At a petrochemical facility, gaseous hydrocarbons leaked from a 3-inch pipe operating under 300 mbar of pressure. The pipe was not being used in the industrial process, instead it served as the tap on another line used at the head of a diluent supply tank the rubber production unit reactor. Consequently, this pipe could not be isolated without first shutting down the unit. During the 80 hours required to locate this leak, 17 tonnes of volatile organic compounds (VOC), including 8.5 tonnes of methyl chloride (CH3Cl or chloromethane), a colourless, easily flammable and irritating gas, were released into the atmosphere. Technicians wearing self-breathing apparatuses temporarily clogged the leak with a polymer strip, and a monitoring protocol was implemented.

The event occurred during an extended cold spell with persistent negative temperatures dropping to -18°C for 8 consecutive days (note that the equipment had been designed for a minimum temperature of -12°C). The damaged section cracked over a 5 to 6-cm stretch due to formation of an ice plug and leaked at the time of thawing. Water accumulation in the pipe resulted from the presence of wet gas combined with the absence of product flow in the unused tap. An inspection performed 8 years prior had not revealed any noteworthy loss of metal thickness in this section, which had not been considered highly critical.

It took extra time to identify the leak due to: a colourless gas, a pipe section placed in a high position, the unit’s gas detectors remaining idle, and insufficient information furnished by the control room technician. This technician was responsible for managing the inventory of diluents for each piece of equipment on an hourly time scale yet had no overview of total daily diluent consumption for the entire unit. This consumption indication could only be obtained by a detailed analysis, which placed the beginning of the leak at more than 3 days prior to the accident. The day before, abnormal diluent consumption had been noticed on the visualisation screen; however, low pressure through the section, the pipe’s high position and the colourless nature of the product had prevented accurately locating the leak. A technician discovered it by accident the next day when climbing a scaffolding assembled for on-site works.

This 8.5-tonne release of CH3Cl over an 80-hour period, equivalent to an annual discharge of 150 t, had no impact on either the acute or chronic toxicity levels for neighbours. The damaged pipe was removed a few weeks later, and the facility operator adopted several measures designed to eliminate or monitor the presence of water in pipes featuring the same characteristics and carrying toxic or flammable fluids during periods of intense cold: circumventing the targeted sections, eliminating water in the pipes, or practicing more extensive monitoring during extreme weather events.