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Research Papers: Codes and Standards

Mechanical Behavior of Steel Pipe Bends: An Overview

[+] Author and Article Information
Spyros A. Karamanos

Department of Mechanical Engineering,
University of Thessaly,
Volos 38334, Greece
e-mail: skara@mie.uth.gr

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received July 21, 2015; final manuscript received October 11, 2015; published online April 28, 2016. Assoc. Editor: Chong-Shien Tsai.

J. Pressure Vessel Technol 138(4), 041203 (Apr 28, 2016) (15 pages) Paper No: PVT-15-1165; doi: 10.1115/1.4031940 History: Received July 21, 2015; Revised October 11, 2015

An overview of the mechanical behavior of steel pipe (elbows) is offered, based on previously reported analytical solutions, numerical results, and experimental data. The behavior of pipe bends is characterized by significant deformations and stresses, quite higher than the ones developed in straight pipes with the same cross section. Under bending loading (in-plane and out-of-plane), the main feature of the response is cross-sectional ovalization, which influences bending capacity and is affected by the level of internal pressure. Bends subjected to cyclic in-plane bending exhibit fatigue damage, leading to base metal cracking at the elbow flank. Using advanced finite-element tools, the response of pipe elbows in buried pipelines subjected to ground-induced actions is also addressed, with emphasis on soil–pipeline interaction. Finally, the efficiency of special-purpose finite elements for modeling pipes and elbows is briefly discussed.

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Figures

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Fig. 1

Pipe elbows (bends) in (a) piping systems of industrial plants and (b) buried gas pipelines

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Fig. 2

Pipe elbow geometry, cross-sectional displacements, and biaxial state of stress

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Fig. 3

Schematic representation of ovalization in (a) in-plane closing moments, (b) out-of-plane opening moments reverse ovalization, and (c) out-of-plane bending

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Fig. 4

Flexibility factor of pipe elbows with respect to the elbow parameter λ

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Fig. 5

Elastic analysis of a 90-deg standalone elbow (λ = 0.23); variation of (a) longitudinal stresses and (b) hoop stresses at external pipe wall with respect to the cross-sectional height; comparison of the analytical solution of Rodabaugh and George [6] with finite-element results

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Fig. 6

(a) Experimental setup for testing a 160-mm-diameter pipe elbow under in-plane and out-of-plane bending and (b) schematic representation of the 90 deg specimens [7]

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Fig. 7

Elastic flexibility of pipe elbow specimen (D/t = 53.3 and R/D = 3) under in-plane closing bending moments; comparison between numerical and experimental results (values of pressure p are in MPa)

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Fig. 8

Moment–rotation diagrams for nonpressurized 60 deg elbows (D/t = 90); comparison between test data and numerical results [9]

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Fig. 9

Schematic representation of elbow experiments [9]

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Fig. 10

Deformed shapes of 60 deg elbows (D/t = 90); (a) flattened configuration under closing moments and (b) and (c) buckled shape under opening moments [9]

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Fig. 11

Finite-element simulation of 60 deg elbow deformation under in-plane closing bending (D/t = 90): (a) ovalized shape under in-plane bending moments and (b) buckled shape under opening bending [9]

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Fig. 12

Final configuration of a pressurized 30 deg elbow specimen (D/t = 55) subjected to opening bending moments

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Fig. 13

Response of a thin-walled 90 deg elbow (D/t = 90) under in-plane opening bending, for three levels of internal pressure: (a) moment–rotation diagram and (b) ovalization–rotation diagram

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Fig. 14

Thin-walled 90 deg elbow (D/t = 90): deformed cross-sectional shapes and spread of plastic deformation under closing bending moments for zero pressure

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Fig. 15

Response of 90 deg elbows under in-plane closing bending moments and three levels of internal pressure: (a) thin-walled elbow (D/t = 90) and (b) thick-walled elbow (D/t = 20)

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Fig. 16

Deformed thick-walled elbow (D/t = 20): cross-sectional shape and spread of plastic deformation under in-plane closing bending moments for zero pressure

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Fig. 17

Out-of-plane loading of an elbow of moderate thickness (D/t = 53): (a) force Q versus cross-sectional flattening at central cross section and (b) deformed shape of specimen obtained experimentally and numerically

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Fig. 18

Response of a thin-walled 90 deg elbow (D/t = 90) under out-of-plane bending moments and three levels of internal pressure (0%, 20%, and 40% of py)

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Fig. 19

Buckled shapes of thin-walled 90 deg elbow (D/t = 90) under out-of-plane bending moments for (a) zero pressure, (b) pressure level 20% of py, and (c) pressure level 40% of py

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Fig. 20

Deformed configuration of central cross section under out-of-plane bending: (a) thin-walled elbow (D/t = 90) and (b) thick-walled elbow (D/t = 20)

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Fig. 21

Schematic representation of the state of stress on an arbitrary location at the “intrados” of the curved pipe portion due to out-of-plane torque.

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Fig. 22

Response of elbows with D/t ratio equal to 20, 55, and 90, subjected to out-of-plane bending moments: (a) zero internal pressure and (b) pressure 20% of py

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Fig. 23

Moment–rotation diagram of elbows under in-plane closing bending in the presence of external pressure up to 40% py (D/t equal to 55 and 20)

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Fig. 24

Moment–rotation diagram of elbows under in-plane opening bending in the presence of external pressure up to 40% py (D/t = 55 and 20)

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Fig. 25

In-plane bending loading of 8-in. diameter elbows: (a) experimental setup and (b) finite-element model for numerical simulations [42,43]

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Fig. 26

Failure of 8-in. diameter SCH 40 elbows: (a) fatigue crack at elbow flank; (b) cross-sectional ovalization, and (c) finite-element simulation results

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Fig. 27

Load–displacement diagram for 8-in. diameter SCH 40 (pressure 3.2 MPa and Δl = ±200 mm); comparison of test and numerical finite-element results [43]

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Fig. 28

Accumulation of hoop strain (ratcheting) at the elbow critical location (flank); comparison between experimental and numerical results [43]

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Fig. 29

Pressure effects on the accumulation of longitudinal and hoop strain (ratcheting) at the elbow critical location (flank), obtained from numerical simulations [43]

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Fig. 30

Schematic representation of a buried pipeline bend subjected to axial force

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Fig. 31

Coupled response of soil–pipeline system for a 30-deg bend subjected to axial (pull-out) force; finite-element simulation

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Fig. 32

Pull-out force–displacement diagram for buried pipeline bends embedded in cohesive soil conditions: (a) soft-to-firm clay (clay I) and (b) stiff clay (clay II)

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