Research and Application of Pulsed Eddy Current Testing for Ferromagnetic Metallic Components

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1 Research and Application of Pulsed Eddy Current Testing for Ferromagnetic Metallic Components G. T. Shen, 1, J. Li 1, 2 and X. J. Wu 3, 1 China Special Equipment Inspection and Research Institute, Beijing, China. More info about this article: shengongtian@csei.org.cn 2 Department of Mechanical Engineering, Tsinghua University, Beijing, China. 3 School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan, China. Abstract Ferromagnetic metallic components with tens or hundreds of millimetres thermal insulation coatings are common in the large complete equipments. It is hard to get the information about the wall thinning of the components without removing the coatings. To ensure the long cycle operation of the large complete equipments, pulsed eddy current (PEC) testing has being researched and applied. Firstly, the reasons why PEC testing is the appropriate candidate are pointed out after the background is introduced. Secondly, the principle and the signal characteristics of PEC testing are investigated. Thirdly, the PEC testing system and the standards have been developed. Finally, the practicability and reliability of PEC testing in assessing the wall thinning of ferromagnetic metallic components are illustrated by two typical field applications. Keywords: Pulsed eddy current testing, ferromagnetic metallic components, on-line 1 Introduction Large complete equipments are widely used in petroleum and chemical industry, and they are increasing required with the development of industrialization. Ferromagnetic metallic components are common in the large complete equipments, because their excellent mechanical property. For saving energy and ensuring safety, tens or hundreds of millimetres thermal insulation coatings are wrapped on the components. The wall thinning caused by corrosion is inescapable on account of the extreme operating condition such as high pressure, high temperature or deep cooling. Periodic testing is critical in the maintaining of large complete equipments, but it is costly and time-consuming. It hopes that the period beteewn the testings as large as possible, on the premise that the large complete equipments are running well. Hence, the on-line testing is essential to the maintaining. The existing testing methods mainly include ultrasonic testing and radiographic testing. However, they have many disadvantages. To the ultrasonic testing, it is not only removing the coatings, but also polishing the surface for coupling well. After testing, professional measures should be done to recover to the original appearance. Otherwise, the inspected point will form the new corrosion and becomes worse. To the radiographic testing, the strict protection is indispensable during testing, and it is hard to inspect the components with large diameter. Therefore, it urgent needs a method, which is efficient [ID148] 1

2 and low-cost, to test the ferromagnetic metallic components with tens or hundreds of millimetres thermal insulation coatings. As an electromagnetic testing method derived from the eddy current testing in NDT and the transient electromagnetic method used in geology, pulsed eddy current testing has the potential to meet the requirements mentioned above. Based on this understanding, systematic research has been done and plenty of achievements have got in recent years. The significant works will be introduced in this paper. 2 Testing principle The principle of PEC testing is shown in figure 1. A probe including a sender coil and a receiver coil is located above the specimen. The rectangle current is loaded on the sender coil. When the rectangle current is high level, a steady primary magnetic field is generated by the sender coil. When the rectangle current turns to zero, the primary magnetic field will rapidly reduce to zero at the same time. According to Faraday's law and Lenz's law, eddy current will be induced in the surface of the specimen, to slow the reduction of primary magnetic field. After that, eddy current will diffuse downward and outward, and its amplitude will fade to zero. The variation of the secondary magnetic field produced by the variation of eddy current will be converted to voltage by the receiver coil. The voltage vary with time is the PEC signal. Generally, PEC signal is log-log ploted for qualitative analysis as shown in figure 2. Due to the diffusion of eddy current is related to the specimen thickness, the information about specimen thickness could be extracted from PEC signal. Figure 1: PEC testing principle. For the ferromagnetic metallic component, the diffusion process of eddy current has been investigated in our previous researches by simulation [1]. It found that the maximum of eddy current density moves with the eddy current diffusion and attenuation. Before the maximum of eddy current density moves to the half of specimen thickness, the eddy current diffusion and attenuation are independent to the specimen thickness. After that, the eddy current diffusion and attenuation will relate to the [ID148] 2

