UK Health & Safety Executive, May 6, 2006
In the early hours of 3 December 1984 a relief valve on a storage tank containing highly toxic methyl isocyanate (MIC) lifted. A cloud of MIC gas was released which drifted onto nearby housing.
Prior to this, at 23.00 hrs on 2 December, an operator noticed the pressure inside the storage tank to be higher than normal but not outside the working pressure of the tank. At the same time a MIC leak was reported near the vent gas scrubber (VGS). At 00.15hrs a MIC release in the process area was reported. The pressure inside the storage tank was rising rapidly so the operator went outside to the tank. Rumbling sounds were heard from the tank and a screeching noise from the safety valve. Radiated heat could also be felt from the tank.
Attempts were made to switch on the Vent Gas Scrubber but this was not in operational mode.
Approximately 2,000 people died within a short period and tens of thousands were injured, overwhelming the emergency services. This was further compounded by the fact that the hospitals were unaware as to which gas was involved or what its effects were. The exact numbers of dead and injured are uncertain, as people have continued to die of the effects over a period of years.
The severity of this accident makes it the worst recorded within the chemical industry.
Failings in technical measures
* The flare system was a critical element within the plant’s protection system. However, this fact was not recognised as it was out of commission for some three months prior to the accident.
* Plant Modification / Change Procedures: HAZOP, identification of safety critical elements
* Hazards associated with runaway reactions in a chemical reactor are generally understood. However, such an occurrence within a storage tank had received little research.
* Reaction / Product Testing: laboratory testing
* The ingress of water caused an exothermic reaction with the process fluid. The exact point of ingress is uncertain though poor modification/maintenance practices may have contributed.
* Design Codes – Plant: ingress of unwanted material
* Maintenance Procedures: training and competence levels
* Plant Modification / Change Procedures: HAZOP
* The decommissioning of the refrigeration system was one plant modification that contributed to the accident. Without this system the temperature within the tank was higher than the design temperature of 0°C.
* Plant Modification / Change Procedures: HAZOP, decommissioning procedures
* The emergency response from the company to the incident and from the local authority suggests that the emergency plan was ineffective. During the emergency operators hesitated when to use the siren system. No information was available regarding the hazardous nature of MIC and what medical actions should be taken.
* Emergency Response / Spill Control: site emergency plan, emergency operating procedures/training
FP Lees, ‘Loss prevention in the process industries – Hazard identification, assessment and control’, Volume 3, Appendix 5, Butterworth Heinemann, ISBN 0 7506 1547 8, 1996.
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