Demystifying mist explosion hazards
Résumé
It is essential that chemical and petrochemical industries take into account potential fire and explosion hazards as part of their risk assessments. Such hazards are sometimes related to flammable mists that can be generated by fluid releases from pressurized pipes or vessels, from condensation of hot vapours, or from splashing, etc. A recent study showed that around 10% of notified releases in the UK Offshore Hydrocarbon Release Database involved flammable mists. Analysis of the French and German ARIA and ZEMA databases also identified dozens of previous significant mist incidents. Despite this, there is limited guidance on hazardous area classification for flammable mists. Whilst classification of flammable gases and dusts is well established, there is a need for scientific evidence to support the classification of flammable mist hazards. To address this knowledge gap, this paper presents an extensive study of diesel, biodiesel, and light fuel oil mists. These high-flashpoint liquid fuels were chosen as they have a high industrial interest and were involved in many reported incidents. A Venturi-based mist generation system was used to ensure the control of the fuel concentration, the fuel/air ratio and a well-defined droplet size distribution. Mists with mean droplet diameters ranging from 5 to 100 μm were thus obtained. Experiments were carried out in a modified apparatus based on the standardized 20 L explosion sphere where the experimental conditions, including the air injection pressure to the ignition delay time, were varied. In the first set of tests, the minimum ignition energy, lower explosive limit and explosion severity of the fuels were determined at 40°C and atmospheric pressure. In order to examine the impact of the fuel vapour/liquid ratio on the explosion behaviour, tests were undertaken with an increasing sphere temperature from 30°C to 80°C. Results showed that the surrounding temperature had a significant effect on the thermo-kinetic explosion parameters Pex (explosion overpressure) and dP/dtex (rate of pressure rise). The value of dP/dtex increased markedly from approximately 24 bar.s-1 at T = 30°C to approximately 314 bar.s-1 at T = 80°C for a diesel mist concentration of 123 g.m-3. These findings were compared to numerical data calculated using combustion software. The influence of the temperature on the evolution of the droplet size distribution and the evaporation rate of droplets was also studied. In addition, a specifically designed metallic reservoir was used to pressurize and preheat the fluid before injection to mimic industrial leaks that could trigger hazardous explosions. The influence of such parameters on the mist explosion risk assessment was then assessed. The work presented in this paper improves our understanding of the physics behind mist explosions of high-flashpoint fuels. It also demonstrates the possibility of classifying hazardous mist areas using certain dimensionless numbers, such as the Ohnesorge and Reynolds numbers (which characterize droplet hydrodynamics) as well as the Spalding number (which characterizes droplet thermal behaviour).