Leakage Failure Reason of Reducing Joint Circumferential Weld for Natural Gas “Z” Pipeline

The leakage reason of the circumferential weld of the “Z” type reducing joint at a natural gas wellhead was analyzed by means of macro morphology observation, chemical composition analysis, electrochemical performance test, corrosion product analysis, etc. The uniform corrosion of the flange surface and localized corrosion of the weld were also comprehensively analyzed by combining 3D modeling and corrosion simulation test. The results showed that the chemical composition and structure of flange, reducer joint and pipe body materials were normal, the selection of welding rod matched the base metal, and there were no incomplete penetration and incomplete fusion defects. After the flange was assembled, there was an assembly gap between the two flange end faces, which was the reason why the corrosion medium in natural gas entered the gap and led to uniform corrosion on the flange surface. The butt weld misalignment of flange and reducer joint was large, and the weld reinforcement of reducing joint and pipe body was obvious, which led to turbulence and fluid backflow in the pipe fittings and the weld reinforcement, which aggravated the loss of weld metal under the effect of fluid scouring and corrosion, thus forming grooves in the weld joint, resulting in rapid thinning of weld wall thickness, and finally perforation and leakage on the weld.