Abstract
A failure was initially found at a butt weld on a 4 inch crossover pipe in a particular plant, and subsequent inspection identified material losses in other locations. A systematic metallurgical analysis was thus conducted to characterize the degradation and identify the mechanism(s) of the failure and the material loss. The metallurgical characterization indicated that the degradation and failure were the result of metal dusting due to the coke deposits collected at these specific locations. Metal dusting resulted in reduced wall thickness of the crossover components by forming cavities and trenches. In the case of failure of the 4 inch pipe, original welding procedures at the time of construction, especially pertaining to inter-pass parameters were not followed and resulted in a reduction of corrosion resistance of the austenitic materials, forming weak points to allow metal dusting to initiate. The butt weld was weakened due to metal dusting trenches along the heat affected zones (HAZ) and a stripe-type defect in the butt weld, which was the result of an improper welding practice. The butt weld failed at a critical wall thickness which could no longer sustain the operating pressure at the time of furnace startup. This critical wall thickness at the time of failure was estimated by conducting a stress analysis, and found to be comparable to the remaining wall thickness measured on the failed butt weld.
These degradation locations shared a common feature in that they were physically isolated from the process stream flow; as a result, coke formed during ethane cracking process could preferentially be collected at these locations. During the de-coking process, a localized environment enriched with carbon monoxide was believed to be established due to the incomplete oxidation and gasification of deposited coke within these isolated locations. This belief was confirmed to be true as high concentration of carbon monoxide was always detected in the bulk de-coking gases for the initial period of decoking process. The formation of carbon monoxide during the de-coking promoted carburization and metal dusting process at these isolated locations of the crossover components.
