On 24 April 2013, there was a release of orthodichlorobenzene (ODCB) from an ODCB cold system after fixing screws failed and a cover plate was torn from a manual locking valve.
The ODCB released was collected in a basin and pumped into a special emergency tank via a waste water pit.
There was no outgassing of the ODCB (low temperature and water cover in the waste water pit).
– The malfunction was detected when a volume alarm was activated in the ODCB cold system.
– The ODCB released was drained into the basin in a controlled manner and then pumped to the special emergency tank.
– As a precaution, the plant fire service was deployed to precipitate any aerosols.
– An air pollution measuring vehicle was deployed at the plant boundary downwind from the leak.
– The production plant was shut down to make it safe.
– As a precaution, the plant fire service was deployed to precipitate any liquid aerosol.
As a precaution, a warning was issued in the areas downwind from the leak (own production areas and neighbouring companies).
– A D1 signal was transmitted to the district control centre.
– An air pollution measuring vehicle was deployed at the plant boundary downwind from the leak.
– The leaked ODCB was pumped into the special emergency tank.
The manual valve was replaced, as was the ODCB which had leaked out.
Members of the plant fire service and plant staff who had replaced the manual valve were decontaminated in the basin.
A pipe section with traces of the ODCB was rinsed and the rinse water was disposed of in the biological waste water treatment plant.
The release of the orthodichlorobenzene (ODCB) was caused by broken cover screws on a manual valve in the ODCB cold system due to chloride-induced stress corrosion cracking.
The local concentration of the damage is due to the partially restricted, critical corrosion conditions resulting from high local chloride contamination and a low pH value (TDI residue dust, sea air containing chloride, nearby hydrochloric acid electrolysis plant) in connection with prolonged humidification (rainwater, cleaning water and temperature of the ODCB cold system), which prevents rapid drying.
- Installation of a filter at the point of discharge of the TDI residue to minimise possible chloride contamination through TDI residue dust.
- Replacement of (similar) cover screws on all valves of the ODCB cold system.
Using screws made of a different material was considered but deliberately ruled out, since, on the one hand, there is a high potential for corrosion on contact with the higher-alloy casing material when screws made of non-alloy and low-alloy material are used and, on the other, there is a high risk of confusion during maintenance when higher-alloy materials are used, in particular in the case of standard components such as screws, due to their very similar appearance.
- Proper functioning of the cover screws will in future be ensured by periodically inspecting the screws on the valves of the ODCB cold system in the relevant part of the plant, in accordance with an inspection schedule agreed with the authority.
The nature and scope of the inspection programme will be periodically assessed in close coordination with the authority on the basis of the results obtained, and will be improved if necessary.
- In other areas of the TDI plant, a representative sample of valves of the same design, distributed over the entire system, was selected to identify any damage to cover screws.
- A representative sample of valves of the same design, distributed over the entire system, was selected to identify any damage to flange screws.
- Furthermore, the company group instigated an inspection of cover screws on selected valves in other establishments on the site and in other TDI plants worldwide.
- The operator was also kept informed of the examination of the case, the causes identified of the damage and the corrective measures taken.
