Mist can explode, but still no standard! Proposal of a combustible sprays test method

Hydrocarbon mists can explode even at temperatures well below their flashpoints. Nevertheless, fuel mist hazards are often disregarded and improperly classified due to the lack of tools and standards to assess and investigate their occurrence. The aim of this study is to propose a new test method to determine the ignition sensitivity and the explosion severity of fuel mists using a single existing standard equipment already available in laboratories or industries. The standardized 20L explosion sphere was selected and modified: a Venturi mist generation set-up and an automated system controlling the inlet concentrations and allowing an optimum evaluation of explosion results were implemented. Once the technical feasibility was established, seven fluids (ethanol, isooctane, kerosene, diesel, lube oil, light fuel oil, and biodiesel) of both industrial interest and different physicochemical properties were chosen to be tested under a wide range of conditions. Extensive studies were performed on their droplet size distribution (DSD), turbulence level, flammability, and explosion severity. To study the behaviour of the mist before ignition, particle image velocimetry and in-situ DSD measurements were performed. Then, sensitivity studies were carried out to examine the influence of parameters, such as the ignition source (spark or chemical igniters), the ignition energy, the DSD, the initial turbulence, the chemical nature of the fuel, the liquid temperature, and the sphere temperature, on the thermo-kinetic explosion parameters Pmax and dP/dtmax. Results showed that this new procedure permits to measure sensitivity parameters such as minimum ignition energy and lower explosive limit, in addition to the above-mentioned severity parameters. This study establishes that it is indeed possible to assess fuel mist flammability and explosibility using a standardized apparatus based on the 20L explosion sphere, and thus allows a comparative ranking of hydrocarbon mists.