Special Features of Metal Lamination in Hydrogen Sulfide Absorber of Catalytic Reforming Unit and Hydraulic Cleaning

Abstract

The paper analyzes the technical state of the hydrogen sulfide absorber of the catalytic reforming unit and hydraulic cleaning containing hydrogen and hydrogen sulfide with its operation time of 168,200 h (21 yr). 17G1S manganese-silicon steel is used in the absorber structure. In operation, in the vessel shell, a cone-shaped region of subsurface metal lamination is developed across the entire periphery. The region is under constant observation using the methods of nondestructive testing. In case the region of the damaged metal intersects the longitudinal welded joint, the vessel shell is being repaired. A low-temperature (to the boiling point) hydrogen-related fracture of metal in the petroleum refining due to hydrogen saturation and its accompanying cracking are considered to be the most critical types of corrosion of oilfield equipment. In view of this, a set of laboratory tests using specimens of metal templates cut out of the damaged shell region has been performed to evaluate the serviceability of the absorber, as well as to determine the effect of hydrogen sulfide on the structure and mechanical characteristics of the metal. It is found that the process of lamination is detected inside the metal intersection and is caused by the macroinhomogeneity of rolled metal, i.e., the axial segregation zone, which is the most critical region of the deformed metal. X-ray phase analysis lends credence to the metallurgical nature of lamination of the shell plate. The results of metallographic examination imply that the subsurface hydrogen-induced cracking is caused by aggregation of brittle refined nonmetallic inclusions, mainly of MnS, and it propagates chiefly over sulfide films along the rolled metal direction. The results of experimental investigations on the specimens are provided. From the fractographic investigations on the fractured surface of specimens after mechanical testing it is established that the process of fracture is ductile occurring via microvoids coalescence in the fracture of bridges between them. The key mechanism of microvoids formation is the inhomogeneity of the plastic strain in the micro-volumes around the nonmetallic inclusions.