An FE-Based Approach for the Limit Load of Pressure Equipment with a Localized Thinning Defect

• Main Authors: Ying-Ying Chen, 陳盈螢
• Other Authors: Chen-Hua Wang
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/04688426759486760532

碩士 === 國立高雄第一科技大學 === 環境與安全衛生工程研究所 === 99 === Nowadays, the design and construction of pressure vessels often require that some sort of engineering standards or codes, such as CNS 9788 or ASME BPVC be followed. However, in these design codes, one of the key assumptions is that its thickness is uniformly distributed over the entire structure, which implies that defects due to long term effects of corrosion are not considered. In other words, the same formula that was used in the design stage is no longer applicable for assessing the fitness of equipment. API RP 579 Fitness-for-service (FFS) is published by American Petroleum Institute (API), aiming for a standardize defects analysis procedures for process equipment. Although, ASME VIII and FFS were developed for different purposes, one for new equipment while the other for aging equipment with defects. However, there exists a common theoretical background, in which they both utilize the limit load of equipment and determine the remaining life of equipment based upon the residual strength factor (RSF). In this study a finite element-based structural assessment methodology was developed on ANSYS, which is designed specifically for handling localized defects on the surface of pressure equipment. In order to compute the limit load of equipment with defects, the solution process must switch back and forth between two finite element models, one designated as the primary model and the defect zone as a sub-model. Solving for the limit load requires the load to be added incrementally until the convergence can no longer be achieved. The method developed here happens to comply with API 579 and can be treated as a Level 3 method. In order to verify the correctness of results as well as the validity of our method, two defect types were considered and a series of comparisons were made. Issues involved include handling of the profiles of defects, effects of the material property on the limit load, and the size effect of a defect. Overall speaking, the method developed in this study has been found to successfully evaluate the limit load of structures of several distinct geometries. With this method, a full finite model of the entire equipment can be analyzed, while focus can be placed on the defect zone for better resolution and accuracy. Most importantly, the higher accuracy of results can be achieved in much shorter time and requires less computer memory. Keywords: local thin area, finite element method, limit load, RSF, FFS