Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
60Thickness Measurement Technology using Cable-lessInner UT for Boiler Heat Exchanger Tube

MIKIYASU URATA*1KIYOTAKA AOKI*2

NAOYA URATA*3

TAKAYUKI WADA*3

SENICHI TSUBAKIZAKI*4 SHINTARO MATSUMOTO*5

Heat exchanger tubes in the main equipment of thermal power plant boilers, such as
economizers and reheaters, have concerns about the occurrence of local thinning caused by wearof the external surface and widespread corrosion thinning. As a technology for the accurate
detection of such damage, Mitsubishi Heavy Industries, Ltd. (MHI) and Mitsubishi Hitachi PowerSystems, Ltd. developed the cable-less inner UT system, which is an ultrasonic water immersion
method that can measure tube thickness from inside the tube. This paper describes a summary of
the cable-less inner UT system and an example case of heat exchanger tube thickness measurement
performed on an actual boiler.

|1. Introduction
It is important for the enhancement of the operating ratio of thermal power plant boiler
equipment to perform inspection sufficiently taking into consideration the form of damageoccurring depending on the operation mode such as frequent starts/stops and quick load changes, aswell as daily maintenance. In addition, more reasonable and higher-precision inspection has beenrequired because of the reduction in maintenance budgets resulting from the liberalization of theelectricity market.
Heat exchanger tubes in the main equipment of thermal power plant boilers, such as
economizers and reheaters, have concerns about the occurrence of local thinning caused by wear
and widespread corrosion thinning. As a technology for the accurate detection of such damage,
MHI and Mitsubishi Hitachi Power Systems, Ltd. developed inner UT technology(1), which canmeasure the tube thickness from inside the tube. Inner UT technology has been applied to many
inspection works and users have gained confidence in the technology, and the number of cases
where inner UT technology is used has been steadily increasing. With such a background, wedeveloped a cable-less inner UT system that enables a smaller size and a higher measurement speed
than conventional inner UT technology, with the aim of further enhancing the efficiency of heatexchanger tube thickness measurement. A summary of the developed cable-less inner UT system
and an example case of heat exchanger tube thickness measurement performed on an actual boiler
are described below.

|2. Cable-less inner UT system
Figure 1 shows the schematic diagrams of the conventional inner UT system and thecable-less inner UT system. The cable-less inner UT system is a small ultrasonic sensor that can beinserted into a heat exchanger tube. The cable-less inner UT system can measure the tube thickness
over the whole circumference and the full length while traveling with the use of water pressure.
Because the cable-less inner UT system requires no signal cable, the large cable feeding device and*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 Chief Staff Manager, Nagasaki Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Hitachi Power Systems,Ltd.
*5 Chief Staff Manager, Boiler Engineering Department, Boiler Products Headquarters, Mitsubishi Hitachi Power Systems,Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
61pressure pump could be eliminated, and thus the total weight of the system was reduced by 90% incomparison with the conventional system. In this manner, the size of the system has beensignificantly reduced.

Figure 1

Schematic diagram of conventional inner UT system and cable-less inner UT system

Figure 2 shows the developed small ultrasonic sensor. The small ultrasonic sensor is reducedin size according to the limitation imposed by the pipe inner diameter and consists of the ultrasonicprobe, a very small pulser-receiver, a memory device for recording of measured values, and anelectronic circuit and a battery for the control of such components. The measured values can besent to a computer through a USB connection after inspection, and then MHI's unique dataprocessing system can visualize the measured waveforms and output as a thinning state diagram.
The system adopts multi-channel ultrasonic probes that are placed at equal intervals in thecircumferential direction of the sensor. The number of channels (number of probes) used is selected
depending on the tube inner diameter.

Figure 2 Ultrasonic sensor for Cableless Inner UT system

In this manner, the cable-less inner UT system can measure the tube thickness from the innersurface and therefore needs no tube grinding work or scaffolding, in contrast to general thickness
measurement technologies that measure the tube thickness from the outer surface. Therefore, thecable-less inner UT system can efficiently measure the thickness of the tubes entirely in a boiler
heat exchanger tube consisting of a tube bundle. The cable-less inner UT system can also be used
for finned tubes, which allows tube thickness measurement of almost all of our boiler heatexchanger tubes.

|3. Case examples of tube thickness measurement
3.1

Case example of mockup panel measurement

Figure 3 shows an external view of a mockup panel that simulates a boiler heat exchanger
tube. The mockup panel consists of a tube with an inner diameter of 34 mm and a bending radius of
57.5 mm. Several parts of the tube are artificially thinned to make thinning flaws. The minimum
thickness values of the thinning flaws are 2.0 mm, 2.7 mm and 3.8 mm. Table 1 and Figure 4shows the measurement results. Figure 4 shows the analysis results of measurement data with theuse of a data processing system and the minimum thickness values are indicated as a chart. It wasMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)62

confirmed in Table 1 that thinning parts can be detected and the tube thickness can be measured at
an accuracy of +/-0.1 mm against the actual thickness measured from the outer surface of the tube.
Figure 5 shows the traveling state of the small ultrasonic sensor. It was confirmed that the smallultrasonic sensor can smoothly pass through the tube and bends that have a bending radius of 57.5mm.

