While re-starting the acrylonitrile units of a synthetic fibre plant, the facilities were flooded with water for 2 hours to test the resistance of equipment. A leak was observed in a steel pipe. Since chlorine was present in the heat insulators covering the pipes, corrosion due to chlorine was immediately suspected to be cause. Several samples were taken and analysed. However, the results excluded the above assumption as the cause of accident. Further investigations revealed that the leak occurred in an area that was often cleaned with sodium hydroxide to remove the frequent polymer deposits. This was confirmed by an expert analysis carried out by a third party. The steel pipe was replaced and as an additional precautionary measure the operator had to test the equipment resistance again by flooding the facilities with water or nitrogen to apply a maximum service pressure for 24 hours under the supervision of the inspection authorities. Since no further leaks were detected, the operator was allowed to re-start the units.

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