On a steelmaking site, a 30-m segment of steel mill gas pipeline connecting the converters to a 60,000cu.m gasometer collapsed at noon subsequent to a 200-mbar internal pressure drop. The factory was operating normally (steel production with gas transfer) when a short-circuit on a cable feeding the coking plant caused at 11:30 am an electrical outage at the steel mill. The positive safety features were successfully activated: closure of the gasometer input and output valves as well as the steel mill output valve, switching of the 3-channel valve with routing of gas to the flare, formation of the safety water joint. The gas (60°C temperature) being transferred to the gasometer was trapped in the 310-m long, 2.4-m diameter pipeline and cooled to ambient temperature, initiating the pressure drop that caused the accident. The vacuum breaker installed to overcome this phenomenon malfunctioned, its sectional valve being closed for an unknown reason. The collapse occurred on the pipe’s weakest section (8-mm thick vs. 10 mm elsewhere on the network), i.e. at the gasometer intake. Damage to the pipeline was estimated at €160,000. The financial loss due to the lack of steel mill gas and forced substitution with natural gas was evaluated at €320,000 for the 2 weeks it took to repair the pipeline. Subsequent to this accident, the operator was required to take the following steps relative to the vacuum breaker: perform an inspection and set up an annual maintenance programme, locate the device valves and deactivate the sectional valves left open, and verify valve positions and water supply status on a weekly basis. Moreover, the operator was requested to disseminate accident feedback to all plant personnel as well as to personnel with maintenance subcontractors.