强磁性體 小口徑配管 檢査用 보빈形 자기카메라에 관한 硏究

Abstract

Small-bore pipes (smaller than tens of mm) made of Cr-Mo base low alloy steel are usually used in boiler and heat exchanger piping systems in thermal power plants because they have excellent high-temperature steel, oxidation-resistance and hydrogen embrittlement resistance and are able to withstand high pressure and temperature. However, those pipes are subject to fretting wear and sliding wear since they are used in high temperature, high-pressure, floating environment, and vibration contact with tube support plates, for a prolonged time. Therefore, it is necessary to develop non-destructive test and evaluation system to detect and evaluate shape and sizes of those wear defects. In this paper, an inspection system will be developed to inspect the wear defects that occur in the small-bore pipes using in boiler and heat exchanger piping systems of thermal power plants. The system has 22 magnetic sensors circumferentially arrayed with interval 16.4° (1.2 mm) in a sensor probe to measure electromagnetic field distribution around the defects and then the defect could be inspected and evaluated. To verify the effectiveness of the inspection system, artificial Short circumferential groove, Wear scar, and Sliding wear on outer diameter of a small-bore pipe will be examined. The magnetic source of the inspection system has three types that using: a permanent magnet, an alternating bobbin coil and combined a permanent magnet with an alternating bobbin coil. By using only a permanent magnet, the system works based on magnetic flux leakage testing (MFLT) principle, and it can detect a minimum depth of 0.46 mm, 0.23 mm, and 0.92 mm (20%, 10%, and 40% of pipe wall thickness) for wear scar, short circumferential groove, and sliding wear, respectively. By using only a alternating bobbin coil (320 mA – 600 Hz input current), the system works based on eddy current testing (ECT) principle, and it can detect a minimum depth of 0.46 mm, 0.46 mm, and 0.92 mm (20%, 20%, and 40% of pipe wall thickness) for wear scar, short circumferential groove, and sliding wear, respectively. However, by using combined a permanent magnet and alternating bobbin coil (320 mA – 600 Hz input current), the system works based on partial saturation eddy current testing (PSECT), the system can detect a minimum depth of 0.46 mm, 0.46 mm, and 0.69 mm (20%, 10%, and 30% of pipe wall thickness) for Wear scar, Short circumferential groove, and Sliding wear, respectively. It shows that the PSECT method provides better results than the MFLT and ECT methods. In addition, the quantitative evaluation algorithm of defect depth will be proposed. The depth evaluation average errors are 18.14% for the depth of Wear scar, Short circumferential groove and Sliding wear.