Ferromagnetic tubes are widely applied in fossil power plants such as waterfall tubes, overheat tubes or reheating tubes. Outer surface corrosion, wearing, depth reduction, cracks are common defects during manufacturing and long term operation. Eddy current detection is a traditional method for the examination of surface and near surface defects. However the efficiency is restricted by lift-off effect and skim effect. Far field eddy current overcomes the above disadvantages with high efficiency and fast examination speed and high sensitivity. In this paper, numerical simulation has been carried out to study the far field eddy current Detection for defects in small diameter ferromagnetic tubes in fossil power plants.
Condenser tube tubes are widely applied in condenser in thermal power plants. Leakage often occurs in condenser tube groups due to the quality of condenser media or high serving stress. Condenser tube leakage not only brings economic losses to power stations, but even influences the safety operation of the power generation units. This paper simulates and experimentally studies the ferromagnetic heat exchanger tubes based on the detection mechanism of far field eddy current. The results show that the position of the far field is with the increase of the wall thickness and the inner diameter of the tubes, and the detection sensitivity of the far field is high for the same size defect, but the difference of the magnetic flux intensity varies equally when the defects with the same size are located at the inner and outer walls. This is consistent with the experimental results of real tube defect detection and meets the principle of far field eddy current detection. It is concluded that far field eddy current testing technology can better determine and identify ferromagnetic tube defects, and has a good application prospect for online nondestructive testing and safety performance evaluation of service tubes.
Tube leakage frequently occurred in on-service fossil power plants. Traditional NDT testing methods, including RT, UT, MT and PT are not applicable for the examination of large amount tubes. Fat eddy current examination is a new technique for testing ferromagnetic tubes defects, such as inner corrosion, thickness reduction and cracks. Far field eddy current simulation and verification for different specification tubes has been carried out. The result shows that the position of far field eddy current moves backwards with the increase of wall thickness and tube ID. To the defects with same size, examination sensitivity is higher in far field eddy current area. When the defect with same size locates in the inner and outer surface of the tube, the change of magnetic intensify differences is approximately the same, which is in accordance with the experimental result and is satisfied with far field eddy current examination theory. It has been concluded that defects in ferromagnetic tubes could be judged and recognized by far field eddy current examination technique, which shows prospective for the on-line examination and safety evaluation for in-service tubes in fossil power plants.
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