Failure Analysis And Safety Assessment On In-Service T530 Zero-Order Evaporator With Defects

For Zero-order evaporator, the main production equipment for petrochemical company, lots of inner-wall cracks perpendicular to the weld joints have been detected during the periodic inspections, which have made detrimental effects on the operating properties of the evaporator. In which several cracks which exceeded the maximum allowable size, sometimes the crack is too long, so that the defective evaporator must be replaced as result. To justify the periodicity of inspections and guarantee the reliability of the equipment operation, it is necessary to conduct the safety assessment on the T530 Zero-order evaporator.Normally, the T530 Zero-order evaporator is operated at a pressure of 1.317 MPa and at the temperature of 180℃, and the liquid in the evaporator contains such corrosive media as formic acid, acetic acid, OH—and Cl—. According to the operating environment, the characteristic and forming reason of cracks were studied. The failure analysis results show that cracks concerned above are stress corrosion cracking (SCC). The corrosive media inside the T530 vessel under the service temperature caused intergranular corrosion cracking. Then, cracks expended along the axial trend under both the operating pressure and the welding residual stress. So the structural safety assessment of T530 vessel needs to take the SCC operating condition into account. The stress distribution without cracks is calculated by finite of element analysis.The stress distribution is used to determine the failure areas. Numerical simulation results showed that the region of the highest principal stress located at the joint of ellipse head and cylinder. At this part, the highest principal stress reaches to 81.230MPa, and the highest welding residual stress 237.672MPa, approaches the yield strength of the base material. The structural safety assessment of T530 vessel was performed according to the loading level and the in-service condition. The GB/T19624 procedure was applied to the safety assessment and the results demonstrate that some cracks greater than the critical crack size will leads to component failure.Significant efforts have been made to improve the structural safety assessment during past years. However these evaluation procedures require not only the expert knowledge on the fracture mechanics but also a large amount of calculations, for which a safety evaluation system for pressure vessel which provides an efficient way of assessment was developed in this research. The system consists of various evaluation modules such as determination of the crack dimensions and crack orientation, plastic collapse estimation, SCC collapse estimation and remaining life estimation. The present system is constructed on Windows XP environment using Visual Basic 6.0 language. Three cases have been performed on this system and the results show that this system is accurate and efficient system.