The Statistics And Analysis For Defects Of In-service Sphere Tank

The sphere tank has been widely used as one large-scale storage tank with the development of petroleum chemical industry. The risk for the safe usage of sphere tank and threats the routine production of enterprise, because of the existence of defects in the tank. To find some laws about the types, the dangers and the parts easy to occur, it is necessary to process the statistics, classification and analysis scientifically for such defects. which may be benefit to the manufacture, the usage, the inspection and the maintenance of the sphere tank.The present shows some laws about defect types, the corresponding location and the potential hazards of in-service sphere tanks by statistical method, mathematic analysis, summary and conclusion based on the sample of twenty tanks inspected by Ningbo Special Equipment Inspection Institute. And analysis of the simple rule of defects by the method of theoretical analysis, data calculation and finite element analysis, etc. It can be summed up as follows:(1)The cracks are the main defects for in-service sphere tanks,mostly occurred near the fillet welds of support leg, inner surface of the tank and girth welds, another area is unusual;(2)At Inner surface cracks ,including spherical crack of stress corrosion with the tendency, which is 2.4 times to those without the stress corrosion tendency;(3)The back repair rate is high for the fillet weld cracks of the leg, the inner surface cracks and the girth cracks as well as the nozzle fillet welds. And comparatively low for outer surface cracks, longitudinal seam cracks and butt welds of polar plate. Also the tanks with SC is higher than the tanks without SC. (4)The low heat input in welding, large intensity of restraint and assemblage induced residual stress are the main factors leading to more cracks and high back-repair rate. Stress corrosion induced cracking (SSIC) dominates the service condition of inner surface. Poor welding quality and stress concentration are two important factors controlling the crack amount and back-repair rate of the leg fillet welds.