| Generally, defects such as cracks and corrosion easily occur in boiler tubes under the condition of high temperature, high pressure and flue-gas flow. Besides, wall thinning could be more common in the environment where Flow Accelerated Corrosion(FAC) exists. Pipes would be invalid and tube explosion or even serious accidents may happen after that. Therefore, timely and periodical nondestructive testing(NDT) to evaluate the wall thinning is vital for the boiler tubes in service. According to the structure characteristics of common boiler tube, the research on eddy current nondestructive testing for it in this thesis is divided into two parts: the straight pipe section and the bend pipe section. The main research work is as follows:1. An analytical model of eddy current testing for straight boiler tube with cylindrical coil is established. Firstly, based on the classical Dodd-Deeds model, a partial differential equation of the magnetic vector potential A is obtained through the Maxwell equations and boundary conditions of electromagnetic field. Then, the solution containing modified Bessel functions and trigonometric functions to the aforementioned equation is found by using the method of variable separation. Afterwards, based on Cheng’s matrix iterative method, range of the unknown coefficients for the resultant solution is extended to arbitrary layers. Thus integral-form analytical model of straight multi-layer conductors is established and a calculation example for appointed number of layers is given. Finally, the integral-form analytical model of straight single-layer boiler tube is obtained by using the established model.2. In order to overcome the difficulties in solving the integral model, a series-form analytical model of eddy current testing for straight boiler tube is established. First, solving range is limited to a cylindrical region with a certain radius by using Truncated Region Eigenfunction Expansion(TREE) method, and thus the infinite integral-form solution is transformed to the infinite series-form solution. Then, based on the superposition principle, series-form analytical models for multi-layer conductors are established. Next, calculation program of multi conductor model is compiled by using MATLAB, and the calculation result is compared with the existing literature. Finally, a series-form analytical solution of eddy current testing for a single-layer boiler tube is established through the multi-layer model.3. A three-dimensional finite element simulation model of eddy current testing for boiler elbow is established. First, conventional solving steps of ANSYS and the specific usage methods of its electromagnetic module are introduced. Then, a three-dimensional finite element model including a detection coil, a 90 degrees elbow and an air layer is established by using the SOLID97 3D electromagnetic analysis element. Finally, influences caused by the inner and outer surface on the coil impedance are studied through changing the detection position and the axial radius of elbow.4. A test rig for eddy current testing is built to verify the analytical model and finite element model. First, based on NI PXIe system, a signal generating, receiving and processing module is built. Then, preamplifier module containing filter circuit is designed and made. In addition, the features of different modules are briefly introduced. Finally, the test rig is realized after the connection and debug of all modules, and the above-mentioned theoretical models are verified by experiments.The research work could provide theoretical support for eddy current testing of boiler tubes, also be helpful to the further development and application of eddy current testing technology. |
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