At 8:24 p.m. on a Sunday, the fire safety system (FSS) went off in the chilled water room inside the utility tunnel of a building at a medical equipment manufacturing plant. One of the two security officers went to see what was happening. Upon seeing a cloud, he contacted emergency services and the site’s on-call manager. The cloud had formed because the casing of the equipment used to introduce a biocide into the water system of the plant’s cooling towers had burst. The facility had been shut down since the previous Friday evening so that the cooling towers could be replaced by adiabatic cooling towers. The security guard who went to see what was happening was taken to hospital for tests.

A subsequent inquiry revealed that the incident was due to a combination of three factors:

• In order to replace the cooling towers by adiabatic cooling, the water circuit had been drained and blown out so that a glycol/water solution could then be added. This meant that the biocide feed system had to be isolated by closing two valves on the circuit. However, one of these valves was mistakenly left open. When the glycol/water solution was introduced, it mixed with the biocide. A technician noticed that one of the valves had remained open and he closed it, trapping the mixture of biocide and glycol/water solution between the two valves. This incompatible mixture produced an exothermic reaction that lasted for more than 24 hours, causing the pressure and temperature to rise and ultimately causing the pipe to rupture.
• The equipment used to introduce the biocide was missing a valve, which had been replaced by a cap ever since an incident in 2016.
• Isolating the treatment loop for more than 24 hours is a rare operation for which the risks had not been assessed.

The operator wondered what conditions had led the technician in charge of the work to forget to close one of the valves. It checked the robustness of its maintenance work preparation process. A solution to avoid forgetting any equipment retrofits was to implement a tracking procedure. A risk assessment procedure for maintenance work is now applied at the site and is expected to be extended to partial equipment shutdown operations and the use of equipment in degraded mode.