###### Abstract  
In this paper, a nondestructive inspection system is proposed to detect and quantitatively evaluate the size of the near- and far-side damages on the tube, membrane, and weld of the water-cooled wall in the fluidized bed boiler. The shape and size of the surface damages can be evaluated from the magnetic flux density distribution measured by the magnetic sensor array on one side from the center of the magnetizer. The magnetic sensors were arrayed on a curved shape probe according to the tube's cross-sectional shape, membrane, and weld. On the other hand, the couplant was doped to the water-cooled wall, and a thin film was formed thereon by polyethylene terephthalate. Then, the measured signal of the flexible ultrasonic probe was used to detect and evaluate the depth of the damages. The combination of the magnetic and ultrasonic methods helps to detect and evaluate both near and far-side damages. Near-side damages with a minimum depth of 0.3 mm were detected, and the depth from the surface of the far-side damage was evaluated with a standard deviation of 0.089 mm. circulating fluidized bed combustion boiler; water-cooled wall tube; magnetic sensor array; magnetic flux density; flexible ultrasonic probe 26 January 2021 8 March 2021 11 March 2021 11 March 2021 122 11 March 2021 122 11 March 2021 122 11 March 2021 122 11 March 2021 122 11 March 2021 122 122 122 122 122 122 122 122 122 122 122 122 12  
water-cooled wall tube, leakage and secondary damage due to the leakage may occur. It is also very important to periodically monitor and maintain the thickness of the water-cooled wall since damage to the tube, membrane, and welding portion of the water-cooled wall can cause a decrease in power generation efficiency. Recently, numerous NDT methods have been developed for the inspection of damage on the water-cooled wall. Ultrasonic testing (UT) applies an acoustic medium to the inspection area of the water-cooled wall and measures the change in the reflection time of the ultrasonic wave according to the thickness change when the ultrasonic wave is incident [3]. Although it is possible to perform a precise inspection of the inner surface of the tube, it is difficult to measure the surface corrosion, and the incident angle of ultrasonic waves may vary according to the manual inspection of the operator, resulting in an error in thickness measurement. Phased array ultrasonic testing (PAUT) has been developed to reduce mechanical errors while scanning the probe on the specimens and signal enhancement. It provides excellent results of damage detection and quantitative evaluation of the damage size, such as depth and length [4; 5]. However, the UT and PAUT methods require continuous supplement of the coupling materials such as water for the propagation of ultrasonic wave between the probes and the tube, and the surface of the tube should be cleaned before the inspection. Therefore, it is difficult to operate in the inspection of the water-cooled wall tubes in the power generation, and it also requires high technical trained operators to use the UT and PAUT systems. Electromagnetic testing methods, including eddy current testing (ECT), remote field eddy current testing (RFECT), and magnetic flux leakage testing (MFLT), are the fast, reliable, and easy operation methods for the inspection of damages in the tubes. These are non-contact inspection methods that do not require the coupling material during the inspection. ECT is a widely used method for the inspection of heat exchanger tubes and boilers of nuclear power plants [6; 7; 8; 9; 10; 11]. This method is highly sensitive to the surface cracks, but it is limited to detecting deep defects due to the high concentration of eddy current on the specimen surface in the skin depth effect. Especially, the eddy current has more difficulty penetrating the wall thickness of the water-cooled tube because it has high magnetic permeability.