available (such as with long hose}, the blasting rate will be slow, making 

 removal of mild scale, old rust, paint or heavy marine growth expensive or 

 incomplete. 



The efficiency of blasting at 655 kilopascals (95 pounds per square 

 inch) is only 50 percent of that of 690 kilopascals (100 pounds per square 

 inch). Lower pressures also often result in other problems, such as the 

 need to compensate by using larger sized abrasives with their attendant 

 deep anchor pattern. 



(c) Air Supply and Couplings . These should be related to the 

 size of the job but generally should be as large as possible in order to 

 supply a constant high volume of air at proper blasting pressure. For 

 large jobs large air lines and couplings should be used in order to minimize 

 friction loss through the hose. Universal type couplings are recommended 

 for use on air lines. 



(d) The Sandblasting Machine or Pot . For successful low cost 

 blasting, the sand pot must also be given careful consideration. There are 

 several types available. All furnish a regulated amount of abrasive to a 

 high-pressure airstream for the blasting function. However, pots with only 

 one chamber are normally used for intermittent blasting, while a two- 

 chamber pot is better adapted to continuous work. Many sizes as well as 

 designs are available. Proper size is determined by the abrasive capacity 

 desired. All sand pots used for abrasive blasting should be equipped with 

 a separator to remove any moisture or oil contamination. The separator 

 trays must be emptied and maintained on a regular basis. 



(e) Moisture Traps and Oil Separation . A moisture trap 

 prevents condensation from forming on the lines and flowing onto the work 

 surface. It should be placed as close as possible to the sandblast pot. 



An oil separator should be placed on air lines between compressor and blast 

 machine to prevent oil contamination of the blasted surface. Although 

 these items are frequently disregarded, they are important in obtaining a 

 good coating job. If moisture or oil contamination are allowed to remain 

 in the compressed air, they will contaminate the surface and degrade the 

 quality of the coating job. 



Water contamination will oxidize and corrode the surface, resulting in 

 reduced quality of the final coating system. Oil contamination will reduce 

 the adhesion properties of the final coating system and can cause delamina- 

 tion. 



(f) Size of Hose With External Couplings . Sandblast hoses are 

 often too small. The inside diameter of the hose should be three to four 

 times the orifice size of the nozzle. Avoid any coupling or pipe-fitting 

 connection that fits internally into hose. This can reduce the inside 

 diameter enough to reduce the air- carrying capacity by more than 50 percent. 

 Use only externally fitted quick couplings. 



(g) Size and Type of Nozzle to be Used . Abrasive consumption 

 and the volume of air used are directly related to the size nozzle being 

 used. Most commonly used nozzle sizes are 6.4-, 8-, and 19-millimeter (1/4, 

 5/16, and 3/4 inch) inside diameter. The larger the nozzle the larger the 



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