By C H Anand, Reliance Group of Industries, Jamnagar
In our delayed coker unit one evening, one of our operators noticed fumes from the drain connection of feed line in one of our coke drums. The drum was in its coking cycle. It was found that the leakage was partially coked up when insulation was removed. The leakage was from the sockolet to nipple weld. After a round of discussions, it was decided to shutdown the block for repairing the connection.
When our technician tried to remove the bolting of the end flange of the connection, the connection got broken from the leaky portion. Upon initial inspection of the broken piece, it was observed that the broken portion had become wafer thin; losing its 90% of thickness within 18 months of operation. Detailed analysis revealed that, the metallurgy used for the failed nipple was carbon steel, while the parent line is 9 Cr steel. Corrosion tests revealed the cause of failure as high temp: sulfur corrosion which had eaten away the carbon steel nipple within months. The corrosion rate was as high as 3 to 3.5mm/year which closely matched the theoretical values presented in relevant literature.
Had the operator missed seeing the light fumes, there could have been a fatal fire due to hot feed coming out at 490 deg C Lesson learned: 100% material checking of small bore connections in alloy steel lines could have prevented the unwanted production losses and a “near miss”. Such connections in alloy steel lines are “ticking time bombs.”