B5/2003Y Explosive fire at melt shop in Tornio on September 19, 2003
The stainless steel manufacturing melt shop in Tornio is part of AvestaPolarit Stainless (Outokumpu Stainless as of January 12, 2004). The melt shop line number 2 that had been built the previous year was in its first annual maintenance stoppage. During the stoppage back pressure valves were to be installed to pipelines conveying the so called raw oxygen and pure oxygen. Therefore they had been closed and made gasfree. Most of the work had already been completed, and the foreman of the factory and the foreman of the subcontractor started preparing the start-up of the oxygen systems.
The men entered the valve room in the third floor of the smelting house, where they successfully opened the main shut valve of the raw oxygen line. Thereafter they started opening the main shut valve of the oxygen line. The valve was apparently 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. At that moment an explosive fire broke out. It kept burning strong as the oxygen was leaking from the damaged piping. Both the foremen that had been opening the valve and a welder who had come to the scene just before the accident were killed.
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 equalise 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 certain information about the use of the by-pass line was found in the investigation. Whether or not the by-pass line was used will have affected the cause of the fire.
During the investigation ten basic causes for fire were looked into. In this case the only possible reasons seem to be friction and particle impact. On the basis of the investigation the fire may have ignited in three different ways. 1. The slide bearings of the valve disc stem may have been damaged, and two steel surfaces may have come into contact in the damaged area. When the valve was being opened, these two seized surfaces may have become hot and set fire on the bearing material which ignites more easily than steel. 2. The valve may have been opened ajar without equalising the pressure. In this case the particles in the fast oxygen flow of the valve gap would have struck valve parts, become hot, and ignited the valve to burn in the oxygen. 3. A foreign object may have gotten stuck in the valve and formed a friction pair that enabled friction heating when the valve was being opened. The third option is possible even though the by-pass line had been used, but the first two necessitate the opening of the valve without equalising the pressure.
In order to avoid any similar accidents the Investigation Commission recommends that dangerous work assignments be identified and instructions for carrying them out be compiled. Potentially dangerous malfunction, fault, and abnormality situations should similarly be defined and detailed instructions to deal with them should be drawn up. Abnormal situations that have occurred should also be registered. The board of investigation further recommends that EU directives should require the valve manufacturers to compile instructions concerning the usability of valves with different gases. Among other things, the instructions should take into consideration the circumstances and purpose of use. In order to guarantee the cleanness of oxygen lines the board of investigation recommends that a plan and instructions should be drawn up for ensuring the sufficient cleanness of the piping. Furthermore, any valves connected with dangerous operations should be included in the preventive maintenance systems of the companies.
The investigation report is available only in Finnish.