Flash point mists

When air is used to clear a line that had contained hydrocarbons, the potential for forming a flammable mixture of air with hydrocarbon vapors and/or mist is created. The elevated pressure which can be developed in a line or system broadens the flammable range, lowers the autoignition temperature, and can convert a previously fuel-rich mixture into a flammable mixture by lowering the vapor mole fraction of the fuel. Even though the temperature of a hydrocarbon liquid in a line may be below its flash point, mist formation can occur due to the sparging action of the air and can result in a flammable quantity of material being forced into the vapor space. 

Diketene is a flammable Hquid with a flash point of 33°C and an autoignition temperature of 275°C. It decomposes rapidly above 98°C with slow decomposition occurring even at RT. The vapors are denser than air (relative density 2.9, air air = 1). The explosive limits in air are 2—11.7 vol % (135). In case of fire, water mist, light and stabilized foam, as well as powder of the potassium or ammonium sulfate-type should be used. Do not use basic extinguisher powders and do not add water to a closed container. 

Volatile Solids, NFPA 325, Quincy, Mass.). Pressure particularly affects flash point and the uppei flammable limit (UFL) see later section entitled Effect of Temperature, Pressure, and Owgen. Mists of high-flash-point liquids may be flammable the lowei flammable limit (LFL) of fine mists and accompanying vapor is about 48 g/m of air, basis 0°C and 1 atm (0.048 oz/fP). 

In view of the above adverse effects a safety factor should be applied where flammability is assessed using flash point. For pure liquids in containers the vapor should be considered potentially flammable if the liquid temperature is upward of at least 5°C below the reported flash point. For mixtures whose composition is less certain, such as petroleum mixtures, the safety factor should be about 15°C relative to the flash point [55]. Where combinations of adverse effects are identified the safety factors should be increased accordingly. A simple but very conservative approach is to assume that all liquids having a flash point <141°F may produce a flammable atmosphere under some ambient conditions, even where no mist or froth production is involved. A more practical approach is to assume that liquids handled in air at least 5-15°C below their closed cup flash points will not present ignition risks unless... 

When a liquid is dispersed into droplets the surface area is increased, which enhances the rates of heat and mass transfer. For a particular liquid dispersed at constant concentration in air the MIE varies with approximately the cube of surface average droplet diameter, hence the MIE decreases by a factor of about 8 when the surface average diameter D is halved (A-5-1.4.4). Ease of ignition is greatly enhanced for finely divided mists with D less than about 20 /rm, whose MIE approaches that of the vapor. Below 10 /rm a high flash point liquid mist (tetrahydronaphthalene) was found to behave like vapor while above about 40/rm the droplets tended to burn individually [ 142]. Since liquid mists must partially evaporate and mix with air before they ignite, the ease with which a liquid evaporates also affects MIE (Eigure 5-1.4.4). 

Fire Hazards - Flash Point Not flammable Flammable Limits in Air (%) Not flammable Fire Extinguishing Agents Not pertinent Fire Extinguishing Agents Not To Be Used Not pertinent Special Hazards of Combustion Products Phosphoric acid mist may form in fires. Toxic oxide of nitrogen may form Behavior in Fire No data Ignition Temperature Not pertinent Electrical Hazard Not pertinent Burning Rate Not pertinent. 

When the reboiler was brought back on line, the water was swept into the heat transfer oil lines and immediately vaporized. This set up a liquid hammer, which burst the surge tank. It was estimated that this required a gauge pressure of 450 psi (30 bar). The top of the vessel was blown off in one piece, and the rest of the vessel was split into 20 pieces. The hot oil formed a cloud of fine mist, which ignited immediately, forming a fireball 35 m in diameter. (Mists can explode at temperatures below the flash point of the bulk liquid see Section 19.5.)... 

Flammable liquids should never be splash-filled, even though they are oelow their flash points. The splash filling may form a mist, which can be ignited by a static discharge. Mists, like dusts, can be ignited at any temperature (see Section 19.5). 

Most everyone knows that dusts—fine particles of solid—can explode, but not everyone is aware that mists—fine droplets of liquid—can explode just as easily and that they can explode at temperatures fai below the flash point of the bulk liquid or vapor [19]. 

Confined Explosions. In situations where the vapors are confined within a building, vessel, or other such enclosure, flammable materials with flash points below the temperature within the enclosure may have the potential for an explosion. Similarly, in confined situations, combustible materials, regardless of temperature, can pose a potential for explosion if dispersed as an aerosol, mist, or dust. 

Spray and mists releases generally behave like a gas or vapor release. The fuel is highly atomized and mixed with air. Sprays or mists can easily be ignited, even below the flash point temperature of the material involved, since mixing of the fuel with the air has already occurred. 

Anon., Sichere Chemiearbeit, 1992, 44(6), 70 Jahresbericht, 1991, 72 A substantial explosion, followed by a fire causing fatal bums to the experimenter, occurred when working with butyllithium in tetrahydrofuran cooled by a propan-2-ol/solid carbon dioxide freezing bath. It was considered that the explosion might have been fueled by isopropanol vapour in air. This explanation was rejected since the alcohol should have been below its flash point. However, making up such cooling baths, containing flammable solvents, does disperse considerable quantities of vapour and mist before the mix is fully cooled. There is a period of potential hazard, as there may be when the bath warms above the flash point once more. 

Liquids can form explosive atmospheres at temperatures below the flash point appropriate to the pressure in question if agitation is sufficient to form a mist. A destructive instance is reported [21],... 

Mists are dust clouds in which the particles happen to be liquid. Should that liquid be combustible, even though it is nowhere near its flash-point, explosion is possible. 

Where published values are not available, the Nf of liquids and gases is developed from flash point data and the St of dusts or mists is determined by dust explosion testing. The Nf combustible solids depends upon the nature of the material as described in the left column of Table V. 

Mists are dust clouds in which the particles happen to be liquid. Should that liquid be combustible, even though it is nowhere near its flash-point, explosion is possible [1] [2]. Mist explosions attract increasing study [3]. It is possible that many vapour cloud explosions have had a mist component. The editor surmises that, under appropriate circumstances, evaporation of volatile mist by the heat of a vapour (or mist) explosion might generate a larger pressure pulse than simple thermobaric effects on air. Foams are inverse mists and should show similar explosive potential. 

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