At 10:12 am., a leak of titanium tetrachloride (TiCl4) during maintenance work on an evaporator occurred during the shut-down of a titanium oxide (TiO2) installation.
Because of the solid impurities in that stage of the process, the installations are not emptied and circulation pumps are kept running to prevent the blocking of equipment, even when the installations are shut down for maintenance.
During the shut-down , employees carried out an inspection of the level measurement device of the TiCl4 evaporator. On disconnecting the signal wiring of the level measurement device, the process computer responded to the absence of signal as empty level and consequently opened the control valve to fill the evaporator. The evaporator was not isolated prior to the start of the work due to an ineffective work permit system, so it filled with TiCl4 without anyone noticing Meanwhile, the work continued and the level measurement device was removed. The workers observed excess vapors to continue their work and went to the control room to check the vacuum system. At this moment, the evaporator overflowed, releasing TiCl4 on the second and first level of the reactor building.
Upon contact with water (air humidity), TiCl4 produces HCl : a thick toxic white cloud formed inside the reactor building. Two subcontractors that were working on the first floor didn’t manage to locate an emergency ladder near their workplace. They died, trapped in an unknown installation with zero visibility. One employee and 2 emergency responders were injured. Roads surrounding the facility were closed and neighbouring inhabitants were sheltered in place. The neighbouring plant was evacuated.
The accident was due to several factors :
- a defective work permit system – the installation was not isolated prior to the work.
- A defective information system for subcontractors – victims did not receive safety information about the installation ( they did not use the safe evacuation route).
- No interlock system to prevent overflow of evaporator. The high level alarm was silenced by the panel operator without action.
- No difference between no signal and zero signal made in the process computer.