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Research Papers: Materials and Fabrication

A Method to Approximate the Steady-State Creep Response of Three-Dimensional Pipe Bend Finite Element Models Under Internal Pressure Loading Using Two-Dimensional Axisymmetric Models

[+] Author and Article Information
J. P. Rouse

e-mail: eaxjr@nottingham.ac.uk

T. H. Hyde, W. Montgomery

Department of Mechanical, Materials and
Manufacturing Engineering,
University of Nottingham,
University Park,
Nottingham NG7 2RD, UK

A. Morris

E.ON New Build & Technology,
Technology Centre,
Radcliffe-on-Soar,
Nottingham NG11 0EE, UK

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received March 12, 2013; final manuscript received September 10, 2013; published online December 12, 2013. Assoc. Editor: Pierre Mertiny.

J. Pressure Vessel Technol 136(1), 011402 (Dec 12, 2013) (12 pages) Paper No: PVT-13-1046; doi: 10.1115/1.4025720 History: Received March 12, 2013; Revised September 10, 2013

Pipe bends are regions of geometric discontinuities in the pipe systems used in power plants and most industry recorded failures have been located around similar regions. Understanding these potential locations of weakness is therefore of great interest for the safe and economic operation of piping components. Increased predictive accuracy would assist in component design, condition monitoring, and retirement strategy decisions. Modeling of piping components for finite element analysis (FEA) is complicated by the variation of the cross section dimensions (changes in wall thicknesses or cross section ovality) around the pipe bend due to the manufacturing procedure implemented. Quantities such as peak rupture stress and creep rupture life can be greatly affected by these geometric variation (Rouse, J. P., Leom, M. Z., Sun, W., Hyde, T. H., Morris, A., “Steady-state Creep Peak Rupture Stresses in 90 Pipe Bends with Manufacture Induced Cross Section Dimension Variations”International Journal of Pressure Vessels and Piping, Volumes 105–106, May–June 2013, pp. 1–11). Three dimensional (3D) models can be used to approximate to the realistic level of detail found in pipe bends. These simulations may however be computationally expensive and could take a considerable amount of time to complete. Two dimensional (2D) axisymmetric models are relatively straight forward to produce and quick to run, but of course cannot represent the full geometric complexity around the pipe bend. A method is proposed that utilises multiple 2D axisymmetric pipe bend models to approximate the result of a 3D analysis through interpolation, thus exploiting the greatly reduced computing time observed for the 2D models. The prediction of peak rupture stress (both magnitude and location) is assessed using a simple power law material model. Comments are made on the applicability of the proposed procedure to a range of bends angles (90 deg, 60 deg, and 30 deg), as well as the effect of the stress exponent (n) and tri-axial (α) material constants. Provided that peak stresses do not occur at the bend/straight interface, the magnitude and location of the peak rupture stress can be predicted by the 2D axisymmetric interpolation method with a typical percentage difference of less than 1%.

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Figures

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Fig. 1

Illustrations of (a) bend position angle (φ) and (b) circumferential position angle (θ) of the pipe cross section

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Fig. 2

Example of the variation of normalised wall thickness with circumferential position (Fig. 1(b)) at a specific bend position angle (φ)

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Fig. 3

Variations of In(φ) and Ex(φ) with bend position for a main steam type of pipe bend geometry. Note the symmetry plane (In(φ) or Ex(φ) = 1) is shown by a dashed line.

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Fig. 4

Variations of TNOM(φ) with bend position for main steam type of pipe bend geometry

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Fig. 5

Example 3D FE mesh (truncated straight pipe section) for main steam type of pipe bend

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Fig. 6

Example 2D axisymmetric mesh for main steam type of pipe bend

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Fig. 7

Illustration of 2D to 3D interpolation procedure, showing the 5 cross section locations (A′–E′) represented by 2D axisymmetric models

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Fig. 8

Plots of normalized local peak rupture stresses versus bend angle positions for (a) uniform 90 deg pipe bend, main steam type, (b) uniform 60 deg pipe bend, main steam type, (c) uniform 30 deg pipe bend, main steam type, (d) uniform 90 deg pipe bend, hot reheat type, (e) uniform 60 deg pipe bend, hot reheat type, (f) uniform 30 deg pipe bend, hot reheat type

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Fig. 10

Example contour plots of normalized rupture stress the proposed 2D interpolation procedure (shown for the geometry HR_1, α = 0.3, n = 6)

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Fig. 11

Equivalent von Mises stress (normalized to internal pressure) contour plots for the model MS_1 (α = 0.3, n = 6), taken at cross section planes defined in Fig. 7. Both 3D ((a), (b), and (c)) and 2D ((a), (b), and (c)) meshes are shown.

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Fig. 9

Plots of normalized local peak rupture stresses versus bend angle positions for the pipe bend models (a) HR_1, (b) HR_2, (c) HR_3, (d) MS_1, and (e) MS_2

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Fig. 12

Plots of normalized local peak rupture stresses versus bend angle position for (a) hot reheat design bend and (b) main steam design bend.

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Fig. 13

Plots of normalized local peak rupture stress versus bend angle position for the models (a) HR_1 90 deg pipe bend, (b) HR_1 60 deg pipe bend, (c) HR_1 30 deg pipe bend, (d) MS_1 90 deg pipe bend, (e) MS_1 60 deg pipe bend, and (f) MS_1 30 deg pipe bend

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Fig. 14

Comparison of rupture stresses predicted by 2D axisymmetric and 3D models for a hot reheat type pipe bend (HR_1), with stress exponent values of (a) n = 4, (b) n = 6, (c) n = 8, and (d) n = 10

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Fig. 15

Comparison of rupture stresses predicted by 2D axisymmetric and 3D models for a main steam type pipe bend (MS_1), with stress exponent values of (a) n = 4, (b) n = 6, (c) n = 8, and (d) n = 10.

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Fig. 16

Comparison of rupture stresses predicted by 2D axisymmetric and 3D models for a hot reheat type pipe bend (HR_1), with triaxial material constant values of (a) α = 0, (b) α = 0.3, (c) α = 0.7, and (d) α = 1

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Fig. 17

Comparison of rupture stresses predicted by 2D axisymmetric and 3D models for a main steam type pipe bend (MS_1), with triaxial material constant values of (a) α = 0, (b) α = 0.3, (c) α = 0.7, and (d) α = 1

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