive friction, impact or abrasion sparks or rapidly generated hot surfaces, shall be excluded. In particular, friction shall be avoided between aluminium or magnesium and iron or steel. Friction and impact between titanium or zirconium with any hard material shall be avoided.
— Category 2 The requirements for Category 1 should be complied with whenever possible. Sparks or rapidly generated hot surfaces shall be excluded in the case of normal operation and in the case of malfunctions.
— Category 3: It is sufficient to implement protective measures against incendive friction, impact or abrasion sparks or rapidly generated hot surfaces during normal operation.
— All categories: Equipment intended for use in explosive gas/air, vapour/air and mist/air atmospheres which can produce mechanically generated sparks or rapidly generated hot surfaces shall be excluded if the possible explosive atmosphere can contain one or more of the gases acetylene, carbon disulphide, hydrogen, hydrogen sulphide, ethylene oxide, unless there is proven evidence that there is no explosion risk.
The requirements for tools which can be present in explosive atmosphere should be in accordance with Annex A (EN ISO 80079-37).
It is possible in some cases to protect light metals from mechanical contact with rust by coating. If coated with non-conductive materials such as plastics precautions against static electricity can be necessary. The coating should not contain high percentages of aluminium.
The likelihood of mechanically generated incendive sparks or rapidly generated hot surfaces can be reduced, for example, by wetting. Possible reactions with the wetting medium should be considered, see EN ISO 80079-37.
