Development of an Eddy Current Probe for Detecting Outside Circumferential Cracks near an Expansion of Heat Exchanger Tubes

Eddy current testing (ECT) is a conventional inspection method of heat exchanger tubes. Many types of probes have been developed depending on such tube conditions as tube shapes and materials. However, the vicinity of a tube expansion is still a difficult area to inspect. This is because there is a step-shape variation circumferentially on the inside surface and there is a ferromagnetic tube sheet around the outside surface so that large noise is observed. A high signal to noise ratio (S/N) is required to inspect the area because fatigue cracks can occur near the tube expansion. In this study, we chose test pieces having an outside circumferential notch with a depth of 20% tube thickness (20%t) and set the target of the detection performance at S/N= 3. In order to realize this target, we utilized the difference of eddy current flows at a local notch and a tube expansion, and devised a new arrangement for exciting and pick-up coils. In our arrangement, the pick-up coil having a winding direction along the circumference is placed at the center of two exciting coils. The exciting coils induce an eddy current flow in the axial direction at the center and the pick-up coil can detect only a circumferential component. By using this probe, large noise having an axial component is suppressed and only a notch signal having a circumferential component can be detected. In order to confirm the probe performance, we compared it with a general bobbin type probe. In the sensitivity evaluation for an outside circumferential notch with 20%t, the signal amplitude was improved 5.7 times. In the noise evaluation test using a test piece with tube expansion, the noise amplitude was decreased 1/8 times. We could successfully detect the outside circumferential notch (20%t) near a tube expansion at S/N=3.5, verifying the detection performance. INTRODUCTION Heat exchangers are widely used in power plant systems. In the heat exchangers, hundreds of tubes are regularly arranged to keep effective heat transfer between circulating water flows. However, tube conditions such as surface morphology can vary depending on the environment of use. Therefore, regular inspection of heat exchangers is needed. Eddy current testing (ECT) is well known inspection technique for heat exchanger tubes. In ECT, a conventional bobbin probe arranged with an exciting coil and a pick-up coil is inserted into a tube at one end by using an air gun and then a winder pulls the probe through the tube at a speed of about 1m/s by winding up an attached cable[1]. Signals from the probe are acquired during the scan and inspectors check the signal waveform. Even though the ECT has been successfully applied to straight or U-shape areas of tubes, the vicinity of a tube expansion is still a difficult area to inspect. This is because there is a step deformation circumferentially on the inside surface and there is a ferromagnetic tube sheet around the outside surface which results in large noise being observed. However, fatigue cracks can occur and extend circumferentially from the outside surface near the tube expansion. Therefore, a new probe making it possible to detect outside circumferential cracks near the tube expansion is required. In order to solve this problem, we utilized the difference of eddy current flows at a local crack and a tube expansion, and devised a new arrangement for exciting and pick-up coils. In this study, we chose test pieces having an outside circumferential notch with a depth of 20% tube thickness (20%t), and set the target of the detection performance at S/N= 3. For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=70 6th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components October 2007, Budapest, Hungary