Page 222 - Dust Explosions in the Process Industries
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194  Dust Explosions in the Process Industries


       The construction materials of the mixer had been selected to eliminate the formation
     of mechanical sparks. This was probably why both the screw and the internal wall of the
     surrounding grounded steel tube were lined with rubber.
       During operation, the 5.2 m3 vessel was flushed with nitrogen, the concentration of
     oxygen in the vessel being controlled by a direct reading oxygen analyzer. According to
     the foreman’s statement, the oxygen content at the moment of explosion was within the
     specified limit.
       After the explosion, the central screw part of the mixer,  with the mixer top, was
     retrieved, as shown in Figure 2.31, about  12 m away from the location that the mixer
     had prior to the explosion. More detailed investigation of the part of the screw shielded
     by &e steel tube Eevealed, as shown in Figure2.32, that  the screw wings had been






















     Figure 2.31   Top of 5.2 m3 premix and 3.3 m long mixing screw with surrounding steel tube (see
     Figure 2.30), as  found after the explosion  12 m away from location of the mixer prior  to the explo-
     sion (Courtesy of E.  Berg, Dyno Industries, Cullaug, Norway).























     Figure 2.32   Section of screw after splitting and removal of surrounding steel tube, showing bidi-
     rectional deformation of the screw wings from the explosion center. Part of rubber lining of steel tube
     removed from upper half of tube (Courtesy of E. Berg, Dyno Industries, Cullaug, Norway).
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