Development of PC-Based Leak Detection System Using Acoustic Emission Technique

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1 Key Engineering Materials Online: ISSN: , Vols. 70-7, pp doi:0.408/ 004 Trans Tech Publications, Switzerland Citation & Copyright (to be inserted by the publisher) Development of PC-Based Leak Detection System Using Acoustic Emission Technique Min-Rae Lee, Joon-Hyun Lee and Young-Chul Park Department of Mechanical Design Engineering, Pusan National University School of Mechanical Engineering, Pusan National University, San 0 Changjeon-dong, Kumjeong-gu, Pusan, , Korea School of Mechanical Engineering, Dong-A University, 840 Hadan -dong, Saha-gu, Pusan, 60-74, Korea Keywords: Acoustic Emission (AE), On-Line Monitoring, PC-Based Leak Detection, Source Location, Wavelet Transform (WT) Abstract. This research is focused on developing an advanced AE source location system including new concept waveform processing. In the present research, the source location system is designed to detect leak location more accurately at faster speed with less cost. Basically, this system is comprised of two different source location modes such as one-dimensional mode (leak detection in pipeline) and two-dimensional mode (source detection in plate). Generally, since the AE waveforms obtained from the various sources are very complicated, it is difficult to distinguish the original signals clearly. Wavelet transform is a powerful tool for processing transient signals with temporally varying spectra that helps to resolve high and low frequency transient components effectively. In this paper, the source location results are also presented by employing the wavelet transform. Introduction Leaks occur in all piping systems with many different reasons, and are usually an indication of the physical deterioration of the system. Economic loss, safety, and environmental issues associated with pipeline leaks are a growing concern around the world. Since leaks are inevitable, every pipeline operation should have a monitoring procedure for leak detection. There are different methods for leak detection at several levels of cost and performance. The present paper is described the development of a leak detection system and method using acoustic emission (AE). In the last decades, AE has been widely used as a nondestructive test method for pipeline and pressure vessel structures [,]. However, the location of AE source is particularly important when monitoring large structures and on of the limitations of the technology is the inability to accurately identify the position of emitting defects. In the present paper, the research of development of reliable source location technique for large structures such as thick or thin plate is described. Acoustic waves propagate in shell or thin plates as guided or Lamb modes. The velocities of these modes are dispersive in that they depend not only on the material elastic properties and density, but also on the frequency. Accurate characterization of Lamb wave dispersion is important in many acoustic based nondestructive evaluation techniques. It is necessary for ultrasonic measurements in shell or thin plates to determine elastic properties and for flaw detection and localization. In acoustic emission technique, if not taken into account, highly dispersive Lamb mode propagation can lead to large errors in source location [,4]. In commercially available AE system, the arrival time is determined by first threshold crossing technique [5]. In the thin plate case, when the propagating wave is dispersive, error could occur in the source location, since the AE pulse changes the shape due to dispersion. In this study, the wavelet transforms (WT) using the Gabor [6] wavelet is applied to the time-frequency analysis of dispersive plate waves. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/05/6,8::44)

2 56 Advances in Nondestructive Evaluation Source Location Method Methods Based on the Arrival Time. The linear source location method has been applied to detect leak in the pipeline by using the transit time difference between two sensors. In order to obtain more accurate leak location, the effective measuring position X is compensated by means of special calibration by calculation, in according with the following formula [7]: X = α x () X = X + α ( ) x x () I I X = X + α ( ) x x () II II The effective measuring position X, X, X and X 4 of the four AE sensors are plotted in Fig.. These effective measuring positions are obtained from the location coordination, x, x, x and x 4, of the measuring position by taking the various damping coefficients, α, α, α. In the segment I, it is thus generally true that X = α x. In the segment II, it is true that X = X ( ) I + α x xi. In the segment III, it is that X = X ( ) II + α x xii. Figure shows bar diagram in which the aforementioned AE rms values are shown through effective measuring position X. Two compensating straight lines, S and S, that are inclined opposite one another can be drawn through the end points of the bars. The intersection of these two straight lines is ascertained, therefore it is identified the actual leak point. However, assume that an AE signal occurs somewhat on the pipe and that resulting elastic waves propagating in the both directions at the some constant velocity. If the difference signals at sensor and sensor were zero, it would indicate a precisely midway between the two sensors. The time difference between signals is equal to the time taken to cross the entire pair separation or; L T = (4) C where L is the distance between sensors and C is the constant wave velocity, and T is the time difference. The source location would then be location at sensor. Generally, the source location d is given by d = ( L T C ) (5) (a) Effective measuring position (b) Time-of-arrival method Fig.. Simplified depiction of the linear source location.

