Mechanical sparks and friction are one of the most common causes of ignition of flammable gas and dust clouds. Mechanical sparks can arise due to processes involving friction, grinding or impact and can occur e.g. in case of disturbances in rotating equipment causing the rotating parts of the machinery (repeatedly) to come into contact with each other at relative high speeds. Also, application of hand tools such as drills, grinding machines, welding and cutting torches and hammers can lead to the generation of mechanical sparks. Single impact sparks may arise due to collisions between two objects.
Accident statistics concerning dust explosions occurring in Germany indicate mechanical sparks and friction as the ignition source in 32.7 % of the incidents (Jeske and Beck, 1997). Billinge (1979) presented an overview of 66 incidents where gases or vapours were ignited by something referred to as frictional ignition. 68 % of the reported incidents were caused by impact, 20 % by friction and 12 % by cutting and grinding operations. Generally, speaking it is unclear whether the ignition occurred by mechanical sparks or by a hot surface generated by the impact/friction. The fraction of gas and vapour explosions ignited by mechanical sparks and frictional heat is unknown.
Mechanical sparks are caused by friction, rubbing or single or multiple impacts of objects of similar or dissimilar materials causing small parts of this material to be torn loose. These small pieces of material are due to the energy absorbed hot and may during their flight start burning resulting in very high temperatures. Melting and evaporation of this mechanical spark may cause it to burst increasing its surface area and thereby its incendivity. This article reviews work performed addressing ignition of gas and dust clouds by mechanical sparks and friction. The review starts from the work performed in connection with coal mine explosions performed in the 1950 s and 1960 s and works published since then. The majority of the reviewed literature concerns experimental work but also theoretical studies have been included in the review.