Mists can explode

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]. 

For example, a material had been oxidized many times without incident in 1 - and 4-L vessels, in an oxygen atmosphere, at a temperature of 80°C, and at a gauge pressure of 225 psi (15.5 bar). The flash point of the solvent at this temperature and pressure was 130°C. The next step was to scale up to a 48-L vessel. The rate of reaction was limited by the rate at 

The introduction of the efficient agitation system was a process modification (see Section 2.6), but its consequences were not foreseen. 

Another incident occuned when contractors were employed to clean several black oil tanks, 4,500-m capacity, so they could be used for the storage of kerosene. The details of the contract were agreed verbally. After removing solid residues and the heater coils, the tanks would be sprayed w ith hot water and a detergent. Floodlights were suspended through roof manholes and were to be removed before the tanks were sprayed. 

The first tank was cleaned without incident. By the time the contractors started on the second tank, a new foreman was in charge. To soften the deposits, he disconnected the steam coil, blew live steam into the tank, and then sprayed kerosene onto the walls, section by section, using a spinner mounted on a tripod. The report does not make it clear whether or not the kerosene was heated. While men were moving the tripod, a fire started in the tank, followed by an explosion. Three men were icilled. one by the fire and tw o by falling bits of the external concrete cladding. 

---

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.)... 

Additional hazardous conditions can occur with aerosols or fine mists of combustible high flash point material. Mists of combustible liquids can readily explode. Mists can explode, just as combustible dusts will explode, even if the fluids in the mists are at temperatures well below their flash point. 

SAFETY PROFILE Moderately toxic by intraperitoneal route. Probably an irritant to skin and mucous membranes. Dangerous fire hazard a powerful oxidizer. Store in a cool place away from fire hazards, sparks, open flames, and out of the direct rays of the sun. Dangerous explosion hazard this material may explode by heat (over 38°) or contamination. Any contaminant that acts as an accelerator to the polymerization or decomposition of this material can cause an explosion. Heat or contact with certain fumes or mists can cause it to explode. To fight small fires, use CO2 or foam extinguishers. Water spray or mist may also be used. Dry chemical is effective. When heated to decomposition it emits toxic fumes of Cr. See also PEROXIDES, ORGANIC. 

SAFETY PROFILE Mfldly toxic by ingestion. Moderately flammable by heat, flame (sparks), oxidizers. Can explode on heating. To fight fire, use water, fog, mist, alcohol foam, dry chemical. When heated to decomposition it emits acrid smoke and fumes. See also PEROXIDES, ORGANIC. 

When shock waves are deflected or reflected by pipe elbows, restrictions, pipe end flanges, closed shut-off valves or such like particularly high temperatures are produced. In outlet pipes of air compressors and in connected vessels lubricant mists can be ignited by compression and explode. ... 

When a flammable liquid is sprayed as fine droplets into the air, a flammable mixture can result, which may burn or explode. The mist or spray may be formed by condensation of saturated vapors or by mechanical means [40]. As the particle sizes of the liquid become greater than 0.01 mm diameter, the lower flammability limit of the material becomes lower while above 0.01 mm, the LEL is about the same as the vapor. Mechanical engine crankcase explosions of oil mist in air are hazardous, and current practice is to apply explosion relief valves to the crankcase. 

Mixtures of fine combustible dusts with air in certain proportions can be ignited and they either burn or explode with velocities of propagation lower than those for explosive gases. Andreev Belyaev (Ref 2, pp 148-53) treat briefly this subject, but do not give any rates of burning. More information on expln of dusts is given under Detonation and Explosion of Dusts, Mists and Vapors Refs 1) Baum, Stanyukovich Shekhter(1959)> 384-85 2) Andreev Belyaev (I960), 139-40... 

If the temperature of a liquid is below its flash point, flammable concentrations of vapor cannot exist, but conditions still can exist for flammability if mists or foams are formed. A suspension of finely divided drops of a flammable liquid in air has many of the characteristics of a flammable gas-air mixture and can bum or explode. A mist may be produced by condensation of a saturated vapor or by mechanical atomization. Normally, the diameter of drops in a condensed mist is less than 0.01mm, whereas in a mechanical spray it usually is greater than 0.1 mm. 

A useful rule is that mists of flammable or combustible liquids in air can burn or explode at temperatures below their flash points. 

Mists of oxidizable liquids may form that can burn or explode at temperatures outside the normal flammable range. 

DOT CLASSIFICATION 8 Label Corrosive SAFETY PROFILE Moderately toxic by ingestion, skin contact, intravenous, and subcutaneous routes. Mildly toxic by inhalation. A skin and severe eye irritant. Excessive absorption can cause urticaria, vomiting, diarrhea, blurred vision, and weakness. Combustible when exposed to heat or flame can react vigorously with oxidizing materials. Explodes on contact with dicyanofurazan. To fight fire, use alcohol foam, mist, dr) chemical, water spray. When heated to decomposition it emits highly toxic fumes of NOx. 

In Southeast Asia during the 1970s, symptoms began within minutes after an exploding munition (air-to-surface rocket, aerial bomb, cylinder) caused a yellow, oily, droplet mist to fall on individuals within 100 m of the explosion site. The falling droplet rain was inhaled, swallowed, and collected on skin and clothing contaminated the terrain and food and water supply and caused humans and animals to become acutely ill and to die after a variable period.7 Massive cutaneous contact was prevalent when the sources of exposure were sprays or coarse mists that were used deliberately to contaminate humans and the environment. Although the suspected trichothecene mycotoxin attacks in Southeast Asia would have involved multiple routes of exposure, we can postulate that the skin would have been the major site for deposition of a aerosol spray or coarse mist. 

---