Figure 3External view of mockup panel
Table 1Thickness measurement results
Thinning A

Thinning B

Thinning C

Actual measured value

2.0mm

1.5mm

2.9mm

Measured value

2.0mm

1.5mm

3.0mm

Actual measured value: Thickness measurement value obtained by cross sectional inspection
Measured value: Thickness measurement value obtained with cable-less inner UTFigure 4 Thickness measurement results(chart diagram)

3.2

Figure 5

Passage situation of sensor

Case example of measurement of heat exchanger tube in actual boiler

A verification test was performed during a scheduled inspection of an actual boiler in orderto evaluate the practicability of the cable-less inner UT system. The verified panel was a reheatertube with a minimum inner diameter of 57 mm and a minimum bending radius of 50 mm. Figure 6to Figure 8 show the measurement results. The validity of the measurement values was evaluatedby comparing them with the measurement values obtained by the conventional inner UT systemthat has been used for many applications.

Figure 6 Measurement results of maximum thinning part(conventional inner UT system)

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
63

Figure 7 Measurement results of maximum thinning part(cable-less inner UT system)

Figure 8 Measurement results (panel diagram)

As a result of verification, both the minimum measurement value of the conventional inner
UT system and that of the cable-less inner UT system were 1.9 mm, and the cable-less UT system
can measure the tube thickness at an accuracy equivalent to the conventional inner UT system evenon an actual boiler, as shown in Figure 6 and Figure 7. In addition, the cable-less UT system cananalyze the measurement data with the use of the data processing system that can display theminimum thickness value for every 100 mm section using different colors on a panel drawing as
shown in Figure 8. It was confirmed that the thinning position detected by the cable-less UT systemcorresponds with that detected by the conventional inner UT system. Moreover, the sensor of thecable-less UT system could pass through both straight tubes and bent tubes smoothly, and therefore
perform tube thickness measurement at a speed about twice that of the conventional inner UTsystem.

|4. Performance comparison and applicable scope
Table 2 shows the results of a comparison between the conventional inner UT system andthe cable-less inner UT system. The cable-less inner UT system successfully reduces the total
weight by 90% in comparison with the conventional inner UT system, while maintaining tube
thickness measurement accuracy equivalent to the conventional system. It can also shorten the timerequired for preparation for an inspection. It was also verified, in an actual heat exchanger tube
inspection, that the cable-less inner UT system can measure a tube thickness at a speed about twicethat of the conventional inner UT system, and therefore it is expected that the cable-less inner UTsystem can shorten the inspection process.
Table 2Comparison results of conventional inner UT system and cable-less inner UT system

Thickness measurement
accuracy
Positioning accuracy
Measurement speed
On-site preparation time

Conventional inner UT system

Cable-less inner UT system

Evaluation±0.1 mm±0.1 mm

Equivalent±100 mm
Normal inspection:
100 to 150 mm/s
(Max. 200 mm/s)
2 days±100 mm
Normal inspection:
200 to 280 mm/s
(Max. 300 mm/s)
1 day

Equivalent
Approximately double
Reduced by 50%

Table 3 compares the applicable scope between the conventional inner UT system and thecable-less inner UT system, and Figure 9 shows the correlation between the passable tube innerdiameter for the sensor used and the tube bending radius. The minimum passable bending radius ofMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)64

the sensor of the cable-less inner UT system is 35 mm when the tube inner diameter is 30 mm, witha minimum tube inner diameter of 25 mm. Due to development of the small sensor, almost all ofour boiler heat exchanger tubes can be measured.
Table 4 shows the standard inspection process. In contrast to the conventional inner UT
system that takes seven days to complete the whole process, the cable-less inner UT system canshorten the period by three days, and therefore it is estimated that measurement can be performed
in four days including the preparation and withdrawal of the equipment. In this manner, thecable-less inner UT system is expected to shorten the inspection process due to the simplification of
the equipment components and the increase in measurement speed.
Table 3

Applicable scope of conventional inner UT system and cable-less inner UT system

Heat exchanger tube thickness
Heat exchanger tube inner diameter
Heat exchanger tube curvature radiusObjective panel typeFigure 9

Conventional inner UT system
1.5 mm – 20 mm
22 mm – 60 mm≧35 mmHorizontal type and hanging type

Cable-less inner UT system
1.5 mm – 20 mm
25 mm – 60 mm≧35 mmHorizontal type

Applicable conditions of tube inner diameter and tube bending radius
Table 4 Inspection process
DayWork item
Setup and adjustment ofequipment

Day 1Day 2

Day 3Day 4Day 5Day 6Day 7Day 8

Reduced by 1Measurement
Disassembly and removalof equipment

Reducedby 1.5 days
Reduced by 0.5 day： Conventional inner UT system： Cable-less inner UT system|5. Conclusion
MHI has been working on the development of various inspection technologies according to
the form of various parts and different types of damage to boilers in order to enhance the operating
ratio of thermal power generation plant boiler equipment. This paper presented the cable-less innerUT system, which is a measurement technology for boiler heat exchanger tube thickness. Among
many boiler heat exchanger tube thickness measurement technologies, this technology is the only
high-efficiency technology that can perform thickness measurement of an entire heat exchanger
tube at a high speed and with high accuracy, and is expected to significantly contribute toimprovement in the operating ratio of boilers. MHI will continue to work on the enhancement ofsuch systems in the future.

References1.

Urata, M. et al., Nondestrusctive Inspection Technology of Heat Exchanger Tube by INNER UT, The
Japan Society of Mechanical Engineers