The Influence of the Internal Pressure and In-Plane Bending Moment Loadings on Pipe Bends (original) (raw)
Circular thin-walled pipe bends are frequently used as a key part in pipeline connection either in the vertical direction or the horizontal direction due to their high flexibility. The high flexibility of pipe bends is due to the ability of their cross-section to ovalize when subjected to internal pressure and/ or bending moments that lead to high-stress concentrations at bend locations within the pipeline system. Moreover, the surface geometric characteristics of bends may cause some unbalanced outward forces caused by the induced internal pressure loading only which leads to an outward resultant force that tends to straighten out the bend causing a rise within the deformations and stress levels. This phenomenon was known as "The Bourdon effect". In addition to that, external bending moment load acting on the pipe bends may result from either occasional loadings such as; seismic loads, soil settlement, and/ or secondary loadings exerted on the pipe due to thermal expansions resulted in additional stresses. These additional stresses resulting from bending loads acting on the pipe bend are accounted for in the design codes using stress intensification factors (i) and flexibility factors (K). These factors are presented in the current American code ASME B31.3.Although they have been derived for a 90-degree pipe bend subjected to in-plane closing bending moment with long bend radius(R), they cannot be used for other loading cases such as in-plane opening moment or out-of-plane bending moment. Previous studies showed that the direction of bending moment affected the distribution and magnitude of stress levels found on the bend. However, previous studies considered only small pipe sizes of NPS 16 (406mm) and smaller under bend angles of 90 degrees or less. This paper extended the investigation on smooth pipe bends with initial circular cross-sections and uniform wall thickness with large pipe size from NPS20 (508mm) up to NPS 72 (1829mm) under a wide range of bend angles (Ø)(from 30° up to 160°). The loading considered in this study is the internal pressure and the in-plane opening/closing bending moment. In this respect, an extensive parametric study is conducted using a numerical finite element analysis (FEA) simulation using ABAQUS software to model Pipe bends with different nominal pipe sizes (NPS), bend angles (Ø), bend wall thickness (t), and various bend radius (R). The results showed that as the bend angle increases, the flexibility of the bend increases as well leading to higher stresses on the pipe bend. Finally, from the finite element analysis results depicted through curves, it could be concluded that the codes do not cover the stress distribution for large pipe bends accurately.
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