Design and Analysis

Three-Dimensional Fracture Analysis of Circumferentially Cracked Boiler Tubes

[+] Author and Article Information
E. Citirik

 Cameron International, Inc., Houston, TX 77041erc5@lehigh.edu

J. Pressure Vessel Technol 134(3), 031202 (May 18, 2012) (6 pages) doi:10.1115/1.4005857 History: Received March 01, 2010; Accepted November 28, 2011; Published May 17, 2012; Online May 18, 2012

A comprehensive methodology is developed to understand and characterize the fracture behavior of circumferentially cracked boiler tubes in this study. Weld overlay is applied on the coal-fired boiler tubes in order to prevent the degradation of corrosive and erosive environment that the boiler tubes are exposed to in the power plants. Finite element modeling and analysis are employed for all of the computations including steady-state and transient stress intensity factor (SIF) calculations in this study. Circumferential cracking has been one of the failure modes in waterwall boiler tubes, which results in high maintenance and replacement costs. Thermomechanical stresses and corrosive environment are basically the two remarkable contributors that bring about this failure mode. The former one is investigated and quantified in this study in order to explain the fracture behavior of weld overlay coatings during the power plant operation. Periodic soot blowing operations cause cyclic transient thermomechanical stresses on the weld overlay coating that results in crack propagation and fatigue failure. Three-dimensional fracture analysis of circumferentially cracked boiler tubes is examined using enriched finite element method in this study. Transient temperatures and thermomechanical stresses are computed using ANSYS for five different periodic crack spacing values (h), which are 2, 4, 6, 10, and 20 mm in the axial direction. 3D fracture analysis was performed, and stress intensity factors were computed using FRAC3D , which is Finite Element Analysis (FEA) software developed at Lehigh University. The maximum stress intensity factor is obtained at the deepest penetration of the crack in the model which has the largest periodic axial crack spacing, h = 20 mm. The stress intensity factors due to welding residuals decrease as the axial crack spacing, h, decreases. The FEA methodology developed in this research would provide the engineers with the ability to understand the fracture problem and predict component life and improve the reliability of the weld overlay coated boiler tubes utilized in the power plants.

Copyright © 2012 by American Society of Mechanical Engineers
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Figure 1

Picture of the boiler tube panel section, h = periodic crack spacing

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Figure 2

2D cross section of the weld overlay coated boiler tube geometry utilized in the FEA computations (dimensions are in millimeter)

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Figure 3

Temperature distribution (in °C) of h = 20 mm model at steady-state

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Figure 4

3D finite element model of a circumferentially cracked boiler tube (h = 10 mm)

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Figure 5

Sequence of images depicting temperature distribution during soot blowing operations in 100 s (The fireside: t = 0 s is at 350 °C and t = 100 s is at 400 °C)

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Figure 6

Mechanical boundary conditions in the x-y plane, front view of the model and in the z-y plane, right view of the model. u = 0 and w = 0 represent the symmetric boundary conditions

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Figure 7

Steady-state axial thermal stresses, σzz , in the boiler tube (units are in pascal)

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Figure 8

Sequence of images depicting stress distribution during soot blowing operation (units are in pascal)

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Figure 9

Superposition approach for computing KI values due to residual stresses from welding

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Figure 10

KI values along the circumferential crack front due to residual stresses caused by welding residual stresses. (h/2 = 5 mm and h/2 = 10 mm results are identical)

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Figure 11

KI values due to residual stresses caused by welding at the deepest point of crack penetration (a; crack depth = 0.64 mm, h; crack spacing—varying from 2 mm to 20 mm)

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Figure 12

Transient stress intensity factors for axial crack spacing, h = 20 mm




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