Pollution
Humain
Environnement
Economique

At 11:30 am., an explosive fire occurred in the melt shop line n°2 of a steelworks factory due to the rupture of a main valve on an oxygen gas pipeline.

The melting shop line number 2 had been built the previous year and was in its first annual maintenance shutdown. During the shutdown, back pressure valves were to be installed to pipelines conveying the so-called raw oxygen and pure oxygen. The pipes had therefore been closed and purged by air. Most of the work had been completed, and the foreman of the factory and the foreman of the subcontractor started preparing the start-up of the oxygen systems. Control work was thus being done on valves on the main oxygen distribution pipeline which runs from the exterior of the building to the melt shop.

The men entered the valve room on the third floor of the melting house, where they successfully opened the main shut valve of the raw oxygen line. They started opening the main shut valve of the oxygen line, but the valve was stuck because the bolt pin holding the hand wheel in the valve stem snapped. The men asked for pipe tongs, with the help of which they started turning the valve hand wheel stem. They managed to turn the stem about one turn, corresponding to a 9° turn of the disc, when the explosive fire broke out. It kept burning fiercely as the oxygen was leaking from the damaged piping. The foreman of the factory, the foreman of the subcontractor and a subcontracted welder, who had come to the scene just before the accident were killed on the spot.

The closure of a valve stopped the oxygen flow and the fire was extinguished within 30 minutes by internal and external fire brigades ; 10 500 cubic meters of gaseous oxygen and 4 tons liquefied oxygen burned before the shut down valve was closed. The damage were limited thanks to building conception (fire containment compartments and non-flammable roof structures).

Judging by the burn marks, the fire started from the oxygen line main shut valve; a butterfly valve with a nominal diameter of 300 mm. Gaseous oxygen compressed to a pressure of 35 bar was flowing in the pipes. There was a by-pass line beside the valve. Due to safety reasons, this line was supposed to be used to equalize the pressure on both sides of the main shut valve before opening the latter. After the accident, the by-pass valve was found to be in a closed position, although no specific information about the use of the by-pass line was found in the investigation. The exact cause of the oxygen ignition is unclear : it might have ignited due to friction, impuries in the oxygen or a foreign object stuck in the valve, the first 2 option necessitating the opening of the valve without equalising the pressure.

The operator made a new safety study, improved written instructions for the use, control and maintenance of the pipelines as well as general safety procedures.