3 Two dimensional method Key Engineering Materials Vols In two dimension source location, an array with a minimum of three sensors is needed to pinpoint the source position. The concept of the two-dimensional source location is similar to the linear approach as shown in Fig.. In Fig., x 0 and y 0 are the locations of the sources, x i and y i are the locations of sensor i, d i is the distance between the source and sensor i, D and D are the distance between sensor and sensors and between sensor and sensor. Simple equations can be formed based on the arrival time at each sensor: d d = [( t = [( t D t D t ( t ) V + D ( t ) V + D t t ) V cos( θ θ )] ) V cos( θ θ )] (6) (7) The location of a source in two dimensions can be obtained by solving Eq. 6and Eq. 7 simultaneously for d and θ. (a) Source location with two sensors (b) Source location with three sensors Fig.. Three sensors array with detection with sequences, and. Wavelet Transform Fourier transform has been extensively used for the analyses of dispersive signals. There have been intensive research activities in the application of wavelets in various fields of science and engineering. Wavelet transform (WT) has been introduced to the time-frequency representation of transient waves propagating in a dispersive medium. The continuous WT of function f (t) is defined by [6] t b Fψ ( a, b) = f ( t) ψ ( ) dt a (8) a where a > 0 and the over-bar indicates the complex conjugate. The function Ψ (t) is called the analyzing wavelet and satisfied the admissibility condition. ) Ψ( ω) ω dω < (9)

4 58 Advances in Nondestructive Evaluation Where, Ψ ) (ω ) denotes the Fourier transform of Ψ (t). Although one can be choose any analyzing wavelets that satisfy the admissibility condition, the Gabor wavelet is adopted in this study, since it is known to provide the best time-frequency resolution. The Gabor function is expressed as ω 0 ( ω 0 / γ ) W g ( t) exp t exp( iω 0t) 4 π γ = (0) and its Fourier transform is ) π γ ( γ / ω 0 ) Ψg ( ω) = exp ( ω ω 4 0 ) π ω 0 where ω 0 and γ are positive constants. Here, we setγ = π / ln The Gabor function Eq. (0) may be considered as a Gaussian window function centered at t = 0, and its Fourier transform Eg. () centered atω = ω0. The function Ψ ) (( t b) / a) is then centered around t = b, and its Fourier g transform [ a exp( ibω) Ψ ) ( aω)] is centered around ω = / a. The WT Wf ( b, a) using the Gabor g wavelet thus represents the time-frequency component of f (t) around t = b andω = / a study, we set ω = π such that / a is equal to the usual frequency f = ω / π. 0 ω 0 ω 0 (). In this System Configuration and Experimental Set-Up Figure shows a typical source location installation. Basically, this system is comprised of a sensor, preamplifier and signal analyzer. AE signals are detected with an AE sensor attached on the specimen. The sensor outputs are amplified by a preamplifier, which has a fixed gain of 40 db or 60 db. After passing through a band pass filter of 00 to 00 khz, to remove the electrical and mechanical background noises, the signals are further magnified by the main amplifier (0 db). The AE parameters such as AE events, amplitude, and the duration time of signals, are analyzed in the AD board (CS5 k, 8bit, 0 MHz sampling). The leak test of pipeline was based on a 48.6 mm in diameter, 6 m in length and.87 mm in wall thickness, steel pipeline was installed at the laboratory as shown in Fig. (a). This line was installed for the development of leak detection and location capabilities in piping system. The leak tests in the laboratory were performed with PC-based source location system and commercially available instrumentation and sensors. Especially, the instrumentation consisted of a four-channel PC-based source location system, while the sensors were PAC model R5 and PICO resonant transducers as well as WD wideband transducers. Sensors were mounted with viscous couplant and held in place with electrician tape. The test of two dimensional source locations in steel plate was based on a 500 mm 500 mm width and 50 mm in thickness. The pencil-lead breaks were used on the surface of specimen in order to excite acoustic waves. The sensors were PAC model R5 resonant transducers (50 khz peak sensitivity). The sensors were mounted with viscous couplant and held in place with electrician tape as shown in Fig. (b). (a) Linear source location (b) Two-dimensional source location Fig.. Schematic diagram of PC-based source location system.

5 Key Engineering Materials Vols (a) The source location obtained from new (b) The source location obtained from effective location coordinates X. general AE measurement Fig. 4. The results of source location obtained from new source location and general AE measurement. 0 SENSOR SENSOR SENSOR SENSOR S Amlpituce (V) 0-5 Magnitude S Time (µsec) (a) A typical waveforms obtained from the each sensors Time (ms) (b) A typical magnitude of wavelet transform at 50 khz Fig. 5. Typical waveforms and its wavelet transform results. (a) Source location result obtained from PC-based source location system (b) Source location result obtained from conventional AE system(pac) Fig. 6. A comparison of results between PC-based source location system and conventional AE system.

6 50 Advances in Nondestructive Evaluation Experimental Results and Discussion Figure 4 shows the comparison results obtained from new source location and general AE measurement. As shown in Fig. 4(a), leak location was indicated about,75 mm compared with actual leak point,700 mm. However, leak location obtained from general AE measurement was indicated about,54 mm. Therefore, it is found that new source location is more accurate than common acoustic emission system. Figure 5 shows typical waveforms and its wavelet transform results obtained from sensors,, and 4. The location of a source in two dimensions can be obtained the arrival time of longitudinal wave and transient velocity. In this study, the purpose of applications of the wavelet transform (WT) is to examine the transient waves propagating in plate. The location of the AE source in the surface and the velocity of the flexural wave are determined from the time-frequency analysis. For velocity measurement, two sensors ( S, S ) were placed a known distance apart along a line from the source on the plate. The arrival time difference between the sensors was then calculated. As shown in section, the maximum of the magnitude of the wavelet transform denotes the arrival time of the wave with velocity C. In order to get the arrival times, the maximum of the magnitude of the wavelet transform has to be determined for each frequency scale. That is, the peaks of the magnitude of WT in the time-frequency domain (at 50 khz) are related to the arrival times of group velocity. In this paper, the results were computed based on the WT technique with a Gabor wavelet in the case of thick plate. Figure 6 shows the measured location results of PC-based source location system and conventional AE system. As shown in Fig. 6, the new source location results well agree with the conventional AE system. The arrival times of each frequency component needed in the velocity calculation could be determined from the peak of the magnitude of WT data on the time-frequency plane. Conclusions This research is focused on development of an advanced source location system based on acoustic emission that can provide timely detection of the source location such as cracks and leaks. The 4-channel PC-based source location system was developed to detect leak positions more accurately at faster speed with less cost. In addition, the evaluation of several commercially available source location methods, in particular, those based on timing methods was performed to improve the finding of source locations. The results of new source location were well agreed with the conventional acoustic emission system. In addition, it was confirmed that the wavelet transform using the Gabor wavelet was effective tool to determine source location. That is, arrival times of each frequency component needed in the velocity calculation could be determined from the peak of the magnitude of wavelet transform data on the time-frequency plane. Wavelet transform also may provide source location information, particularly at thin plate, where the velocity may be relatively dispersive. Acknowledgement This work was supported by the Safety and Structural Integrity Research Center (SAFE) under Sungkyunkwan University, Korea, and was also partially supported by the Brain Busan Project in 00 References [] Ronnie K. Miller and Paul Mclntire: Acoustic Emission Nondestructive Handbook, Vol. 5, American Society for Nondestructive Testing (987). [] J. von Stebut and F. Lapostolle: Surface and Coatings Technology (999). [] Noiret D and Roget J: Journal of Comp. Mater., Vol (989), p

7 Key Engineering Materials Vols [4] Habeger CC and Mann RW: Ultrasonics, Vol. 7 (979), p [5] Ziola SM and Gorman MR: Journal of Acoustic Soc Amer, Vol. 90 (99), p [6] Kishimoto K. and Inoue H: Jonrnal of Appl Mech, Vol. 6 (995), p

8 Advances in Nondestructive Evaluation 0.408/ Development of PC-Based Leak Detection System Using Acoustic Emission Technique 0.408/ DOI References [] Noiret D and Roget J: Journal of Comp. Mater., Vol (989), p Title of Publication (to be inserted by the publisher) 0.77/