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THE BEHAVIOUR OF PIPE BENDS UNDER INTERNAL PRESSURE AND IN-PLANE BENDING LOADING

  • Author / Creator
    Abdulhameed, Diana
  • The behaviour of pipe bends differ according to the type of applied load whether it is internal pressure, pure bending or a combined load of pressure and bending. Pipe bends under internal pressure tend to straighten-out and the stress levels and deformations are higher than predicted using simple beam theory. Due to the special geometry characteristic of a pipe bend, outward forces are generated that tend to open the bend generating additional hoop and longitudinal stresses. This phenomenon is known as the “Bourdon effect”. This phenomenon was ignored in past studies and no thorough investigation was found to understand the bend’s behaviour under internal pressure. The behaviour of pipe bends under in-plane bending differs based on the direction of bending moment. The initially circular cross-section deforms into an oval shape when subjected to opening in-plane bending and the bend gains more stiffness. On the other hand, a closing in-plane bending moment deforms the cross-section into a flattened shape where the pipe bend gains flexibility by loading. Therefore, the behaviour of the pipe bend and its flexibility is highly affected by the direction of bending moment applied. Moreover, the combined loading of internal pressure and in-plane bending results in a behaviour that is much complicated than to be solved using theoretical approaches. The bending moment tends to deform the initially circular cross-section of the bend into an oval or flattened shape for opening and closing bending moments, respectively. However, the internal pressure tends to resist the cross-sectional deformation resulted from the bending moment and tends to straighten out the pipe bend due to the generated outward forces. These two behaviours are nonlinear where the stresses cannot be added based on superposition. Past studies proposed a “Pressure reduction factor” that accounts for the reduced stress generated due to adding internal pressure to a closing in-plane bending moment. This factor is used by the current codes without modification for the case of in-plane opening bending or out-of-plane bending moments. Moreover, these factors ignored the effect of the pipe bend angle on the generated stresses which is found to be highly significant.In this thesis, an extensive numerical investigation is conducted on pipe bends under internal pressure. The results show that Bourdon effect increases the stresses on pipe bends by up to 48% when compared to the stresses on a straight pipe. Based on this study, a new “Pressure factor” is proposed to account for the increase in stresses due to the “Bourdon effect”. Moreover, a mathematical model is derived to evaluate the Bourdon outward forces that are beneficial in designing any lateral supports at bend locations. The stress intensification factors presented in current design codes are reassessed for the in-plane bending moment using large deformation finite element analysis. The study is extended to assess the pressure reduction factor presented in the ASME B31.1 code that accounts for the internal pressure effect on in-plane bending moments. Comparing the results presented in this thesis with the CSA-Z662 and ASME B31.1 codes confirms that the ASME piping code underestimates the stresses on pipe bends under internal pressure and bending moment. However, for pure bending, the codes are conservative in some cases and un-conservative in other cases depending on the bending moment direction, the pipe bend geometry and size. New stress intensification factors and pressure correction factors are proposed in this thesis that accounts for the effect of the bending moment direction and the pipe bend angle on the stresses. These proposed factors are beneficial for the piping industry since it considers more parameters and it covers a wider range of pipe sizes and geometry.

  • Subjects / Keywords
  • Graduation date
    Spring 2018
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/R3V698T24
  • License
    Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.