Explosions, dust

It has been somew hat of a surprise to many engineers that fine dust particles are combustible and will explode 

Av = vent area, sq meters V = enclosure volume, cu meters e = 2.718, natural logarithm Pred = maximum pressure developed during venting, bar ga. 

The dust hazard class, K, vessel volume and strength, and the relieving pressure of the vent closure are the key components of the relief determination using the nomographs [27]. Although stated by Ref. [27] to be non-exact 

Applied Process Design for Chemical and Petrochemical Plants 

The dust hazard classes for deflagrations are given in Table 7-28, with data in Tables 7-28 and 7-30A, B, C, D, and E. 

---

When dispersed as a dust, adipic acid is subject to normal dust explosion hazards. See Table 3 for ignition properties of such dust—air mixtures. The material is an irritant, especially upon contact with the mucous membranes. Thus protective goggles or face shields should be worn when handling the material. Prolonged contact with the skin should also be avoided. Eye wash fountains, showers, and washing faciUties should be provided in work areas. However, MSDS Sheet400 (5) reports that no acute or chronic effects have been observed. 

K. N. Palmer, Dust Explosions and Fires, Chapman and Hall, London, 1973. 

Health and Safety Factors. Both pyromellitic acid and its dianhydride irritate skin, eyes, and mucous membranes, and they cause skin sensitization (156). When it comes in contact with moist tissue the dianhydride converts to the acid. Direct contact with should be avoided and protective clothing should be worn in areas where it is used. The LD q for intergastric administration in rats is 2.2—2.6 g/kg (157). In 6-mo experiments, the maximum nontoxic dose was 0.07 mg/kg/d, and it affected the fiver, kidney, and reproductive tract. Precautions against fire and dust explosions as explained in the terephthafic acid section should be foUowed. 

Dust explosions usually occur in pairs. The first explosion involves dust already in suspension. This jars dust from beams, ledges, etc, creating a second cloud to which the explosion propagates, resulting in a secondary explosion. Dust clouds have been ignited by open flames, electric sparks, hot... 

Succinic acid is Generally Recogni2ed As Safe (GRAS) by the U.S. PDA (184) and is approved as a flavor enhancer, as a pH control agent in condiments, and for use in meat products. It causes irritation to the eyes (185), skin, mucous membranes, and upper respiratory tract. LD q in rat is 2260 mg/kg. Succinic acid, like most materials in powder form, can cause dust explosion. 

OSHA and ACGIH have not estabhshed specific airborne exposure limits for PVB and PVF resias however, some products may contain sufficient fines to be considered nuisance dust and present dust explosion potential if sufficient quantities are dispersed ia air. Unformulated PVB and PVF resias have flash poiats above 370°C. The lower explosive limit (lei) for PVB dust ia air is about 20 g/m. ... 

Because biphenyl is often transported in the molten state, a moderate fire ha2ard does exist under these circumstances. Biphenyl, with a flash point of 113°C, has a lower flammability limit of about 0.6% (by volume) at the flash point to an upper limit of 5.8% at 166°C (42). Dust explosions are a ha2ard when vapors from a hot Hquid surface condense in air in a confined space. 

According to the National Board of Fire Underwriters, activated carbons normally used for water treatment pose no dust explosion ha2ard and are not subject to spontaneous combustion when confined to bags, dmms, or storage bins (64). However, activated carbon bums when sufficient heat is appbed the ignition point varies between about 300 and 600°C (65). 

Cellulose esters, like most dry organic materials in powder form, are capable of creating dust explosions (133). The explosion at Bayer s cellulose acetate plant at Dormagen, Germany in 1976 can attest to the explosive potential of dust. Damage to the plant was estimated at between DM 5—10 million (134). 

Frevention of Fire and Dust Explosions in the Chemical, Dje, Fharmaceutical, andFlastics Industrj, NFFM 654, National Fire Protection Association, Quiucy, Me., 1982. 

Whereas an explosion from methane tends to be localized, it may start coal dust explosions resulting in more widespread injury and loss of life. AH coal breaking operations result in formation of fine coal particles some are controlled with water during the mining operation. Breakage associated with hauling disperses dust, and dust accumulations can be made safe by rockdusting. Powdered limestone is spread over the mine surfaces to cover the dust. 

Dust Explosion Prevention-Plastics Industy, Code No. 654, NFPA, 1988. 

Richard Siwek/ M S / Explosion Protection Manager, Corporate Unit Safety and Environment, Ciba-Geigy Ltd., Basel, Switzerland. (Dust Explosions)... 

Prevention and Protection Concept against Dust Explosions. 26-61... 

Dust Explosions Static Electricity Hazards of Vacuum Hazards of Inert Gases Gas Dispersion... 

General References Bartknecht, Dust Explosions, Springer-Verlag,... 

Definition of Dust E losion A dust explosion is the rapid combustion of a dust cloud. In a confined or nearly confined space, the explosion is characterized by relatively rapid development of pressure with a flame propagation and the evolution of large quantities of heat and reaction products. The required oxygen for this combustion is mostly supphed oy the combustion air. The condition necessaiy for a dust explosion is a simultaneous presence of a dust cloud of proper concentration in air that will support combustion and a suitable ignition source. 

For example, in rotary vacuum dryers it is possible to prevent the formation of explosible dust-air mixtures by setting and monitoring a certain partial vacuum (negative pressure). This pressure value must be determined by experiment for each type of dust. With pressures of less than O.I bar, in general, hazardous effects of dust explosions need not be anticipated. If the vacuum system malfunctions, the partial vacuum must be released by inert gas and the instaUation shut down. 

---