Two incidents occurred in a viscose rayon manufacturing process about a month apart during the same step of a semi-monthly cleaning operation in a carbon disulfide recovery box. The operation removes sulfides that accumulate on equipment surfaces during the week. In both incidents, carbon disulfide was released from the recovery box into the vapor exhaust system and the area surounding the recovery box. Ignition occurred and the resulting vapor flash propagated throughout the vapor exhaust system and the operating bay. Both incidents caused extensive equipment and building damage and minor injuries to personnel.
Investigation and recommendations after the first incident, although appropriate, did not eliminate the root cause of the problem. Additional information obtained during the investigation of the second incident showed that a combination of several modifications and subtle changes in that particular process line, in conjunction with the colder temperatures of fall and winter, resulted in the incidents. The changing of the critical balance of temperature, pressure in the closed system, and sudden volatilization of accumulated carbon disulfide in an unlikely location were triggered by startup of the cleaning operation. Once the carbon disulfide vapor pressure vented from the closed system numerous ignition sources were available.
The discussion will focus on the importance of management of change for seemingly small, logical changes, and on the importance of attention to small details in an accident investigation.