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No injuries in explosions at pellet mill

Date of incident: October 2012
Date of incident number: 2012161980244
Employer: Manufacturer of wood pellets

Incident summary
A pellet mill operator, working in the mill's control room, noted a high amperage reading coming from the motor of an auger that moved dry wood materials from a hammermill plenum. The motor was located directly above the enclosed plenum auger. A millwright discovered that the mounting bolts on the motor were loose and needed to be replaced. The shift supervisor approved the hot work to remove the old bolts, and the millwright completed the task using a cutting torch. Approximately 20 minutes later, there was an explosion in the hammermill plenum. Seconds after the first explosion, a dry product silo outside the building exploded. A large portion of the silo's roof was blown off in sections. There were no injuries from either explosion.

Investigation conclusions
Cause

  • Wood dust ignited and exploded: Wood dust in the hammermill plenum ignited and exploded after a torch was used to cut the bolts off a motor located directly above the enclosed plenum auger. Burning material then entered the rotating knife-edge feeder and high-pressure blower line, neither of which had been turned off during the hot work. This resulted in the burning material entering the silo, where wood dust there also exploded, blowing large pieces of the roof off the silo.
Underlying factors
  • Inadequate hazard recognition during hot work: Neither the supervisor, the millwright, nor the operator recognized the hazard of leaving the knife-edge feeder and the high-pressure blower running during the hot work on the plenum auger. Although they took some precautions such as using a spark watch person and wetting down the area, the failure to recognize this hazard and turn this equipment off allowed burning material from the first explosion in the plenum to ignite the dust and cause the second explosion in the silo.
  • Inadequate maintenance: Until July 2012, maintenance was performed primarily on an as-needed basis only. In July 2012, a system of preventive maintenance was established, which included a scheduled 12-hour maintenance shutdown. In spite of this change, the employer did not keep records or ensure that preventive maintenance of the safety systems --- such as spark detection, deluge, and the abort gates --- was done in accordance with manufacturer's specifications. Maintenance at the plant was inadequate to ensure the proper operation of important safety systems and the protection of the facility and its workers.
  • Inadequate engineering in silos' explosion vents: During the construction of the two dry product silos, the employer recognized the need to add explosion venting. However, the design and materials used were not done in accordance with good engineering practices, under the supervision of a professional engineer, or in accordance with accepted standards. This resulted in the installation of explosion vents that were not adequate to protect the silo or the workers during the explosion.
  • Inadequate supervision: A dedicated supervisor position had recently been introduced. The new supervisor was relatively new to the pellet industry. When he was hired, he underwent orientation, which included the hot work policy. However, at the time of the incident he failed to recognize the hazard of allowing the knife-edge feeder and high-pressure blower to operate during hot work. This demonstrated an overall lack of familiarity with the industry and its processes and that he was not adequately trained or supervised in this regard.
  • Insufficient hot work procedure: Hot work procedures, which include the use of a cutting torch, were in place. However, the procedures were insufficient as they lacked specific information and instruction with respect to managing the operating air systems that were in the immediate area of the hot work being done.

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Publication Date: Oct 2012 Asset type: Incident Investigation Report Summary NI number: 2012161980244