In a plastics manufacturing plant, a leak occurred on a union fitting of a ‘sight glass’ during the restart of a secondary ethylbenzene distillation column following shutdown of the styrene unit (metal inspection). A technician visually detected the leak whose flow rate was estimated at 50 to 100 l/h over 15 min. As the temperature of the column was 240 °C, the ethylbenzene vaporised immediately. Once the leak had been located, the column was isolated and the leaking union fitting was retightened. As a precaution, pumping was carried out on the floor at the bottom of the column. The inspection authorities for classified facilities noted that the ‘sight glass removal’ and ‘sight glass reinstallation’ operations had been signed by the sub-contracting company but not counter-signed by the operator. On some charts, it was noted that the operator’s counter-signature was also absent for most of the operations. The inspection authorities for classified facilities ordered the operator to resolve this point.