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Liquid mists and sprays flammable below the flash point - The problem of preventative bases of safety

Puttick, Stephen

Preventative bases of safety (BoS) (i.e. absence of flammable atmospheres or avoidance of ignition sources) are the most economic to establish, so there are clear drivers for using them where possible. However, where they are used they must be robust and maintainable. For example solvents can be used below flash point (with a safety margin) to avoid flammable atmospheres or dusts can handled where possible electro-static discharges are below the MIE (Minimum Ignition Energy) . To establish such a BoS the material must be well characterised relative to the possible ignition sources present. Sprays and mists can be created deliberately in processing (e.g. cleaning vessels; and spraying materials onto substrates) as well as from leaks. A number of workers have identified that mists can be flammable below the flash point. A figure often quoted is that mists can be flammable as much as 60 K below flash point. However, results from Syngenta's legacy organisation shows sprays to be flammable as much as 125 K below the flash point; with no upper limit having been determined. There may be an upper limit but it will not help us for many materials. To use the BoS absence of viable ignition sources the sensitivity of the mist to ignition must be established in an analogy to dust MIE. Little work has been done in this field particularly for hazard assessment. A rig has been built to spray materials at ambient temperature and perform ignition tests with pyrotechnic and electrostatic ignition sources. Electrostatic ignition presents some challenges in making reliable measurements: electrodes are wetted by the spray; the presence of droplets between the electrodes can lead to early breakdown and discharge below the desired voltage. Full scale nozzles use large quantities of fluid and ignitions can contaminate this. Characterisation of the spray will be necessary so that measurements are made at a relevant droplet size possibly with a much smaller nozzle. Several fluids have been tested.