With regard to piping systems of industrial facilities, sometimes they are subject to large deformation with high local plasticity. This results mainly from the detrimental ground movement due to possible liquefaction near seaside when a strong earthquake occurs. Especially the high local plasticity concentrates on elbows (bends) in piping systems. In this paper, the elbow behaviors of in-plane (closing: deflection to decrease elbow radius of curvature and opening: deflection to increase its radius of curvature) and out-of-plane bending are analyzed in a large plastic range using Nonlinear Finite Element Method (FEM) with the shell elements of material and geometrical nonlinear characteristics. The flexibility factor of elbows depends on flexibility characteristics, angular distortion and yield stress of material. Thus, in order to estimate the behaviors of piping systems with large plasticity at elbow by a simplified method, the modified flexibility factor method is introduced from results of FEM. Calculation results obtained using the simplified method are compared with experimental results, and this method gives an index to assess behaviors of piping systems subject to extremely large movement of supports. It is also possible to estimate local plasticity at elbows in piping systems by this conventional analysis.
The leakage evaluation when gas is used is more severe than that when liquid is used in pipe flange connections. In a practical design, it is also necessary to examine the leakage in the connection under liquid internal pressure. This paper deals with the contact gasket stress distributions in the pipe flange connections with a spiral wound gasket and a compressed sheet gasket by using elasto-plastic finite element method (FEM) taking account hysteresis and non-linearity in the stress-strain curves of the gaskets, when bending moments as well as internal pressure are applied to the connections. In the FEM calculations, the effects of the gaskets and the initial clamping bolt force (bolt preload) on the contact gasket stress distributions are examined. The leakage tests for the connections under bending moments were also conducted by using liquid (water). By using the results of the leakage tests and the calculated contact gasket stress distributions, the sealing performance of the connections is evaluated. It is found that the sealing performance of the connection under the bending moment can be estimated when internal fluid is liquid (water).
A design method of piping components for Level 2 earthquake (the possible strongest earthquake with extremely low probability of occurrence) such as bolted flanged joints, expansion joints, and equipment nozzles is described. This design method is provided taking into account their failure modes and degree of safety. The failure modes for each piping component is classified according to the past damage experience due to earthquake, and each criterion is provided against the failure mode. The typical failure modes are gas leakage, fatigue failure, cumulative plastic deformation during and after earthquake for bolted flanged joints, expansion joints and equipment nozzles in piping components, respectively. Specifically, the simplified method of bolted flanged joints is proposed as the convenient design method for chemical plants and petroleum refineries, etc. (here in after calls as plant) The method is derived using gasket factor, gasket dimensions and clamping forces due to bolts for external piping load. This practical method is investigated and verified due to the experimental results on the welding neck type flanges subjected to static bending moment, in which the bolted flanged joints of NPS 4″ and 8″ in size, 3 types of gaskets are used. In addition, the dynamic inertia force effect is also studied by the shaking table tests using cantilever model of bolted flanged joints at fixed side with changing the bolt clamping forces and gasket types.
Many leakage troubles were reported in petroleum and petrochemical refineries for bolted flanged connections in elevated temperature service due to the effect of differential thermal expansion in flanges, bolts, gasket, and so on as an assembly. The leakages were found especially in shut down condition for petroleum refineries and chemical plants, even if the weather seal (rain cover) was installed to prevent the flange assembly from rapid cooling by wind or rain. On the other hand, the leakage mechanism of the bolted flanged connections with weather seal under the condition of wind flow is not still clearly understood. Therefore, the leakage behavior on bolted flanged connections is investigated using the latest computerized fluid dynamics (CFD) and the structural thermal analysis with nonlinear material properties in flange and gasket (FEM: Finite Element Method). The actual environmental effect such as wind is modeled by CFD, and the plasticity is also taken into account by the nonlinear material properties in flange and gasket by FEM. The case studies are performed for 6”, 12” and 18” for Class 2,500 with ring joint gasket. In addition, the two temperature and pressure cycles including first start-up, normal operation, shut-down, and the consequent start-up, normal operation, emergency shut-down conditions are considered. Thus, the leakage phenomena are able to evaluate, and the guideline is proposed for maintenance.
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