Buncefield, 2005: filling blind
A petrol storage tank overflowed for about 40 minutes because both its level protections had failed. The vapour cloud exploded.
What happened
Tank 912 was being filled with unleaded petrol from a pipeline overnight. Its automatic level gauge had been sticking intermittently since a service in August, and on that night it stuck, showing a constant level while the tank kept filling. The independent high-level switch, which should have shut off the flow, was inoperative: a test lever had not been put back in place after testing.
Petrol cascaded over the tank roof, broke up into droplets and evaporated, forming a large vapour cloud in still, cold air. At 06:01 it ignited, causing a vapour cloud explosion far more violent than expected and the largest fire in peacetime Europe.
Sequence of events
Why it happened
- Both layers of overfill protection failed: a gauge known to stick, and an independent switch left inoperative after testing.
- No alarm on “no level change while filling”, and limited control of incoming flow by the site.
- Containment failed: bund walls and joints leaked and fire-water and fuel escaped, polluting groundwater.
- Unexpected severity: a vapour cloud in a moderately congested area exploded far more strongly than models of the time predicted.
Lessons for your plant
- Overfill protection must be truly independent, tested properly, and returned to service after testing (functional safety, SIL).
- Treat repeated instrument faults as warnings, not nuisances.
- Design secondary and tertiary containment (bunds, drainage) to hold fuel and fire-water.
- Do not assume a vapour cloud will only flash: congestion can make it explode.
Sources
- Official · HSE (COMAH Competent Authority): Buncefield, why did it happen? (PDF, copy hosted by IChemE)
- Official · HSE: Response to the Buncefield incident, with links to the Major Incident Investigation Board reports
- Secondary · Buncefield fire: Wikipedia overview
Summary written for training. Figures are as published by the investigating bodies and widely cited sources; see the sources for full details.