Failure analysis on corrosion perforation of a 90°elbow pipe in high H2S/CO2 natural gas well station

摘要

A 90 ° elbow pipe in high H 2S/CO2 natural gas well station started performation leakage after le ss than 9 years operation. To investigate the causes of elbow perforation failure, optical microscope, scanning electron microscope, X -ray diffractometer and fluid dynamics simulation software (Fluent) were used to analyze the causes of corrosion perforati on of elbow pipe in high H 2S/CO2 natural gas well station. The test results of elemental compostion,strength and metallographic structure analysis of the failed elbow pipe material ( L245NCS) showed that elbow material met the requirements of national standards. The main chemical components of corrosion products included FeS and FeCO3. The fluid dynamics simulation of Fluent shown that natural gas entraining fine sand will cause serious erosion and wear to the outer wall of the 90° elbow under operation conditions, but has little influence on the straight pipe section and the inner wall. The com prehensive analysis results show ed that the failure of elbow perforation was mainly caused by the coexisting corrosion of CO2 / H2S, galvanic effect between outer wall and iner wall in coordination with the errosion of sand -containing fluid on the outer wa ll of the elbow. After the corrosion product film of the outer wall of the elbow is eroded and damaged, galvanic corrosion is formed with the straight pipe and inner wall protected by corrosion products of the elbow, and the outer wall becomes the anode zone. The erosion and wear of the sand further accelerate the thinning of the outer wall, and eventually lead to perforation leakage.