Liquid mists ignition hazards

Flammable mist-vapor-air mixtures may occur as the foam on a flammable liquid collapses.28 Thus, when ignited, many foams can propagate flame. An additional hazard can arise from the production of foams by oxygen-enriched air at reduce pressures. Air confined over a liquid can become oxygen enriched as pressure is reduced because oxygen is more soluble than nitrogen in most liquids. Thus, the presence of foams on combustible liquids is a potential explosion hazard. 

An area which deserves special attention with respect to safety is the storage of liquid ammonia. In contrast to some other liquefied gases (e.g., LPG, LNG), ammonia is toxic and even a short exposure to concentrations of 2500 ppm may be fatal. The explosion hazard from air/ammonia mixtures is rather low, as the flammability limits [1334]-[1338], [1343] of 15-27% are rather narrow. The ignition temperature is 651 °C. Ammonia vapor at the boiling point of-33 °C has vapor density of ca. 70% of that of ambient air. However, ammonia and air, under certain conditions, can form mixtures which are denser than air, because the mixture is at lower temperature due to evaporation of ammonia. On accidental release, the resulting cloud can contain a mist of liquid ammonia, and the density of the cloud may be greater than that of air [1334]-[1344], This behavior has to be taken into account in dispersion models. 

DOT CLASSIFICATION 4.3 Label Danger When Wet, Corrosive, Flammable Liquid SAFETY PROFILE Moderately toxic by inhaladon. Corrosive. A severe irritant to skin, eyes, and mucous membranes. Ignites spontaneously in ait. A very dangerous fire hazard when exposed to heat or flame. Forms impact-sensitive explosive mixtures with potassium permanganate, lead(II) oxide, lead(TV) oxide, copper oxide, silver oxide. To fight fire, use water, foam, CO2, mist. When heated to decomposition it emits toxic fumes of CL. See also CHLOROSILANE. 

Always ventilate spaces with normal air and never pure oxygen. Introducing a source of ignition into a space containing a flammable atmosphere will cause an explosion. Consider hazardous most liquids, vapors, gases, mist, solid materials, and dusts space. 

Hazardous material releases in the process industries are either gaseous, mists, or liquids and are either atmospheric releases or from contained pressurized process systems. Gas and mist releases are considered more significant since if combustible, they are readily ignitable since they are in the gas state and due to the generation of vapor clouds, which if ignited, are instantly destructive in a widespread nature. This is in contrast to liquid fires that may be less prone to ignition, generally localized, and relatively controllable. 

---