Hexamethylene diisocyanate (HDI) was sprayed at a chemical plant. The workshop, which had been shut down for several days, was partly dismantled to allow a contractor to perform routine scheduled maintenance work. While this maintenance was being carried out, a scheduled update was taking place on the workshop control software. The update caused the air-to-open (ATO) valves on the loop to open. The department manager knew that the software update would shut down the equipment, but he had not identified any risks for the workshop and for maintenance staff working at the same time as the update. All the equipment had been purged and was no longer connected to the line containing HDI, which is hazardous. However, HDI residue was still inside a heat exchanger upstream of one of the ATO valves. When the ATO valve opened, the HDI flowed through the pipe and on to the upstream valve that was being refitted by the contractor. The HDI sprayed the contractor, who suffered chemical burns.

Since the accident, the operator has updated the workshop’s maintenance work procedure so that maintenance on the computer system and physical maintenance on the facilities are no longer conducted at the same time. The operator is also considering the possibility of controlling automatic opening of the valves to assess the risks of uncontrolled opening.