Gas release at the bulk terminals complex, Chicago, Illinois. 26th April 1974

Accident summary

Bulk Terminals was a storage tank farm with 78 tanks ranging in size up to 4900 m3. At about 12:30 hours on Friday 26 April 1974 a dull thud was heard and fumes were seen rising from the bund surrounding a 3300 m3 tank of silicon tetrachloride. It was discovered that a pressure relief valve on a 6-inch line leading to the tank had been inadvertently closed. The pressure in the system was sufficient to burst a flexible coupling in the line, shifting the piping system and cracking a 3-inch line on the tank wall. Liquid silicon tetrachloride escaped forming an irritant cloud containing hydrogen chloride gas.

The terminal management waited for the owners of the chemical to take emergency action and the fire service did not respond, as there was no fire. The EPA sent lime trucks to neutralise the chemical, but these were refused entry to the site. By 15:00 hours the cloud was 400 m wide, 300-450 m high and 1600 m long.

At 04:10 hours on Saturday 27 April, foam was added to blanket the liquid in the bund but this failed. At 09:00 hours fuel oil was added along with eight truck loads of lime. The vaporisation reduced dramatically and operations began to transfer the liquid from the damaged tank. At 08:00 hours on Sunday 28th April, it began to rain. Power lines were corroded by the hydrochloric acid in the rain, and four pumps became inoperable due to corrosion before a general power failure stopped all pumping.

The materials added into it had reduced the capacity of the bund, and a further pit had to be dug to take the overflow in the event of a full tank failure. It was attempted to seal the leak on the tank using quick drying cement. The first attempt failed and it wasn't until 23:30 hours on Monday 29 April that the leak was sealed. It took until 3 May to empty the tank and until 15 May before emissions had reduced to tolerable levels. One person was killed, 160 hospitalised and 16,000 people were evacuated during this incident.

Failures in technical measures

  • Inadvertent closure of a pressure relief valve allowed the pressure to rise causing the failure of a flexible connection.
  • Relief Systems / Vent Systems: relief valves, bursting discs
  • Design Codes - Pipework: use of flexible pipes
  • The site emergency plan was not developed, leading to a situation where no-one knew whose responsibility it was to take action.
  • Emergency Response / Spill Control: site emergency plan
  • The secondary containment was inadequate considering that blanketing and absorbent materials would need to be added in the event of a major spill.
  • Secondary Containment: Bunds


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.

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