Hazardous Area Classification (HAC) for explosive gas atmospheres is well established, with guidance published in various standards and industry codes of practice. However, the same situation is not currently the case for high flashpoint liquid releases that could give rise to an explosive mist atmosphere. There is a pressing need for clear guidance on mist hazards to allow operators to determine the extent of areas where flammable mists may be present and to select appropriate equipment for use in those areas.
This report provides a survey of the recent literature on flammable mists and pulls together information that will be useful in developing a HAC methodology for explosive mist atmospheres. It focuses on the three fundamental issues: mist flammability, mist generation and mitigation measures.
The first of these is discussed with reference to five measurable parameters: the Lower Explosive Limit (LEL), Minimum Ignition Energy (MIE), Maximum Experimental Safe Gap (MESG), Minimum Igniting Current (MIC), and Minimum Hot Surface Ignition Temperature (MHSIT). Measurements of these quantities in mists are analysed and models for their prediction are discussed.
The second issue of mist generation is examined under four categories: mists produced by pressurised sprays, condensation aerosols, agitation/splashing/sloshing and air stripping. Of these, the primary focus is on spray releases and condensation aerosols, which are considered to be the most likely sources of mists. Measurements undertaken in sprays are described and models are discussed. Mitigation measures are surveyed briefly, which include mist detection, use of fire-resistant fluids or anti-misting additives, inerting and control of static charge.
Finally, tentative proposals are suggested for developing area classification guidance based on the prediction of the flammable mist cloud size.
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