A high-pressure water leak occurred on the automatic drainage pipe during the steam generator start-up sequence in a chemical plant. The gas burners were in operation and the production of steam at 140 bar and 340°C (saturated steam phase) was starting at the separator. In order to drain the separator in order to reach its set point, the technician controlled the opening of the automatic drain valve from the control room. The operation resulted in a leak that caused a powerful emission of steam in the building. Two employees from the maintenance department went to the sick bay for a hearing check but no injuries were reported. Property damage, such as broken windows, was observed. This specific start-up operation is carried out five times a year. The analyses showed that the incident was caused by corrosion of the drainage pipe, a part of which is made of non-alloy steel and is thus more sensitive (lower hardness) to cavitation/vaporisation phenomena. The damage to the automatic drainage pipe resulted in the unavailability of the generator.