4 BULLETIN 681, U. S. DEPARTMENT OF AGRICULTURE. 



the States of Washington, Idaho, and Oregon.^ During the past 

 season the investigation has been extended to Colorado, Texas and 

 other grain-growing sections. 



THE EXPERIMENTAL ATTRITION MILL. 



The investigations conducted with the experimental attrition mill 

 were undertaken for (1) the determination of the possible causes of 

 explosions, and (2) the testing of various preventive measures that 

 have been suggested during the progress of the work. The entire 

 scope of the investigation can be seen in the following outline of the 

 various phases of the problem that were touched upon: 



1. Sparks emitted by foreign substances. 



2. Naked flame. 



3. Carbon arc in attrition miU. 



4. Carbon arc in dust room. 



5. Explosibility of various grains. 



6. Static electricity in attrition mill. 



7. Static electricity as cause of explosions. 



8. Effect of atmospheric humidity. 



9. Effect of moisture content of materials. 



10. Revolving dampers or fire traps. 



11. Relief valve and pipe. 



PRINCIPLES UNDERLYING GRAIN-DUST EXPLOSIONS. 



On account of the general lack of knowledge concerning the theory 

 of dust explosions it may be advisable to insert at this point a brief 

 summary of the chief facts and principles involved. 



It is generally conceded that two main conditions must be present 

 in order that a dust may produce an explosion: (1) Fine, dry dust 

 must be in a state of suspension in an atmosphere containing the 

 proper amount of oxygen. (2) A source of sufficient heat must be 

 brought in contact with the dust. 



It should be noted that carbonaceous dusts are not considered 

 capable of igniting spontaneously, for an outside source of heat is 

 .requhed. The following sources of heat have been listed as the 

 probable causes of many of the explosions in milling plants throughout 

 the country: ^ 



1. Use of open lights, or naked flames, such as lamps, torches, 

 gas jets, lanterns, candles, and matches. 



2. Entrance of foreign material in grinding machines. 



3. Electric sparks from motors, fuses, switches, lighting systems. 



4. Static electricity produced by friction of pulleys and belts, 

 grinding machines, etc. 



As for the first condition, many factors are involved, such as the 

 chemical composition, moisture content, fineness, atmospheric 

 humidity, density or degree of diffusion^, and presence of inert gases. 



1 U. S. Dept. Agr. Bui. 379. 2 Reference 27, Bibliography. 



