Metal dusting in the low-point drains of crossover piping system in petrochemical environment

摘要

In April, 2008, a leak from a welded elbow was found on the coil inlet piping of an ethane cracking furnace in one of NOVA Chemicals plants. Subsequent examinations identified more potential failure locations, e.g. abandoned thermowells, low-point drains. All these locations were identified as containing confined spaces, which were relatively isolated from the process stream and allowed the buildup of coke. Extensive analysis revealed that metal dusting of the 304H stainless steel was responsible for the material wastage and reduction of wall thickness at these specific locations. This paper describes the analysis on the low-point drains and elaborates on the corresponding mechanism of the metal dusting in these confined spaces. Two 304H stainless steel (SS) low-point drains were analyzed to investigate the root causes of the failure. These failed low-point drains were located in the crossover piping system of an ethane cracking furnace. The failure was in the form of material loss, resulting in the thinning of the plug wall and creating a leak path. The analysis confirmed that the material loss was the result of metal dusting due to excessive carbon deposition and carburization. The metal dusting is proposed to occur in the decoking process, which is an oxidizing environment, rather than the cracking process. Excessive carbon content was detected in the void formed by metal dusting and the crevice between the threads. The coke formation was in the form of catalytic coking, promoted by the existing catalysts (iron and/or nickel) in 304H SS. The metallurgical analysis revealed carburization in the steel and local burns were present simultaneously with the metal dusting. The reduction or disappearance of the original protective layer (Cr_2O_3) allowed a diffusion path of the carbonaceous gases, thus creating carburization and metal dusting. The disappearance of the protective oxide (Cr_2O_3) layer was the result of local excessive temperature causing loss of chromium.