Volume 6, Issue 8 (September 2009)
Tribocharging of Particle Contaminants Evaluated as an Ignition Source in Oxygen-Enriched Environments
Contamination particles make an oxygen system more vulnerable to fire hazard as they provide a large surface area with reduced minimum energy of ignition. Adiabatic compression, resonance, solid friction, flow friction, and promoted ignition have acted as the source of ignition in oxygen systems. In this paper, static electricity generated in solid particles is proposed to be another possible source of ignition. It is known that the movement of particles in a system results in the buildup of static charges in them. This static electricity, if supported by suitable geometric configuration and grounding of metal projections, may lead to propagating brush discharge (PBD) and provide the energy needed for initiation of fire. Particles with low auto-ignition temperature, particularly in dispersed state, may help in initiating the fire, while metal particles may aid the propagation because of their high combustion energy. In this paper, we have determined the velocity profile utilizing the FLUENT code and calculated the charge generated in particles, exploring the ignition probability of equipment in oxygen environment through electrostatic discharge. We have also attempted to perform preliminary calculation to determine charge density and the electric field strength required for brush discharge. Computational fluid dynamics analyses on globe valves reveal the presence of probable areas of particulate accumulation in the valve. The decrease in minimum ignition energy (MIE) in the environment of high pressure, high temperature, high purity of oxygen, low humidity, small particles, high dielectric constant, low thickness of insulating layer, and high resistance of the deposited dust layer facilitates occurrence of PBD, which may cause ignition. Adiabatic heating may aid combustion by lowering the MIE of particles. Experiments have been proposed for verification of the hypothesis and analytical calculations presented by the authors. The paper may give a new insight into the possible causes of oxygen fire.