Explosion and fire at Shell, Stanlow. 20th March 1990
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Accident summary
On 20 March 1990 the halogen exchange reactor on the Fluoroaromatics plant was ruptured by the pressure generated by a runaway reaction. The plant was partially destroyed and missiles were projected over 500m. Six employees were injured and one subsequently died from post-operative complications.
A batch had been charged into the vessel and was being heated up as normal. When it reached 165oC, the temperature continued to rise and the operators adjusted the jacket temperature. The display screen in use did not display pressure and they were unaware of a corresponding rise in pressure. By the time they were alerted to the rise in pressure the pressure relief valves had lifted. Before any other corrective action could be taken, the reactor exploded. The pressure in the vessel reached a value of about 60-80 barg compared with the relief valve set pressure of 5 barg.
The resulting blast was enhanced by the formation of a fireball, which occurred when the contents of the reactor ignited within the plant structure. This started local fires and initiated what became a major conflagration in an adjacent unit where vessels containing xylene were damaged by the blast/missile effects. The ensuing fires were brought under control in four hours by the Shell fire team and Cheshire fire service.
The initial cause of the incident was the ingress of excessive water into the process leading to the formation of acetic acid which, upon recycle to the reactor, reacted vigorously with the reactor contents initiating the explosion. Water was present as a part of the process, however a massive incursion led to the formation of a separate layer in the process vessel which was not removed but recycled back into reactor.
Failures in technical measures
- Vital process information, in this case reactor pressure, was not displayed on the VDU the operators were using at the time. Had this been available they would have been able to detect an abnormal rise in pressure sooner and possibly in time to take corrective action.
- Control Room Design[17]: human factors
- The layer of water that remained in the vessel allowed the formation of acetic acid that was carried through the distillation train and into a subsequent batch.
- Raw Materials Control / Sampling[18]: identification of potentially hazardous impurities (especially in exothermic reactions)
- Reaction / Product Testing[19]: effect of impurities
- The pressure in the vessel significantly exceeded the relief valve setting during relief. This indicates that the relief system design was inadequate or that the relief valve did not operate at the correct setting.
- Relief Systems / Vent Systems[20]: relief valves, sizing of vents
References
FP Lees, 'Loss Prevention in the Process Industries – Hazard Identification, Assessment and Control', Volume 3; Appendix 1, Butterworth Heinemann, ISBN 0 7506 1547 8, 1996.
'Chemical reaction hazards', Edited by J. Barton & R. Rogers, IChemE, ISBN 0 85295 341 0, 1997.