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.
Highlights
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]
A magnetizer manufactured in a curved shape according to the cross-sectional shape of the tube, membrane, and welding part magnetizes a portion of the water-cooled wall in the axial direction
The shape of the surface defect can be qualitatively determined from the magnetic flux density distribution measured by the magnetic sensor array deflected from the center of the magnetizer to one side
Summary
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]. 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. 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]. It is 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
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