Ignition hazards arising from broken optical fibres in flammable atmospheres

Abstract

Optical fibres are being used to replace electrical cables for monitoring the immediate environment and controlling equipment, wherever there is a flammable atmosphere in the mining and chemical industries. The optical fibre based equipment was believed to be intrinsically safe, therefore suitable for use in such environments, but recent work has shown this to be untrue. A model is presented, simulating a fibrous agglomerate, in a flammable atmosphere, exposed to laser radiation and re-emitting the energy in the form of heat. Uniform temperature and concentrations are assumed throughout the reaction zone for simplicity, facilitating the inclusion of detailed chemical reaction schemes. Heat transfer is modelled by radiation and Newtonian cooling. Various parameters of the system are altered in an attempt to ascertain the most hazardous situation for a given geometry and gas concentration. The relationships between the various parameters and criticality are then shown, which leads to a tentative prediction of a minimum safe power level at which laser-based control and sensing systems can be operated in a flammable atmosphere of H 2 in air.