Corrosion Failure Analysis of Elbow in LPG Desulfurization Unit

The elbow plays a crucial role in changing the flow direction of the medium in pipeline system and is one of the most commonly used pipeline components in the oil and gas transportation. This paper focuses on the corrosive failure mechanism of the elbow of regeneration tower of LPG desulfurization unit in a refinery. Aiming at the failure elbow, based on the macro and micro perspectives, the physical laws of the inner layer of the elbow, including the distribution of corrosion holes and wall thickness, are summarized and analyzed. The further characterization methods were used to study the corrosion mechanism, including mechanical properties, metallographic examination, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy spectrum analysis (EDS). Taking the thief hole as center, the elbow was divided into 4 rows. It is found that the maximum diameter was 21.1 mm while the minimum was 7.76 mm, and the vast majority of holes were 16 ~ 19 mm. The average size of the middle section was larger, meanwhile, possessed most holes over 19 mm. The corrosion thickness first increased then decreased along the flow direction and reached the maximum in completely destroyed area of the 1st and 2nd row. The corrosion thickness increased gradually along the flow direction of the 3rd and 4th row. Erosion corrosion is the main cause of elbow failure. Fluid erosion plays a dominant role in the failure process while electrochemical corrosion plays a dominant role in the formation of corrosion holes. Besides, the presence of heat-stable salts (HSS) also aggravates the corrosion of elbow.