At around 5:40 p.m., a technician at a thiochemicals plant was overcome by fumes emanating from a pit located near a storage tank. The pit was being used to hold effluent discharged by a unit restarted the day before. The effluent consisted of methanol and methyl mercaptan (also known as MM, or methanethiol). MM is a gas that is toxic in high concentrations and has an extremely strong smell. The technician moved away from the area, but was unable to breathe. Nearby employees rushed to his side and the plant’s firefighters evacuated him to hospital. In accordance with plant procedures, the operator did not implement the internal emergency plan. Instead, it issued an area alert and 240 employees present in the area were evacuated. The local residents complained to town hall about the siren’s noise and the smell of the MM. The production units were shut down and the biodegradable wastewater system was isolated and flushed with water. The wastewater was diverted to a containment pond to enable the wastewater treatment plant used by the chemicals complex to operate. The onsite alert was lifted at 8:15 p.m. and operations resumed after the situation was under control. The plant operator issued a press release.

The accident was caused by miscalibration of the conductivity meter used to measure the water content in the reflux drum (tank) of the MM synthesis fractionating tower. When it was installed a few years earlier, the meter had not been calibrated using the chemicals inside the drum (MM, water, methanol) and the existing physical conditions. As a result, the display in the control room showed a purge volume that was greater than the actual volume. The control-room operators, assisted by management because the plant had been restarted after a major retrofit shutdown, drained the contents of the drum to the methanol recirculation tower without searching for the cause of this abnormally high volume. The MM present in the drained liquid quickly evaporated due to the heat and the difference in pressure between the distillation tower and the recirculation tower. At 5:30 p.m., a sensor detected the sudden increase in pressure inside the recirculation tower. The sensor triggered an alarm in the control room and turned off the heating. Failing to make the connection with the abnormal volume of water detected shortly before, the control-room operators did not realise that MM was inside the recirculation tower and they drained it to the wastewater treatment circuit as per plant procedure. Because the usual wastewater outlet was unavailable, the drained liquid flowed into the biodegradable wastewater system leading to the site’s wastewater treatment plant. The treatment plant’s operators were informed that the COD level could rise because of the methanol. As a section of this system was not sealed, MM and methanol fumes emanating from the hot liquid entered into the waste water pit of the nearby unit.

The operator had not recalibrated the conductivity meter after it was installed because it was only used for control functions, not safety reasons. The site’s instrumentation department recalibrated the conductivity meter the following day. A blind flange was fitted on the biodegradable wastewater system to temporarily isolate the pit. The following corrective measures were implemented:

•  A high-pressure safety device was fitted at the top of the recirculation tower. This device closes the reflux drum’s drain valve and opens the tower’s drain valves to the wastewater circuit.
•  A check valve was fitted between the pit and the biodegradable wastewater circuit.
•  A padlock was fitted on the manual bypass valve on the outlet of the reflux drum to prevent MM being accidentally sent to the recirculation tower.
•  The unit’s operating procedures were amended to take this feedback into account, as sending the reflux drum’s contents to the recirculation tower was a recent modification (less one month).