A Raffinate Splitter was inadvertently overfilled with liquid during startup. As the splitter warmed up, the pressure rose and liquid puked into the overhead line. The pressure safety valves (PSVs) were located in the overhead line approximately 45 m (148 ft) below the top of the tower. The overfill created enough static head to cause the PSVs to lift, discharging a large quantity of light hydrocarbons to the unit blowdown drum which was connected to an atmospheric vent stack (not equipped with a flare). Most of the liquid released flowed to a closed sewer but some puked like a geyser from the top of the stack. The resulting vapour cloud found an ignition source and exploded. Fifteen people in or near temporary turnaround office trailers located close to the blowdown stack were killed and a further 180 were injured. A shelter-in-place order was issued requiring some 43,000 people to remain indoors

Basic Cause:
Light naphtha puking from an atmospheric blowdown stack, forming a vapour cloud which found an ignition source (probably idling diesel vehicle engine) and exploded.