A large explosion and fire that took the lives of two workers at the Bayer CropScience plant last August was caused by a thermal runaway reaction, U.S. Chemical Safety Board (CSB) investigators said in preliminary findings released in April.

Scene of August 28 tank explosion at Bayer CropScience facility.
Photo courtesy of CSB

A large explosion and fire that took the lives of two workers at the Bayer CropScience plant last August was caused by a thermal runaway reaction during the production of an insecticide. The event likely resulted from significant lapses in chemical process safety management at the plant, U.S. Chemical Safety Board (CSB) investigators said in preliminary findings released in April.

The blast on August 28, 2008, in Institute, W.Va., occurred as the runaway reaction created extremely high heat and pressure in a vessel known as a residue treater, which ruptured and flew about 50' through the air, demolishing process equipment, twisting steel beams and breaking pipes and conduits. Two operators died as a result.

Eight workers reported symptoms of chemical exposure, including aches and intestinal and respiratory distress. Among those reporting serious health effects were two employees of the Norfolk Southern railway company; five Tyler Mountain, W.Va.-based volunteer firefighters; and an Institute, W.Va.-based volunteer firefighter. Two sought treatment at a hospital emergency room the next day, were treated, and released.

When he released the preliminary findings prior to a planned CSB public meeting in Institute, CSB Board Chairman John Bresland said, "Our investigation is continuing, but we are here to brief the community about what we know at this point."

According to Bresland’s remarks, the explosion at Bayer could have had additional grave consequences.

"There were significant lapses in the plant's process safety management, including inadequate training on new equipment and the overriding of critical safety systems necessitated by the fact the unit had a heater that could not produce the required temperature for safe operation," the chairman said.

John Vorderbrueggen, the CSB lead investigator, noted the accident occurred after an extended maintenance shutdown of the entire Methomyl section of the Larvin pesticide-manufacturing unit. (Methomyl is used to make Larvin).

Vorderbrueggen said, "Prior to starting up, Bayer had recently upgraded the computer control system for the unit... The control screens were completely different - and Methomyl production equipment control was changed from a keyboard to a computer mouse - yet operators had not been fully trained and prepared to operate the complex process equipment on the new system. Furthermore, the written operating procedures for the unit were significantly out of date and did not adequately address all process equipment startup and normal operating steps."

Vorderbrueggen added that the residue treater had an undersized heater. "According to unit operators, the heater for the residue treater was incapable of reaching the required temperature to begin the controlled decomposition of Methomyl."

As a result of the heater problem, operators used a workaround that involved defeating safety interlocks controlling flow into the residue treater vessel. The CSB found a normalized practice outside of operating procedures of starting to feed Methomyl into the vessel below the required temperature in order to create the necessary heat for the startup. But, bypassing the interlocks made it more likely that too much Methomyl would enter the vessel. Safety analyses and the operating procedure warned that Methomyl concentration above one percent inside the residue treater would likely cause it to violently rupture.

"As a result of equipment deficiencies, improper procedures and lack of training on brand-new computerized control equipment," Mr. Vorderbrueggen said, "the vessel was charged with as much as a 20 percent solution of Methomyl in solvent, whereas the residue treater was designed to safely decompose the chemical at a concentration of less than one percent in solution."

The CSB reported that operators attempted to check the residue treater vent system as the pressure rose. But, the residue treater ruptured, suddenly released 2,500 gal of Methomyl-solvent liquid and chemical decomposition products.

"These equipment deficiencies and procedural deviations were never subjected to formal management-of-change reviews to assess their safety - a key requirement of the OSHA process safety management (PSM) standard," Bresland said.

Bresland noted the CSB investigation is continuing, and that safety recommendations will be issued in the final report, expected later this year.