PSI - Issue 17

Tamás Fekete / Procedia Structural Integrity 17 (2019) 464–471 Tamás Fekete / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Structural Integrity for a particular System

As stated above, for a particular System, the term Structural Integrity denotes the special processes/activities, all built up to ensure the safe operation of the System, up to its technically allowed lifetime. Nevertheless, the Structural Integrity concept for that System would be very complicated to comprehend if neglecting its original design. Therefore, first we present those aspects of the design that are most relevant from the structural integrity aspect of a functioning System (in a more detailed form, see Fekete (2018)).

3.1. Safety Design

The design for safety (rather the term safety design is used hereinafter) of a particular large-scale pressure system is part of the design process for a whole technological unit (e.g. an electric power generation unit, or a chemical technology unit). The unit design is based on design requirements, which incorporate the design goals (main unit capacity, required technology, expected service lifetime etc.), completed with the framework conditions. If all requirements are fulfilled, the completed unit shall operate, at least up to its expected service lifetime. Framework conditions refer to the conditions that describe the expected attributes of the unit. From the perspective of safety design, the most important attributes are those that express: • The required safety features of the System, in terms of damage and overload tolerance. Damage tolerance means that the influence of material flaws is taken into account during the safety assessments of the System. Overload tolerance denotes the requirement that the System must keep its safety not only during the normal operation of the unit, but under specific, extreme, accidental emergency situations as well. Accidental emergency situations are low probability ( ≤ 10 -4 ), short time: (1) internal events, which may cause great and fast changes in the operation of the technology, and/or (2) significant dynamic effects stemming from the environment. • The forecasts for the expected time series of loadings and the environmental conditions respectively, at least for the expected service lifetime of the unit. • The accepted methodology – which should be used for safety design – from the dimensioning calculations up to and including the System design safety computations. This raises the question of design standards (see e.g. ASME (2019), PNAE (1989)) and other safety calculation guidelines (see e.g. Kang and Kupča (2010) and VERLIFE (2013)). When the design standards and the necessary guidelines are unambiguously designated, the particular methodology that is used during the safety design process must follow the accepted methodologies. The stage of safety design can be described as a highly evolving and complex process. In the dimensioning phase, simple and highly idealized geometry (axial or spherical symmetric, both elliptic) models are generated for specifically selected, important parts of the System. The necessary wall thickness(es) of a part are derived from: (1) geometric parameters – necessary volume and/or cross section – required by the working technology, (2) physical parameters (working pressure plus temperature) stemming from the macroscopic thermodynamic features of the working technology, and (3) standardized values of the material parameters to be used in the computations, which are conservatively assessed, and usually can be found in the design standards. The calculation rules for determining the necessary wall thickness are derived from the requirement that the wall must keep its load carrying stability, under certain – time-independent – overloading conditions, but for a very long time. The necessary, particular calculation rules are precisely formulated in the design standards, in the language of allowable stresses, using linearized small-deformation kinematics, and a linear-elastic, homogeneous, isotropic material model. In addition to the fact that the use of these rules is mandatory, their applicability has been validly proven by the industrial experience during the last more than hundred years. During the design process, the initially highly idealized geometric models (of the System components i.e. equipments and pipes) are gradually made more detailed and refined, in order to achieve the final goal of the design, sc. to achieve a geometry model that will be feasible – with the constraint that it must be available for operation, at least for the expected service lifetime – . In refined models, local zones enter the picture, where the local strains and stresses are significantly larger, and can be characterised as substantially far more inhomogeneous than the stress strain fields on the idealized geometry. As it is observed since the early days of engineering, the safety – and also the