Research Of Coal Gasifier Connecting Nipple Flange Cracking Failure Analysis

In this paper, the cracking failure problem happened on a coal gasifier connecting nipple flange in a petrochemical plant was taken as the research object. The reason of the flange cracking failure has been uncovered through the analysis on the material physical and chemical properties and the stress of the flange structure strength. The results of the analysis on the mechanical property, metallographic, fracture morphology and chemical composition of the flange material show that the circular and radial tensile strength and yield strength of the flange material,20MnMo steel, are both lower than the standard values. The toughness of the material near the flange inner wall has significantly decreased. The hardness of the metal at the seam and heat affected zone has exceeded the standard values. The fracture is a brittle fracture and has been badly corroded, and there is high content of sulfur in the corrosion. The grains at the weld are thick, and the microstructure has been deteriorated. There are multiple radial micro stress-corrosion cracks near the weld. The results of the finite element numerical simulation on the flange structural strength show that there is a big stress concentration at the joint between the flange inner wall and the lower surface of the supporting plate, especially the circumferential tensile stress. The Mises equivalent stress near the screw hole and the weld is significantly higher than that in other areas.The causes of the flange cracking failure are as follows:the deterioration of the microstructure at the rule-breaking weld, the large tensile stress at the weld and the corrosive environment of wet H2S caused the micro cracks at the weld, which are the initial cracks. Besides, the large temperature stress, which caused the deformation of the supporting plate that did not coordinate with that of flange at the weld and the screw joint, and the stress at the failure section rose sharply, the cracks kept expanding, and finally formed several long cracks which caused the failure of the nipple flange. At last, in view of the connecting nipple flange failure analysis, corresponding measures were proposed to prevent the flange failure.