Citation

Pedersen GH, Eckhoff RK. Fire Safety J. 1987; 12(2): 153-164.

Copyright

(Copyright © 1987, Elsevier Publishing)

DOI

unavailable

PMID

unavailable

Abstract

Several independent investigations of past dust explosions in the grain, feed and flour industries in the U.S.A. and Europe arrive at the conclusion that 'friction sparks' may have been the initiator of a substantial fraction, up to 50%, of all the explosions recorded. However, this conclusion has to a large extent been based on indirect evidence, and essential details of the suspected ignition process most often remained unconfirmed.In the present investigation a comprehensive series of single-impact ignition experiments were carried out, by generating explosible dust clouds of dried maize starch, in the region of tangential impact between a moving body and a stagnant horizontal anvil.The net energy of the impact was defined as the loss of kinetic energy of the moving body during impact. The influence of the impact energy on the frequency of ignition was determined. It was found that much higher energies were required for ignition at high velocities of impact than at low ones. The reason is that any moving object colliding with a solid surface, reduces the ignition sensitivity of the dust cloud in the vicinity of the impact point by inducing turbulence.Details of the impact and subsequent ignition of dust clouds were studied using a high-speed movie camera. The temperatures of the metal sparks generated from a range of different metals were measured by a four-wavelength optical pyrometer. Typical spark temperatures were [approximate] 1500-2700 [deg]C for mild steel and [approximate]2000-2900 [deg]C for titanium.The overall conclusion from the present investigation is as follows: steel sparks produced by single impacts of net energies of up to 20 J, between steel and concrete, steel or rusty steel, are unlikely to ignite clouds of dried maize starch. In the case of maize starch of 10% moisture, ignition is unlikely even with titanium sparks. Maize starch is amongst the easiest ignitable dusts encountered in the grain, feed and flour industry. Also, most dusts found in practice will contain some moisture. Therefore, it seems unlikely that dust explosions involving dusts of grain, feed or flour can be initiated by heat from accidental single impacts between tramp metal, and anvils of metal, corroded metal, stone or concrete, unless the net impact energies are much higher than 20 J.