PSI - Issue 37

Fekete, Tamás et al. / Procedia Structural Integrity 37 (2022) 779–787 Fekete, T .: The Fundaments of Structural Integrity … / Structural Integrity Procedia 00 (2021) 000 – 000

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frequency, over a long period of time. The physical explanation for the limited service-time of a system made of solid material is that some form of material instability emerges in one of its load-bearing elements, which prohibits its further use. Whether or not a system can continue functioning after the occurrence of an instability depends on the fitness for use criteria of the system. Among the instabilities observed in solid materials, the phenomenon of fast/cleavage fracture – the ultimate form of material instability – is of outstanding importance. The abrupt appearance and global spread of a fast fracture in a large structure result in its final failure: the structure loses its integrity . However, the fundamental question for the designers, operators and users of the system is whether and under what conditions and over what time-period the structure will retain its function in a globally stable manner, or in other words: under what conditions and over what time-period will it retain its Structural Integrity ( SI ). As can be seen from the explanations above, and from the evolution of SI as a discipline so far – see e.g., Toribio (2020) – , SI and fracture are entangled concepts: they cover complementary issues in a given problem domain, they are closely interrelated and cannot be meaningfully addressed without each other. In the remainder of the paper, due to space limitations, selected results of an ongoing research project at Centre for Energy Research, Budapest, Hungary ( CER ), addressing fundamental issues of SI for Large-Scale Pressure Systems ( LSPSs ) are summarized in a nutshell. 2. The basics of the Structural Integrity of Large-Scale Pressure Systems Large-scale, high-performance power generation and heavy chemical systems – i.e., power plant units and chemical plants – and their pressure systems, are built with large geometric dimensions and thick walls to ensure the necessary space for the high-pressure, high-temperature and environmentally hazardous technology and to separate it from the environment. These systems are typically built up from a few large-scale equipment (i.e., pressure vessel) and a piping system connecting them. Typically, the main vessels are ≈ 3 – 5 m in diameter, 10 – 15 m in length, with typical wall thicknesses in the order of 200 – 500 mm, while the larger pipes are typically between 0.3 – 1 m in diameter, 50 – 100 mm in wall thickness and 10 – 100 m in length. These types of systems are called LSPSs . They are not mass-produced; they do not have prototypes for testing purposes and are designed and manufactured to individual customer orders. The heavy components of the system are manufactured individually, and the complete system is then assembled on site. Since the very first systems were built, LSPSs have been made to ever higher thermodynamic parameters and unit power respectively; consequently, they have become increasingly large in geometry and have also become more complex. LSPSs are safety-critical: any failure of an LSPS by a fast fracture would result in an extraordinary risk of human casualties and severe direct environmental damage in the system's immediate environment, and an increased risk of other types of environmental pollution in its more remote environment. They are designed, manufactured, assembled, and operated with particular care. 2.1. Design Safety Calculations of Large-Scale Pressure Systems Since about the mid-20 th century, they have been designed for a limited Service-Lifetime ( SL ). The Design Service Lifetime ( DSL ) of an LSPS is the SL ‘ that the designer intends’ the system ‘ to achieve when subject to the expected service conditions … ’ Nireki (1996). DSL and expected service conditions are laid down in the design specification that serves as basis for the design. The DSL of an LSPS is justified by the Design Safety Calculations ( DSCs ). DSCs of an LSPS are the computations made to predict: (1) the expected behaviour and the structural and material stability conditions of the system under design, based on the expected operating conditions and other necessary information developed during the design project, including its geometry, the necessary data for ageing assessments for the structural materials etc.; (2) an estimate for SL , called SL estim . The DSL is justified if DSL < SL estim . DSCs are largely based on design-oriented standards and guidelines – e.g., ASME (2021) – . Nowadays, the DSCs implement the damage- and dynamic overload-tolerant design concept, which means that they: (a) incorporate fracture mechanics calculations – damage tolerance, Schwalbe and Zerbst (2006) – ; (b) model the system behaviour not only under normal operating conditions, but for various types of accidents with low probability but potentially severe consequences – overload tolerance – . Given that design standards, guidelines – ASME (2021) – and other regulations require DSCs to use conservative assumptions, it can be reasonably concluded that DSCs will predict SL estim conservatively. The DSL of