3 specimen thickness. It is to say, there is a time to separate the PEC signal to two parts. The former part is independent of specimen thickness, and only the latter part is relate to the specimen thickness. Due to the non-zero edge time of practical rectangular wave could impact PEC signal in an ultrashort time, the former part could divide two parts. Therefore, PEC signal could be divided to three parts as shown in figure 2. Figure 2: PEC signal of ferromagnetic metallic specimen. The part I of PEC signal is the synthetic performance of primary magnetic field caused by the nonzero edge of rectangular wave and the secondary magnetic field caused by the eddy current in the specimen. It is hard to extract the specimen information form this part. The part III of PEC signal correspondings the stage that after the maximum of eddy current density moves to the half of specimen thickness. Between the part I and III, it is part II. The part II and III of PEC signal could be describe by (1). U t F 2 l t, t t t ct F l, d e, t t (1) where l is the liftoff of probe, σ, μ and d are the conductivity, the permeability, and the thickness of specimen respectively, c is the undetermined constant, F II(l) is the function about l, F III(l, d) is the function about l and d, t III is the begin time of the part III of PEC signal, it is proportional to σ, μ and the square of d shown as (2). 2 d (2) t Obviously, employing t III could assess the specimen thickness. However, as a smooth transition point, t III is sensitive to the noise. It is hard to get the accurate value. As shown in (1), the information of specimen thickness is contained in the part III of PEC signal, and it could be extracted out from this part. It is noteworthy that the amplitude of the part III of PEC signal is weak. Pre-processing is necessary to amplify and denoise. Besides, the sampling length should be long enough to extract the information of specimen thickness. [ID148] 3

4 3 Development of PEC testing system Based on the above studies, the block diagram of PEC testing system has been designed as shown in figure 3. PEC testing system is composed by a portable computer, an instrument host, a preamplifier, a probe and a battery. The computer is used to control the testing process, signal analysis and data storage. The instrument host is composed by a power amplifier and an A/D card. The power amplifier is used to convert the voltage signal produced by the A/D card to electric current to load on the sender coil. The A/D card is dual functional. It includes a D/A module to produce the desire voltage signal, and an A/D module to acquire the PEC signal. The preamplifier is used to amplify and denoise the PEC signal produced by the receiver coil. The probe includes an excitation coil to generate the primary magnetic field, and a receiver coil to sense the change of secondary magnetic field. The battery is used to supply the power to the instrument host. Figure 3: The block diagram of PEC testing system. In figure 3, the probe and the power amplifier play the key roles in the PEC testing system. Besides, as the brain of the PEC testing system, the software is essential to realize the functions. Next, the development of the probe, the power amplifier and the software will be introduced. 3.1 Probe Generally, the distribution region of the primary magnetic field is proportional to the external diameter of the excitation coil. In order to test the thick walled pipe, the excitation coil with big external diameter should be employed. However, the spatial resolution will become wrose due to the bigger distribution region of the primary magnetic field. It should be a balance between the testing capability and spatial resolution. Therefore, after the common industrial pipe specification has been separated to four series, four regular probes have been developed [2]. Besides, a V-shape probe has been developed for testing on curved surface such as pipe [3], and a saturation magnetization probe has been developed for galvanized iron sheet [4], which is ferromagnetic and commonly used as the cover layer in China. The probes are shown in figure 4. [ID148] 4

5 (a) Regular probes (b) V-shape probe (c) Saturation magnetization probe Figure 4: PEC probes. 3.2 Power amplifier With regard to the probes shown in figure 4, the most of commercial power amplifier would be incompetent. Either the waveform distortion of current loaded on sender coil is unacceptable, or the volume is too big to the field application. To overcome the difficulties, a special power amplifier has been developed as shown in figure 5(a). It is designed with parallel and bridge connection of four units. Each unit adopts output transformerless (OTL) power circuits. The special power amplifier has the advantage of wide bandwidth, large output current, small volume and light weight. When the rectangle voltage with 1 Hz repetition frequency loaded on the sender coil, which is 2.2 Ω and 7.15 mh, the current waveform on the excitation coil is shown in figure 5. It is not difficult to find that the edge is slope, and the edge time is about 800 μs when the amplitude step from 4 A to 0 A. The small edge time makes the part I of PEC signal short enough to avoid disturbing the part III of PEC signal. (a) Configuration (b) Result Figure 5: Testing configuration and result of PEC power amplifier. 3.3 Software In order to realize the functions of PEC testing system, the software has been developed as shown in figure 6. The main interface shown in figure 6(a) includes display&analysis settings region, signal region, result region and control bar region. Its interface is friendly. On the control bar, there are several buttons to provide the interfaces for the most of functions. For example, the first button Parameters provides the interface for parameter setting shown as figure 6(b). [ID148] 5

6 The software has realized the functions of parameter setting, A/D card operation, signal display, signal analysis and report exporting. Specially, the function of signal analysis is easily replaceable to meet the update of the signal processing methods [5-8]. (a) Main interface (b) Parameter setting interface Figure 6: The software of PEC testing system. 3.4 PEC testing system When all parts shown in figure 3 and the software shown in figure 6 have been developed, a PEC testing system has been formed as shown in figure 7. Figure 7: The self-developed PEC testing system. Based on the system, a commercial PEC testing system (Type: ZTJ-PEC-A) is presented [9]. It could test the thickness of the ferromagnetic component with 3 mm~70 mm, -196 C~500 C, curvature radius 25 mm, coating thickness 200 mm, the wall-thinning error 5%, and the least absolute accuracy of measurement 0.5 mm. The commercial PEC testing system could meet the common testing requirements in the industrial field. 4 Development of standards In the above research process, the influence mechanism of the factors met in the field has been revealed [10]. And then, the first national standard, the first energy industry standard in China, the first ISO standard have been formulated successively in recent years. These standards specify the PEC testing technology used to perform thickness measurement on ferromagnetic metallic components with or without the presence of coating, insulation and weather sheeting [11-13]. [ID148] 6

7 5. Typical applications Since the self-developed PEC testing system is available, the system has been applied in the on-line testing of hundreds of pipes and tens of pressure vessels in many power plants and petrochemical plants. The applications have created considerable economic and social benefit. In order to illustrate the practicability and reliability of PEC testing technology in wall thinning testing, two typical testing applications are presented as follows. 5.1 Typical application on pipe In May 2015, tens of pipes were tested by the self-developed PEC testing system. Among them, a tested pipe is shown in figure 8. (a) Testing in field (b) Testing area schematic Figure 8: PEC testing on pipe. The specification of the pipe is DN mm, the material is 20# steel, the temperature is 25 C, and the pipe is covered with coatings, which is composed of rock wool and galvanized iron sheet. In the testig, it is found that the signals along the pipe are abnormal. When take off the coatings from the pipe, the tracing pipe is found under the testing points. Therefore, the PEC testing has been carried again far away from the tracing pipe, and the results of PEC testing and ultrasonic testing are shown in table 1. Testing Results by Results by area PEC testing ultrasonic testing Error (A) (9.56 mm) (B) (9.67mm) (C) (10.43mm) (D) (9.70 mm) (E) (9.72 mm) Table 1: Testing results on pipe. From table 1, the results by PEC testing are basically consistent with those by ultrasonic testing, and the testing error is less than ±0.05 (±5%). It illustrates that PEC testing technology is reliable and practical in assessing the wall thinning of pipe. [ID148] 7

8 5.2 Typical application on vessel In April 2016, several vessels were tested by the self-developed PEC testing system. Among them, a tested vessel is shown in figure 9. (a) Testing in field (b) Testing area schematic Figure 9: PEC testing on vessel. The inner diameter of the vessel is 1100 mm, the wall thickness is 21 mm, the material is Q345R, the temperature is C, and the vessel is covered with coatings, which is composed of aluminum silicate and galvanized iron sheet. When the PEC testing is completed, the coatings are removed and the thickness of each testing area is measured by ultrasonic testing. The results are shown in table 2. Testing Results by Results by area PEC testing ultrasonic testing Error (A) (21.3 mm) (B) (21.8 mm) (C) (21.4 mm) (D) (21.4 mm) (E) (21.3 mm) (F) (22.0 mm) (G) (21.7 mm) (H) (21.5 mm) Table 2: Testing results on vessel. From table 2, comparing with the ultrasonic testing results, although the maximum testing error by self-developed PEC testing system is (-6.57%), and the location of the most thick region is different, the most wall thickness distribution is the same, especially for the thin wall regions. Therefore, the results by PEC testing are credible. It illustrates that PEC testing technology is reliable and practical in assessing the wall thinning of vessel. 6 Conclusions (1) PEC signal of ferromagnetic metallic components could be divided into I,II,III parts according to the sharpe of PEC signal, which is related to the evolution of eddy current density distribution. It should extract suitable feature from the part III of PEC signal to characterise the specimen thickness. [ID148] 8

9 (2) A PEC testing system has been developed, and the standards including the first national standard, the first energy industry standard in China, and the first ISO standard have been developed. (3) The typical applications illustrate that the PEC testing technology is reliable and practical for testing the wall thinning of ferromagnetic metallic components. (4) The achievements presented above are obtained when the specimen is ferromagnetic. Their applicability on non-ferromagnetic materials, such as stainless steel and titanium alloy, should be further studied. Acknowledgments This work is supported by National High-level Talents Special Support Plan, and National Key Research and Development Plan (Grant No. 2016YFC ). References [1] J Li, X Wu, Q Zhang, et al, Measurement of Lift-off using the Relative Variation of Magnetic Flux in Pulsed Eddy Current Testing, NDT & E Int., vol. 75, pp , Oct., [2] Z Xu, Theory and Method for Pulsed Eddy Current Testing of Wall Thinning in Insulated Pipes, Huazhong University of Science and Technology, Wuhan, China, (In Chinese) [3] Pulsed Eddy Current Probe for Pipe, by X Wu, J Li and Q Zhang, Sep. 23, 2015, CN B. [4] Saturation Magnetization Probe for Pulsed Eddy Current Testing, by X Wu, Z Xu, C Huang, et al, Aug. 24, 2011, CN B. [5] C Huang, X Wu, Z Xu, et al, Pulsed Eddy Current Signal Processing Method for Signal Denoising in Ferromagnetic Plate Testing, NDT & E Int., vol. 43, no. 7, pp , Oct., [6] C Huang, X Wu, Z Xu, et al, Ferromagnetic Material Pulsed Eddy Current Testing Signal Modeling by Equivalent Multiple-coil-coupling Approach, NDT & E Int., vol. 44, no. 2, pp , Oct., [7] C Huang, X Wu. An Improved Ferromagnetic Material Pulsed Eddy Current Testing Signal Processing Method based on Numerical Cumulative Integration, NDT & E Int., vol. 69, pp , Jan., [8] J Li, X Wu, Q Zhang, et al, Pulsed Eddy Current Testing of Ferromagnetic Specimen based on Variable Pulse Width Excitation, NDT & E Int., vol. 69, pp , Jan., [9] ZHONG TE JIAN T&D (BEI JING) CO. Ltd, Sep. 30, 2017, < [10] G Shen, J Li, X Wu, Research and Application of Pulsed Eddy Current Testing Technology for Pressure Equipment, Journal of Mechanical Engineering, vol.53, no. 4, pp , Feb., [11] Non-destructive Testing Test Method for Pulse Eddy Current Testing, GB/T 28705, [12] Nondestructive Testing of Pressure Equipment Part 13: Pulse eddy current testing, NB/T , [13] Non-destructive Testing Pulsed eddy Current Testing of Ferromagnetic Metallic Components, ISO 20669, [ID148] 9