Investigation of cracks in the feed pipe of reactor in the Coal-to-Oil hydrogenation unit

Abstract Pressure pipelines, as special pressure-bearing equipment, are widely used in industries such as petroleum, petrochemicals, chemicals, and electricity. They often operate in environments characterized by high temperatures, high pressures, and corrosive media. Once accidents occur, they can have catastrophic consequences. This paper focuses on the analysis of cracks discovered during regular inspections of a feed pipeline in a hydrogenation unit. Macroscopically, there are approximately 12 cracks, with lengths ranging from 10 to 20 mm, propagating from the outer surface inward but not penetrating the pipe wall. Analytical results show that the material composition, hardness, mechanical properties, and metallographic structure all meet standard requirements, with no signs of material degradation. Subsequently, scanning electron microscopy was used to analyze the corrosion products at the crack tips, revealing a high content of chlorine elements. It is inferred that the cracks on the parent material of the pipe are due to chloride-induced stress corrosion cracking, where the cracks initiate from pits on the external surface of the pipeline and propagate inward and axially under the stress corrosion induced by chlorides. Finally, the paper proposes corresponding suggestions for subsequent management, emphasizing the need to control the surface quality of pipelines to avoid deep pits and to inspect and repair the pipeline’s insulation aluminum jacket to prevent rainwater infiltration.