*Article* **Measurement of Thinned Water-Cooled Wall in a Circulating Fluidized Bed Boiler Using Ultrasonic and Magnetic Methods**

**Jinyi Lee 1,2,\*, Eunho Choe 3, Cong-Thuong Pham <sup>4</sup> and Minhhuy Le 4,5,\***


**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 watercooled 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.

**Keywords:** circulating fluidized bed combustion boiler; water-cooled wall tube; magnetic sensor array; magnetic flux density; flexible ultrasonic probe

#### **1. Introduction**

Circulating fluidized bed combustion boilers burn various fuels such as wood, coal, and combustible waste together with solid fluidized media such as sand and ash [1]. In addition, combustion air is injected at high speed through a distribution plate at the bottom of the furnace to burn coal in a gas-solid flow condition inside the furnace. The high temperature of the heated fluid medium particles scatter and circulate in a suspended state to transfer heat to the heat transfer tube. Since heat is transferred through collisional contact with the fluid particles, the heat transfer coefficient is very superior compared to the convection heat exchange method of the existing boiler. However, due to the collision contact between the surface of the water-cooled wall and the fluid particles, which is repeated as the operation time elapses, erosion due to direct exposure to combustion flames, corrosion due to high-temperature combustion and formation of potassium chloride, and acceleration of corrosion due to adhesion could appear. Thus, the life cycle of the watercooled wall is shorter than that of the existing boiler systems. In addition, the lower part where the concentration of the fluid medium is high is a splash area where the fluid medium violently behaves, and the water-cooled wall is severely damaged. These damages intensify in the kick-out area located at the boundary between the lower fireproof part and the water-cooled wall [2]. On the other hand, when abrasion and corrosion occur on the

**Citation:** Lee, J.; Choe, E.; Pham, C.-T.; Le, M. Measurement of Thinned Water-Cooled Wall in a Circulating Fluidized Bed Boiler Using Ultrasonic and Magnetic Methods. *Appl. Sci.* **2021**, *11*, 2498. https://doi.org/ 10.3390/app11062498

Academic Editor: Giuseppe Lacidogna

Received: 26 January 2021 Accepted: 8 March 2021 Published: 11 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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 [6–11]. 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. RFECT [12–16] uses a probe consisting of an excitation coil and a measuring coil that can be inserted into a ferromagnetic heat pipe tube such as the water-cooled tubes. The magnetic energy generated by the interpolation type excitation coil goes from the excitation coil to the outside of the tube and flows in the axial direction, and then back to the inside from the remote field area at a certain distance. The measuring coil can sense the energy delivered without receiving it from the excitation coil. In order to increase the ratio of the signal to noise, it is necessary to increase the cross-sectional area and the number of turns of the excitation coil and the measurement coil so that the spatial resolution of the probe is low. Therefore, there is a limitation in quantitatively evaluating where the damage is occurring on the water-cooled wall tube, the weld, and the membrane. For further improvement of the sensitivity, a giant magnetoresistance (GMR) and Hall sensors were used to measure the low magnetic leakage signal in the MFLT systems [17,18]. This method makes it possible to detect a defect on the surface and near the surface of the water-cooled wall tubes. However, it is still difficult to measure the thickness changes of the tube due to the damages. The combination of the ultrasonic and electromagnetic field has been developed in the electromagnetic acoustic transducer (EMAT) system for the inspection of the water-cooled tubes [19,20]. The magneto-elastic phenomenon and Lorentz force help the EMAT inspect a deeper damage without the need for coupling material. However, the signal is weak and requires advanced signal processing circuits and algorithms. In addition, the EMAT probe has a big size, and thus, it is not efficient to build an array EMAT probe with a high spatial resolution for quantitative evaluation of damage sizes.

This study proposed a combination of the magnetic flux leakage testing and ultrasonic testing methods for the efficient detection and quantitative evaluation of the depth and residual thickness distribution of the near-side and far-side corrosion of the water-cooled wall. A Hall sensor array probe with 48 elements arrayed in an interval of 2.5 mm was

developed to detect the near-side damages and thus make it possible to evaluate the damage size. A flexible ultrasonic probe (FUP) was developed to detect the far-side damages on the tube, membrane, and welding lines of the water-cooled wall. The FUP was incorporated with a flexible membrane that allows the transmission of the ultrasonic wave from the probe to the water-cool tube surface efficiently. Thus, it is not required to largely supply coupling material during the inspection. In addition, the FUPs could be arrayed according to the water-cool plates for fast inspection. For the verification of the proposed method, artificial tapper-type wears and slit-type damages with different sizes were produced on the tube, membrane, and welding lines of the water-cooled wall. Both the detection and size/depth evaluation of the damages will be discussed.
