Fitness of service assessment of a pressure vessel with crack-like flaws

[[abstract]]壓力容器設備存在缺陷時，其對結構的損傷在初期、尚處設計容許的範圍內時，屬潛藏性失效模式，唯有經工廠檢查後才能發現，進而判定其對結構的影響程度。「檢查」時多依賴非破壞性檢測(NDE)技術，至於對結果的詮釋與判定，則需要較多的知識與專業。目前產業界對減薄類型的缺陷，大致上都有能力進行檢查與分析。但對於其它類型缺陷如裂痕類缺陷的評估，仍沒有確定標準的評估方式及判定準則。 裂痕類缺陷的力學評估模式很複雜，即使最單純的線彈性破壞力學(LEFM)模式，遇到發生於特殊形狀的元件、裂痕靠近幾何不連續部位置、或結構受力模式略為複雜的情況時，要計算其對結構強度的影響，便幾乎不可能。 而適用性評估為近年來國際上所建立的缺陷評估分析法之一。配合工廠非破壞性檢測數值、設備承載狀況等基本資訊，判定缺陷大小是否仍在容忍範圍，更進一步分析設備應力狀況，明確掌握結構強度現況。 本研究以設備案例進行分析，以適用性評估與線彈性破壞力學分別評估，同時以有限元素法加以驗證。經評估驗證後，得知適用性評估與有限元素法結果相近，且較接近實際情況；本研究更進一步將適用性評估之Level 1與Level 2流程轉化為程式，使此種評估方法於業界實際應用時能更加快速便利。[[abstract]]For all kind of reasons flaws may happen and are mostly difficult to detect during the life span of a piece of process equipment When flaws are found upon a pressure vessel people would take it for granted and anticipate that flaws be not just inspected but also assessed In fact plants do have scheduled plan for non-destructive testing equipment but a majority of plants lack of the capability to assess flaws other than general metal loss to the vessel wall Furthermore there are de facto standards for designing pressure vessels but rarely do we see a widely recognized technical standards regarding aging equipment with defects or flaws on vessel wall Crack analysis is extremely sophisticated due to the fact that yielding occurs at the crack tip while other portion remains elastic Literature reviews show that for smaller cracks linear elastic fracture mechanics (LEFM) is most often utilized for modeling and simulating the stress field around the crack surface and the crack tip However only planner cracks of simple geometry can be computed which is far from practical at for for plant use In addition the mechanics problems become much cumbersome for special locations such as the weld and discontinuities All these make the stress analysis of a crack almost impossible Fitness for service (FFS) receives growing attention lately in Japan Europe and the United States What FFS provides is a standardized approach toward assessment of the remaining strength and thus the acceptance for continual service Based on LEFM a plant can evaluate a crack and make decision on whether the equipment has enough strength for further use In this thesis several case studies were utilized for comparing results among the fitness for service LEFM and numerical methods Results from FFS are quite closed to those from using the finite element method Also a computer program for level 1 and level 2 was written to test out the feasibility of web-based analysis and diagnosis environmen