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An explosion and a fire that killed two workers at the Bayer Cropscience plant on 28 August 2008 was caused by a thermal runaway reaction during the production of an insecticide.

It is likely that the event resulted from significant lapses in chemical process safety management at the plant, US Chemical Safety Board (CSB) investigators revealed in preliminary findings released at a news conference.

The blast – in Institute, West Virginia – occurred as the runaway reaction created extremely high heat and pressure in a vessel known as a residue treater, which ruptured and flew about 50ft (15m) through the air, demolishing process equipment, twisting steel beams and breaking pipes and conduits.

Aside the two operators that were killed, eight workers reported symptoms of chemical exposure, such as aches and intestinal and respiratory distress, including two employees of the Norfolk Southern railway company, five Tyler Mountain volunteer firefighters and an Institute volunteer firefighter.

Two sought treatment at a hospital emergency room the next day; they were treated and later released.

Releasing preliminary findings prior to a planned CSB public meeting in Institute, John Bresland, the CSB chairman, said: ‘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 explosion occurred within 80ft of a pressure vessel containing more than 13,000lb of methyl isocyanate [MIC], a raw material for the pesticide that the company was making at the time and the same chemical that caused death and injury in the Bhopal accident 25 years ago.

‘As our investigation continues, we will look further into the issues surrounding the safe placement of the tank and its potential vulnerability.

‘We note that other chemical companies, notably Dupont, no longer store MIC in their chemical production and we are looking into other systems that make and then immediately use the MIC, eliminating the need for storage,’ he added.

Bayer Cropscience is a large chemical complex of more than 400 acres that was first constructed in the 1940s.

Until 1986, it was owned by Union Carbide, which produced carbamate pesticides at the site.

It was acquired by Bayer in 2002 and now has more than 500 employees.

John Vorderbrueggen, the CSB’s lead investigator, said that the accident occurred after an extended maintenance shutdown of the entire Methomyl section of the Larvin pesticide-manufacturing unit.

He said: ‘Prior to starting up, Bayer had recently upgraded the computer control system for the unit, replacing a Honeywell system with one purchased from Siemens.

‘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 start-up and normal operating steps,’ added Vorderbrueggen.

He also said that the residue treater, a large pressure vessel, 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 longstanding heater problem, operators had to use a workaround.

This involved defeating safety interlocks controlling flow into the residue treater vessel.

The CSB found a normalised 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 start-up.

However, 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 per cent inside the residue treater would be likely to cause it to rupture.

Vorderbrueggen said: ‘As a result of equipment deficiencies, improper procedures and a lack of training on brand-new computerised control equipment, the vessel was charged with as much as a 20 per cent solution of Methomyl in solvent, whereas the residue treater was designed to safely decompose the chemical at a concentration of less than one per cent in solution.’ The CSB reported that operators attempted to check the residue treater vent system as the pressure rose.

However, the residue treater ruptured, releasing 2,500 gallons of Methomyl-solvent liquid and chemical decomposition products.

Bresland said: ‘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.’ The CSB is also examining operator fatigue as a possible contributor to the accident.

Unit operators worked high overtime levels during the three months prior to the accident, averaging almost 20 hours a week of overtime.

Operators regularly worked 12-hour days – sometimes up to 18 hours – with very few days off.

Bresland said: ‘We are concerned about the potential for operator fatigue, which can, of course, be an important factor in major accidents.

‘Our preliminary information clearly indicates that, as I told a congressional committee investigating the Bayer accident, the accident could have been prevented,’ he added.

The pressure vessel containing the highly toxic MIC is surrounded by a large wire-rope protective mesh shield designed to prevent impact.

Bresland said that the CSB is planning to study the design basis for the protective shield to determine whether the MIC tank is appropriately located and protected.

He noted that the CSB investigation is continuing and that safety recommendations will be issued in the final report, which is expected later in 2009.

US Chemical Safety Board

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