Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
1High-efficiency Inspection Technology forInner Surface Corrosion of Lengthy Tubes by ECTMIKIYASU URATA*1KIYOTAKA AOKI*2

NAOYA URATA*3

TAKEHIKO YAMAGUCHI*4

KENTARO JINNO*4

SHINTARO MATSUMOTO*5

In boiler heat transfer tubes for thermal power plants, corrosion thinning caused by caustic
gouging may occur on the tube inner surface. Mitsubishi Heavy Industries, Ltd. (MHI) andMitsubishi Hitachi Power Systems, Ltd. have established inner UT technologies using theultrasonic method for continuous thickness measurement of heat transfer tubes, but they areaffected by scale which is comprised of rust and deposits produced by corrosion, resulting indifficulty in the detection of thinning of the tube inner surface. The author et al. studied theapplication of ECT (Eddy Current Testing) as a method of inspecting the overall
length/circumference of the tube without being affected by scale, extracted the flaw detection
conditions for detecting thinning parts through electromagnetic field simulation and developed a
special ECT probe.

|1. Introduction
In boiler heat transfer tubes for thermal power plants, corrosion thinning may occur on thetube inner surface caused by caustic gouging, etc. (Figure 1). Boiler heat transfer tubes, however,are densely installed to form tube banks. Therefore, in many cases, thinning parts cannot be easily
accessed depending on their locations. We have established inner UT technologies using theultrasonic method for continuous thickness measurement of heat transfer tubes, but the effect ofscale produced by corrosion has made it difficult to detect thinning of the tube inner surface.
Furthermore, it is impossible to inspect finned tubes from the outer surface. Therefore,conventionally, a part of the tube was cut and a borescope, etc., was inserted and the state of thedeposition of surface scale was observed to evaluate the presence or absence of thinning orabnormalities. This evaluation by visual inspection is qualitative, however, and it was difficult toquantitatively evaluate thinning.

Figure 1

Example of caustic gouging*1 Materials Research Department, Research & Innovation Center*2 Chief Staff Manager, Materials Research Department, Research & Innovation Center*3 Nagasaki Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Hitachi Power Systems, Ltd.
*4 Electricity & Applied physics Research Departmen, Research & Innovation Center*5 Manager, Nagasaki Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Hitachi Power Systems, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
2For corrosion thinning which occurs on the inner surface of boiler heat transfer tubes, westudied the application of ECT (Eddy Current Testing) to inspect the overall length/circumferenceof the tube without being affected by scale produced as a result of corrosion. In this report, the ECTprobe and the inspection system that were developed based on electromagnetic field simulation andthe actual boiler heat transfer tubes are introduced, and the results of the verification of thedetectability of caustic gouging thinning and the practicality of the inspection system are described.

|2. Development of ECT inspection system
Figure 2 shows the developed tube-inserted ECT inspection system. In this system, thedeveloped ECT probe is inserted into the heat transfer tube to inspect thinning over thecircumference and length of the tube while being moved by the driving force of a cable conveying
device and hydraulic pressure. In ECT, eddy current is generated in a test specimen that has
electrical conductivity, and from the change of the eddy current, flaws on the surface of the test
specimen are detected. It is a detection method using electromagnetic reaction and is hardly
affected by scale. The use of this feature enables the detection of corrosion thinning of the innersurface over the length/circumference of the heat transfer tube and quantitative evaluation ofthinning depth, which were previously impossible.
Figure 3 shows the developed ECT probe. There are various types of sensor coil, and thisprobe adopts the differential coil method in which the effect of the magnetic properties and noise
generated by the sensor tilt are small and the defect detectability is high. This means that to detectthe difference between the signal components in the tube circumferential direction and in the tube
axial direction, noise can be reduced while signals generated by defects are maintained. In addition,a float structure was incorporated in the signal cable, improving the driving force and allowing
passage through a tube with a long length.
Figure 4 indicates the state of the eddy current generated on the thinning part in anelectromagnetic field simulation. In the simulation, local thinning and relatively wide-ranging
thinning were simulated, and the distribution of the eddy current in a single coil was evaluated.
Based on these results, the arrangement of a small-sized coil was optimized and a multi-channel
structured probe which allows the detection of thinning over the tube circumference without
missing anything was designed. The coil part of this probe has a flexible structure, which allows it
to pass smoothly through discontinuous portions such as welding penetrations or tube bends.

Figure 3

Figure 2

ECT inspection system

Developed ECT probe

Figure 4 Electromagnetic field
simulation|3. Detected state of thinning partTo evaluate the thinning detectability of the developed ECT probe, a verification test wasconducted using a sample with artificial flaws processed on the tube inner surface. Figure 5 shows
the detected state of the artificial flaws. The artificial flaws had the same depth (0.5 mm) and varied
in width and length. The developed ECT probe was able to detect all the artificial flaws.

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
3

Figure 5

Example of detection of artificial flaws

Figure 6 shows the inspection results of the boiler heat transfer tube obtained in the trial
operation of the actual unit and the investigation results for the cross section. The developed ECTprobe was able to detect the corrosion thinning part of the actual boiler tube inner surface withdeposited scale. In the investigation of the cross section, corrosion thinning that was 1.8 mm indepth was observed at a point at the thinning depth of 1.6 mm evaluated by ECT. Figure 7
provides the comparison results of the thinning depth evaluation value by ECT and the thinning
depth value measured by the investigation of the cross section. The error in the thinning depth
evaluation by ECT was ±0.3 mm, and it shows that among the measurement techniques using
electromagnetic reaction, this method allows a relatively high-precision evaluation of thinning
depth. In this system, the inspection results are displayed with a color view of the tube inner surface
developed by the original analysis system, enabling the thinning distribution to be visuallyunderstood. Furthermore, the thinning depth is evaluated from the obtained signal amplitude, etc.

Figure 6

State of corrosion thinning detected on the actual tube

Figure 7

Evaluation precision for thinning depth

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
4|4. Conclusion
An outline of the tube-inserted ECT inspection system, which allows the inspection ofcorrosion thinning of the inner surface of boiler heat transfer tubes over the tube length and thecircumference and a quantitative evaluation of the thinning depth, was presented. This system isour original inspection technology which enables a quantitative evaluation of corrosion thinninggenerated on the inner surface of magnetic material tubes without removing scale and is applicable
to an actual plant. This system has already been applied to two plants and its effectiveness has beenverified. However, this technology adopts a hydraulic system, and therefore, the size of the systemis large and a great deal of time is required for preparation for inspection, such as the transport andinstallation of a large-sized pump and water storage tank. Therefore, the adoption of a pneumatictransfer system instead of a hydraulic system is currently under study. Through substitution with apneumatic transfer system, the device can be installed inside a boiler furnace and an increase inwork efficiency and labor saving can be expected. In the future, we will continuously conduct
verification tests to increase the practicability of the technology.