Dust Control in the Flour Milling Industry

[:fa]سیستم های غبارگیری فن سیکلون[:]
Fine particles of grain and foreign material, as well as flour that is dispersed in the atmosphere or lying on the floor and equipment, are termed “dust.” “Dust control” normally means the suppression of collection of micro-fine particles in the mill. 

Suction is used to cause negative pressure to remove dust from wheat-handling equipment and eliminate dust buildup. Control of dust in the flour milling operation is normally advantageous because the dust is valuable, and its collection has economic justification. In addition, it reduces the possibility of a dust explosion; mill sanitation is improved; and potential infestation problems are avoided. The fully automatic, lights-out mill of today creates additional needs for measures of fire detection and dust-explosion suppression. The U.S. Occupational Safety and Health Administration (OSHA) published grain handling standards. Under this Standard, measures are described for control of fires, grain-dust explosions, and other safety hazards associated with grain-handling facilities. Dust explosions in grain elevators and flour mills claim lives and cause injuries every year. Before the issuance of the final grain-handling-facilities standard, between 1958 and 1987, an average of 7.3 deaths and 29.1 injuries related to grain explosions occurred per year. After the standard, between 1988 and 1998, an average of 2.3 deaths and 13.2 injuries occurred per year.

Four components are required to support an explosion:

 1) suspended grain dust,

 2) an ignition source,

 3) an enclosed space, and 

4) oxygen. 

If one of these four components is missing, an explosion is prevented.  Two separate explosive phases (primary and secondary) occur during a major dust explosion. The primary explosion is caused by confinement of airborne dust and its contact with a heat source that ignites it. The first explosion creates an air-shock wave, traveling at about 300 m/sec, which stirs up layered dust along gallery corridors, tunnels, and vertical shafts in the elevator. A flame front, traveling at about 3 m/sec, follows the initial pressure wave, igniting airborne dust as it progresses through the structure. Dust from the primary explosion may be carried along with the pressure wave, providing even more fuel for secondary explosions. The secondary blasts send additional pressure waves through the structure. According to literature, grain industry research has measured pressures of 13.8 kPa (or 2 lb/in.2) during the primary explosions and pressures of more than 550 kPa (or 80 lb/in.2) during secondary explosions. A dust particle of 30 µm has a terminal velocity of 2.4 m/min (8 ft/min). 

Particles of this size and smaller remain suspended if the air velocity in the space is higher. A spark that originates from a piece of metal moving with the wheat and hitting a hard surface can supply the necessary ignition of the suspended particles, causing a dust explosion. Good housekeeping, regular maintenance, and adherence to work regulations can reduce explosion potential. A comprehensive list of safety measures recommended to prevent dust explosions was documented and described recent dust explosions in grain facilities and preventive practices to avoid them.