Combustion hazards posed by the pressurised atomisation of high-flashpoint liquids

Study of the revised Institute of Petroleum Model Code of Safe Practice for area classification of petroleum installations has raised the question of whether pressurized systems of high-flashpoint liquids (e.g. diesel, kerosene) should be considered hazardous in respect of their potential to atomize to produce flammable aerosol systems. This work collates relevant scientific studies to assist in investigation of the possibility of combustion hazards posed by these systems. It is envisaged that it will serve as an initial assessment in an area where industry acknowledges that insufficient information exists. A simple method is presented for characterizing liquid leaks in terms of their potential to atomize, which is considered a necessary condition to give rise to a flammable atmosphere. A cloud of very finely atomized liquid near stoichiometric concentration is shown to be hazardous with regard to a ‘low-energy’ spark. Empirical formulae for estimating the initial ‘average’ droplet size and the functional form of the volume of fuel contained in droplets that are less than a particular droplet size for this type of leak give an indication of the nature and potential severity of the hazard. These correlations need to be validated for use outside their ranges of applicability or to be updated. Information collated here facilitates what is likely to be a very conservative assessment of the hazard potential. Identification of areas where there is a dearth of information is just as important, and it is hoped that this paper will provoke further interest leading to improved methodologies for hazard quantification. However, at this stage, it has to be concluded that, even under low pressures, the possibility of creating aerosols that result in flammable systems cannot be eliminated for commonly-handled high-flashpoint liquids. Two practical illustrations, a large-scale study and a real incident, are given to support this conclusion. Finally, in the specific context of area classification, a simple solution to some potentially hazardous aerosol leaks is suggested