The ignition of flammable atmospheres by small amounts of metal vapor and particles

Abstract

The discharges between electrodes of certain low-boiling-point metals are known to be capable of igniting flammable gases at much lower levels of arc energy than with other metals, and it has been observed that during the course of the discharge these low-boiling-point metals emit relatively large amounts of metal vapor and metal particles. This current investigation was carried out in order to determine whether or not the small quantities of metal vapor or particles which are ejected into the surrounding gas during a discharge are capable of causing an ignition. In one group of experiments, metal wires were exploded by discharging a capacitor bank through them and some of the explosion products were allowed to pass through a small hole into a chamber containing a flammable mixture of methane and air. The results showed that all but the metals with the lower boiling points readily caused ignitions. Schlieren photographs of the development of the ignitions indicated that whereas individual particles of aluminum readily caused ignitions, individual particles of the other metals examined (Cd, Cu,Fe, Pb, Sn, W, and Zn) did not. In another group of experiments, small quantities of vapor and particles were produced by focussing the output of a ruby laser onto the flat surface of samples of both metals and nonmetals. The amount of laser energy necessary to produce sufficient vapor to ignite a number of flammable atmospheres was determined for these materials, and it was found that this amount increased considerably as the boiling point of the material decreased. It was concluded that the case with which are discharges between low-boiling-point metals cause ignitions was not due to the ignition of the flammable atmosphere by the metal vapor and particles emitted during the discharge.