Just seven months after a devastating report on workplace safety practices at DuPont Co.’s chemical plant in Belle, W.Va., the U.S. Chemical Safety Board today is releasing another report about DuPont, this one detailing problems that led to a November 2010 explosion that killed one and injured another contract welder at a DuPont plant outside of Buffalo, N.Y. CSB officials said the explosion was caused by the ignition of flammable vinyl fluoride inside a large process tank, a hazard which had been overlooked by DuPont engineers:
The accident occurred at the DuPont chemical plant in Tonawanda, a suburb of Buffalo, which employs approximately 600 workers. The facility produces polymers and surface materials for countertops, sold under the trade names Tedlar® and Corian®. The process for making Tedlar involves transferring polyvinyl fluoride (PVF) slurry from a reactor through a flash tank and then into storage tanks. The tanks were also inter-connected by an overflow line. The CSB found the company erroneously had determined that any vinyl fluoride vapor that might enter the tanks would remain below flammable limits.
Days before the incident the process had been shut down for tank maintenance due to corrosion on tank agitator supports. The fill lines were locked out for safety. Tanks 2 and 3 were repaired and the process restarted, but work on tank 1 was delayed because the necessary parts were not available. Finally, a contract welder and foreman were engaged to repair the agitator support atop tank 1. Although tank 1 remained locked out from the main process, the overflow line remained open which connected tank 1 to tanks 2 and 3. The CSB determined that flammable vinyl fluoride flowed through the overflow line into tank 1 and accumulated to explosive concentrations. Investigators found that while a facility hot work permit was issued for the task, the DuPont personnel who signed it were not sufficiently knowledgeable about the Tedlar chemical process.
Although DuPont personnel monitored the atmosphere above the tank prior to authorizing hot work, no monitoring was done inside the tank to see if any flammable vapor existed there. The CSB investigation found the hot work ignited the vapor as a result of the increased temperature of the metal tank, sparks falling into the tank, or vapor wafting from the tank into the hot work area.
The explosion blew most of the top off the tank, leaving it and the agitator assembly hanging over the edge. The welder died instantly from blunt force trauma, and the foreman received first-degree burns and minor injuries.
Regular readers might recall that, after investigating three January 2010 accidents at the DuPont Belle plant — including one that killed longtime plant worker Danny Fish — CSB experts recommended that DuPont take a long, hard look at its corporate safety policies and practices:
“These kinds of findings would cause us great concern in any chemical plant — but particularly in DuPont with its historically strong work and safety culture,” said CSB member John Bresland, a longtime chemical plant manager and one-time DuPont lab technician. “In light of this, I would hope that DuPont officials are examining the safety culture company-wide.”
In explaining the Buffalo incident, CSB Team Lead Johnnie Banks said:
Our investigation found that DuPont’s process hazard analysis incorrectly assumed that vinyl fluoride in the Tedlar process could not reach flammable levels in the slurry tanks. And, critically, DuPont personnel did not properly isolate and lock out tank 1 from tanks 2 and 3 prior to authorizing the hot work. The flammable vapor was able to pass through the overflow line into the tank the welder was working on, unknown to him or to the operators who signed off the hot work permit.
The CSB also determined that DuPont should have included the three storage tanks as part of the Tedlar process covered by OSHA Process Safety Management rules. Yet on the day of the accident, a compressor failure led to higher concentrations of vinyl fluoride vapor in the polyvinyl fluoride slurry. Furthermore, a U-shaped seal loop on the flash tank overflow line had a “fishmouth” split in the pipe that could emit vinyl fluoride vapor. Engineers concluded further operation with the broken seal loop presented no hazards, but the CSB determined the pipe split provided a potential pathway for flammable VF gas to enter the tanks.
To learn more about the dangers of “hot work”, watch the CSB video on the issue: