비자성체 열교환기 전열과 비파괴검사 시스템 개발

Abstract

A heat exchanger is one of the key components in petrochemical plants, district heating, and thermal and nuclear power plants. In the heat exchanger, heat energy is transferred from a high-temperature medium to a low-temperature medium through a heat exchanger tube, which is a metallic separator. The heat exchanger tube used in high-temperature and water chemistry environments, such as high-pressure feedwater heaters in nuclear power plants, which are made of austenitic stainless steels (STS304), a non-magnetic material. Despite the use of heat- and corrosion-resistant material, the high temperatures, high pressures, and vibrations in such environments may lead to flaws. As an indicative example, between 1976 and 1996, out of a total of 166 failure records of the main body of feedwater heaters and internal equipment, 144 cases included a failure due to leaks in the heat exchanger tube; thus, 87% of the total cases involved failures. This indicates that the primary-side and secondary-side media separated by the metallic separator are not mixed, which precludes the inherent function of the heat exchanger tube, i.e., exchange of heat with high efficiency. In order to prevent this problem, a quantitative evaluation of the presence of flaws in the tube as well as the shapes of the flaws should be performed through periodic inspection. In the conventional method of eddy current testing (ECT), a bobbin probe and motorized rotating pancake coil (MRPC) are used to detect the presence of flaws and examine the shapes of the flaws. However, the bobbin probe has limitations in that it cannot distinguish between cracks and volumetric flaws, which poses a challenge in detecting crack-like flaws in the circumferential direction. On the other hand, with MRPC, the probe, driven by a motor, rotates at 900 rpm, and the eddy current distributions in the inner and outer walls of the tube are quantitatively evaluated. However, MRPC also has operational limitations in that the scanning speed is very low, i.e., 0.5 inches/s, the on-site installation of the inspection system is not straightforward, and the signal analysis process is complicated. In addition, since the inner wall of the tube and the probe are in direct contact, the service life of the probe is short due to wear, and the cost of the probe, including the rotating mechanism, is high. Therefore, when MRPC is used to inspect all the heat exchanger tubes at a power plant, the downtime of the plant for nondestructive testing needs to be prolonged, which leads to a major disruption in the power supply. As a result, the use of MRPC is limited to critical parts, such as steam generator tubes, and is not suitable for general non-magnetic tubes. In this regard, to overcome the limitations associated with quantitative evaluation of ECT using the bobbin probe and to consider the field requirements in terms of scanning speed and the service life of ECT with MRPC, this study aims to develop a magnetic camera for nondestructive testing of non-magnetic heat exchanger tubes. The magnetic camera includes a differential bobbin coil and an encircling array magnetic sensor. The differential bobbin coil applies an induced current to the inner wall of the tube and measures the impedance and phase difference according to the presence, type, and size of flaws. Then, the flaw information included in the time-varying magnetic field induced by the exciting coil is measured by the semiconductor-based magnetic sensor with an encircling array. The amplitude and phase difference distribution of the time-varying magnetic field measured by the magnetic sensor array contains information on the shape and size of the flaws as well as information for detecting the presence of flaws. To implement this mechanism, exciting coils, magnetic sensor arrays, a remote DC and AC-stabilized power supply, and parallel multi-channel amplitude-phase signal processing circuits were developed. In addition, dedicated software for the proposed system was developed. To test the developed magnetic camera, an artificial tube specimen made of non-magnetic material (STS304) was fabricated. Each test specimen was processed by simulating flaws commonly detected in feedwater heaters, e.g., slits and wears of various shapes that can be formed due to corrosion, erosion, and vibration. In addition, by simultaneously using both the differential bobbin coil and the encircling array magnetic sensor, the presence, locations, and shapes of the flaws were determined, and the depths of the flaws were quantitatively measured.