Document 15VybqpwQ5ReEL7BrnN6Rwd05

control the production of ozone in the weld ing or cutting area Ultraviolet radiation from the welding or cutting arc can also decompose chlorinated hydrocarbons, such as trichloroethylene and perchloroethylene, to form highly toxic sub stances Since this decomposition can occur even at a considerable distance from the arc, degreasing operations and other work using these chlonnated solvents should be located so that no solvent vapor will reach the weld ing or cutting area Inert-gas shielded arc welding requires that much greater precaution be taken to provide respiratory protection than is ordinarily the case with other welding or cutting processes Depending upon a number of factors, includ ing the particular variety of gas-shielded arc welding to be done, the nature of the mate rials to be welded, and whether or not the work must be done in a confined space, there will be a need for provision of positive ven tilation, local exhaust removal, or approved respiratory equipment, or a combination of them Indoor exhaust and ventilation In spaces of 50,000 cu ft and over, where welding is an essential part of the work, local exhaust or positive ventilation is not required for the protection of welders on uncoated ferrous metals, provided that (a) welding bays are not structurally blocked so as to obstruct cross ventilation, (b) the work is not done inside tanks, boilers, or other closed iron or steel containers, (c) space allowance of 10,000 cu ft is assured each welder, (d) ceiling heights are greater than 16 ft, and (e) the process in volved is other than inert-gas shielded arc welding Where these requirements are not met, the Code specifies mechanical ventilation at a minimum rate of 2000 cfm of air per welder, or four air changes per hour, whichever is the greater, except when local exhaust hoods and booths or respiratory equipment approved by the U S Bureau of Mines are used (Fig 31-11) When welding must be performed in a space screened on all sides, the screens should be so arranged that they do not seri ously restrict ventilation They can be mounted about 2 ft above the floor, unless the TABLE 31-A SPECIFICATIONS FOR LOCAL EXHAUST DUCT DIAMETERS AND FLOW RATES Distance from Arc or Torch (in ) Mtn Air Flow' (cfm) Duct Diameter1 (in) 4 to 6 150 3 6 to 8 275 3'/j 8 to 10 425 4'h 10 to 12 600 5'/j 'Increase by 20 per cent for hoods without flanges sTo nearest 'h in based on velocity of 4000 fpm in duct work is performed at so low a level that they must be nearer the floor to protect nearby workers from welding glare Local exhaust removal may be by means of movable hoods placed by the welder as near as practicable to the work being welded and provided with a rate of air flow sufficient to maintain a velocity in the direction of the hood of 100 fpm at the point of welding when the hood is at its most remote distance from the point of welding The duct diameters and air-flow volumes that will produce this control velocity using a 3-m wide flanged suction opening are shown in Table 31-A Exhaust air should be discharged outdoors Local exhaust may also be by a fixed en closure with a top and not less than two sides, which surround the welding or cutting op erations, and with an air flow sufficient to maintain a velocity away from the welder of not less than 50 fpm Local exhausts for weld ing and cutting are further discussed in Chap ter 29, "Local Exhaust Systems " Medical control Preplacement physical, including chest X ray, examinations are re commended for all persons engaged in weld ing Periodic re-examinations should be made as recommended by the plant physician Eye protection Goggles, helmets, and shields that give maximum eye protection for each welding and cutting process should be worn by op erators, welders, and their helpers These 943