Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)
50Surface Defect Detection Technology
Contributes to Shortening of Inspection TimeM i ts u b i s h i H i ta c h i P o w e r S y s t e m s , L t d .

Welds of a thermal power plant boiler, which may be damaged by thermal fatigue cracking,creep fatigue, etc., are surface-inspected using PT (penetrant testing), MT (magnetic particletesting), etc. However, PT and MT inspections require the removal of the high-temperatureoxidized scale generated on the tube surface, and this preprocessing work takes time. Furthermore,because there are parts with complicated shapes and narrow areas depending on the equipment, thework efficiency of PT and MT may be lowered and that results in long inspection times.
Accordingly, Mitsubishi Heavy Industries, Ltd. and Mitsubishi Hitachi Power Systems, Ltd.
developed surface defect detection technology using Pencil ECT (Eddy Current Testing) as atechnique for efficiently detecting the damage. This paper describes the characteristics of Pencil
ECT and case examples of defect detection implemented on actual boilers.

|1. BackgroundFigure 1 shows examples of damage of boiler welds. Major damage occurring on boiler
welds includes thermal fatigue, creep fatigue, etc. In periodic inspections, PT and MT are applied
as an evaluation method of cracks caused by this damage. However, if high-temperature oxidized
scale, etc., are generated on the surface of the inspection object, polishing such as blasting orgrinding is required, and such preprocessing work takes time. In the case of PT, the detection ofcracks utilizes the capillary phenomenon of penetrant and developer, so it takes time for thepenetration of detecting liquid and for the detection of defects. Furthermore, when the inspection
object has a complicated shape and exists in a narrow space, such as a header tube, the work
efficiency of PT and MT decreases, resulting in longer inspection times. On the other hand, there isa growing need to shorten the periodic inspection process of thermal power plants to improve theoperational rate, and the efficiency improvement of non-destructive inspection implemented during
periodic inspection is required. Therefore, we developed a Pencil ECT (Eddy Current Testing)sensor that can efficiently inspect boiler welds.

Figure 1

Boiler welds and examples of damage

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)51|2. Characteristics of Pencil ECT
Figure 2 presents the principle of ECT. ECT is a technology to generate eddy current in a
conductive specimen and detect defects existing on the surface of the specimen based on changes inthe eddy current. Since ECT is a detection method that uses an electromagnetic reaction, it ischaracterized by being barely affected by scale. Using this characteristic, we developed a Pencil
ECT sensor that can detect defects even when scale is adhered to the surface of the object beingmeasured.

Figure 2

Principle of ECT

Figure 3

Pencil ECT sensor

Figure 3 shows the developed Pencil ECT sensor. The Pencil ECT sensor consists of a
pencil-like holder and a small coil. The small coil is mounted on the tip of the sensor and embedded
in a low friction material. By bringing the tip of the sensor directly into contact with the weld end,
it is possible to detect a defect even in locations where the weld shape is suddenly changed. Inaddition, due to its pencil-like shape, the sensor is easy to hold and makes inspection relatively
easy, requiring no advanced skills for sensor scanning. In some cases where the inspection area has
a complicated shape and is a narrow space, such as a header tube, the magnetizing device for MT
cannot be accessed. Due to its small size, Pencil ECT can access the narrow area easily. Pencil
ECT can be applied to the inspection of cracks occurring on not only welds, but also element pipes,flat plates, etc.
Figure 4 indicates an inspection example of a weld of a header tube in an actual boiler. As
shown in the figures, it was confirmed that penetration indication patterns were detected as a resultof PT implemented at the location where Pencil ECT detected a defect.

Figure 4Inspection example of header tube in actual boiler

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)52

Figure 5 shows a comparison between the conventional PT method and the developed Pencil
ECT for the operation time per header tube. While the operation time of PT is 30 minutes(including treatment of the surface) per location, that of ECT can be shortened to about 5 minutes
under the same conditions, so the operation time is expected to be shortened by about 80%. This is
greatly attributed to the reduction of the preprocessing time. The aim of this sensor is to shorten thepreprocessing time, but time periods for defect detection and crack evaluation can also be saved,
resulting in the improvement of the efficiency of all work operations. In addition, because theinspection records can be stored as digital data, the handling of records is easier than ever and it ispossible to create a database. As such, many advantages can be expected by applying ECT.

Figure 5

Inspection example of header tube in actual boiler|3. Future prospects
We have been working on the development of various inspection technologies for boiler
parts/forms of damage to improve the operational rate of thermal power plant boiler facilities. This
paper described Pencil ECT, which is one of the developed technologies. We will continue theapplication of this technology to actual facilities, and also promote further upgrades to contribute to
the improvement of the operational rate of thermal power plant boiler facilities. The inspection
technologies introduced here will be applied not only to boiler facilities, but also to chemical plants
and various piping facilities.