NDTnet 1998 Aug, Vol.3 No.8

Full-Text - Abstract

Introduction
Heat exchanger tubes in nuclear power plants, when damaged by corrosion attack have to be plugged if the reduced wall thickness has reached a certain limit. Efficiency of the heat exchanger component gets poor, if too many tubes are closed. An alternative repair method is done by inserting a second tube in order to cover the damaged locations. This inserted sleeve is welded to the first tube by laser beam. Afterwards, the welding has to be proofed by NDE-techniques. Unfortunately, this is a very difficult task. The authors have tried to investigate the feasibility of applying remote field eddy current (RFEC) technique by numerical simulation (s. Figure).

Fig.: RFEC-Testing of sleeved heat exchanger tubes

Numerical model
Finite element calculations have been used for understanding the interaction of eddy current field with weld connecting the coaxial tubes as well as for prediction of signal behaviour. The solution domain included non-ferromagnetic sleeve tube (16.6 mm ID, wall thickness 1.2 mm, conductivity 2 MSm/m) inside another one (19.5 mm / 1.25 mm / 2 MSm/m), weld-model in form quite close to reality, gap between the two tubes, exciter and receiver. All calculations were performed on the same mesh (about 10,000 finite elements). For each calculation, the material allocations were changed in the region of the welding. Some of the triangular elements were specified sometimes as weld, air or as tube material respectively.

Results
The signal responses of the receiver were predicted by moving the weld-model along the tubes. Signal trajectories were predicted for various frequencies. It was evaluated the difference between the testing situation calculated for a particular weld and the signal in case of the absence of weld. An optimal difference signal was obtained at a frequency of 7 kHz.

Next series of calcualtions was made at this optimum frequency for welds of different conatact zones between the inner and outer tubes. Difference signal for welds of different contact with have been found to behave nearly linear for widths up to 70% of outer wall thickness.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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