PSI - Issue 46

Tamás Fekete et al. / Procedia Structural Integrity 46 (2023) 189–196 Tamás Fekete / Structural Integrity Procedia 00 (2021) 000–000

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load-bearing components of these systems are unable to absorb the same loads as when they were new; this phenom enon has also been especially observable in systems that are repeatedly subjected to overloads of varying frequency over a long period of time. In general, durability of a system means its ability to tolerate the damaging effects of operation/use for a sufficiently long period of time, with no significant risk of a sudden loss of its safe operability – see Nireki (1996)–. The limited lifetime of a solid system is caused by the fact that after some time, somewhere in one of its load-bearing elements, a material instability –see e.g., Gilmore (1993) and Ivanova (1998)– will be developed that prohibits its further use. A final form of stability loss in solid materials is fast fracture, which is of outstanding importance: the instantaneous onset and high-speed propagation of a fast fracture in a structure leads to its ultimate failure, which is commonly referred to as structural integrity loss . For system designers and operators, however, the fundamental question is not to describe fast fracture in detail, but whether the structure will retain its overall stable function, and if so, under what conditions and over what time-period, or more simply: under what conditions and over which time-period will it maintain its Structural Integrity ( SI ). The objective of SI of operating Large-Scale Pressure Systems ( LSPSs ) is to assess their durability, i.e., the predic tion of their Technically Allowable Lifetime ( TAL ). The efficacy of SI –in addition to the performance of the meas urement/control systems used in industrial practice– also strongly depends on the predictive capabilities of the theo retical framework used in Structural Integrity Calculations ( SICs ). Nowadays, SIC s for LSPS s are largely based on internationally recognized –mainly design-oriented– standards and guidelines that bear footprints of hundred-year-old mechanistic theories –ASME (2021), PNAE (1989)–. For decades, efforts have been made to overcome the current limitations of SIC methodologies, but even today, the outcomes of various Research and Development ( R&D ) activities are not sufficient to address some fundamental issues, e.g., a unified and theoretically sound description for ageing and fracture of structural materials that can be used effectively for industrial applications is still lacking. Currently, long-standing research is ongoing at the Centre for Energy Research ( CER ) in Budapest, Hungary, with the aim, to develop a new, generalized methodology for future SIC s of LSPS s. 2. Notes concerning the current methodology for Structural Integrity of Large-Scale Pressure Systems Pressure systems for high-performance power plant units and heavy chemical systems are built with large geomet ric dimensions and thick walls to ensure the necessary space for the environmentally hazardous, high-pressure, high temperature technology and to isolate it from the ecosystem. These kinds of systems are then referred to as LSPSs . They are typically assembled from a few large-scale equipment –e.g., pressure vessels– and the interconnecting piping system. The representative geometrical dimensions of the main pressure vessels, in terms of diameter × wall thickness × length , are about 3-5 m × 200-500 mm × 10-15 m, while for larger pipes they fall in the range of 0.3-1 m × 50-100 mm × 10-100 m. They are designed for use with specific customer orders. Each of their heavy components is manufactured individ ually and the system is mounted on site. During the last over a hundred years, LSPSs have been designed and manu factured to support ever increasing thermodynamic parameters and to have higher unit power. This has resulted in their increasing geometric dimensions and complexity. Given the fact that a failure of an LSPS through fast fracture would result in an extreme risk of human casualties and severe direct ecological damage in the surrounding area, and an increased risk of other forms of pollution in the more distant ecosystems, LSPSs are safety critical. Therefore, particular care must be taken in their design, manufacture, assembly, and operation. Design Service Lifetime ( DSL ) for an LSPS is the time-interval ‘ that the designer intends’ the system ‘ to achieve when subject to the expected service conditions … ’ Nireki (1996). The Design Specification, as the starting point for the design, includes the design concept , the expected service conditions, and the DSL of the system. Currently, the cutting-edge design concept for LSPSs is the Damage and Dynamic Overload Tolerant ( DDOT ) design concept, where: (a) damage tolerance means that the designed structure must be able to withstand material defects in its struc tural materials, including cracks –Schwalbe and Zerbst (2006)–; (b) dynamic overload tolerance implies that: (1) the system must be able to be safely operated under the intended normal operating conditions , and (2) the system must withstand various accident scenarios with a low or very low probability of occurrence, the possible onset of which cannot be excluded solely on physical and systems engineering grounds. The DDOT design concept means that an