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The Risk

Bhopal: 2000 killed by cyanide gas

Submitted by Locke » Sun 16-Mar-2014, 08:43

Subject Area: Safety

Keywords: Bhopal, safety

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Around midnight on December 2, 1984, for unknown reasons, a substantial volume of water found its way into a Methyl Isocyanate (MIC) storage tank in Union Carbide’s Bhopal, India pesticide plant. The reaction of water and MIC formed carbon dioxide at high pressure, resulting in the passage of MIC vapour through the pressure release system, up the stack of a vent gas scrubber and into the atmosphere.
At atmospheric pressure MIC decomposes into various components, the most toxic of which is cyanide gas. The immediate death toll from the cyanide release was in excess of 2000 people. Estimates of death and injury in the following days and years range from 30,000 to 500,000. Bhopal therefore represents the world’s worst industrial accident.
The disaster at Bhopal came about through a series of human errors - critical decisions made by human beings. Given the potentially lethal consequences of an MIC release, the storage system had four separate safety protection measures. Based on the account provided in the book Five Past Midnight in Bhopal by D. Lapierre and J. Moro, safe operating procedures required that:
1. All MIC tanks were to be operated at 50% of capacity to allow for injection of a solvent to inhibit any reactions.
2. Tank contents were to be kept below 15° centigrade by a refrigeration system with a high temperature alarm provided in the control room.
3. Should the tanks become over pressured, the pressure relief system featured a scrubber designed to extract toxic chemicals from exhaust gases by injection of caustic soda.
4. Finally, should all else fail, all exhaust gases could be burned off by an ignition system installed at the top of a 34 meter flare stack.
The critical decisions that contributed to the accident were as follows:
1. The MIC tank levels were maintained above 50% and there was no awareness of the solvent injection measure.
2. The refrigeration system had not operated for months as plant management believed it wasn’t necessary.
3. The scrubber had been off line for maintenance for a week.
4. The flare igniter was also out of service for maintenance.
Four separate safety measures were therefore defeated by two common cause faults:
1. Corporate memory loss resulting in lack of competence in process technology (Union Carbide forgot about its own safe operating procedures) and
2. Lack of safety awareness (the ability of human beings to recognise and deal with an unsafe situation).
To abstract these causes further, the disaster was the result of poor education and poor attitude in human beings with decision-making power in a safety-critical environment.
Preventive action is well-known in this case. Four measures:
1. Configuration management. Identify all documents describing operations technology, make sure they are up-to-date and available to operations staff.
2. Regular training and reflection. Conduct weekly safety meetings focusing on some aspect of safe operating procedures. Give staff the task of finding something wrong with the plant every week.
3. Audit. Conduct periodic audits to verify that safe operating procedures are being followed.
4. Corrective action. Take action to rectify non-compliances.


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