Non-Destructive Examination of Underground Pressure Vessels Using Acoustic Emission (AE) Techniques

European Mechanical Science 2017, Vol. 1(1): 1-8 Original Paper EUROPEAN MECHANICAL SCIENCE Non-Destructive examination of underground pressure vessels using acoustic emission (AE) techniques§ Deniz Karaduman1, Durmuş Ali Bircan2, Ahmet Çetin3 Çukurova University, Department of Mechanical Engineering, 01330 Balcalı, Adana, TURKEY 1,2,3 Abstract The methodology of Acoustic Emission (AE) for detecting and monitoring damages, cracks and leaks in different structures is widely used and has earned a reputation recently as one of the most reliable and wellestablished technique in Non-Destructive Testing (NDT). Besides evaluation of fracture behavior, crack propagation and fatigue detection in metals, composites, wood, fiberglass, ceramics and plastics; it can also be used for detecting faults and pressure leaks in pressure vessels, tanks and pipes. As a relatively “clean” form of energy, Liquefied Petroleum Gas (LPG) is widely used for industrial applications and domestic heating. Periodic inspection of buried tanks used for LPG storage is complicated and limited because of their underground location. This situation prevents “conventional” NDT techniques from being used. So, AE testing which fulfills all safety requirements, is the most appropriate and cost-effective technique that can be used for periodic inspection and proof testing. In addition of a general presentation on the AE technology and its applications, this study provides comprehensive evaluation of AE testing techniques of underground LPG tanks during service in accordance with TS EN standards. Some representative results and data obtained from a performed AE test are also provided. Keywords: Acoustic Emission (AE), Non-Destructive Testing (NDT), Underground LPG Tanks, In-Service Monitoring. 1. INTRODUCTION Acoustic Emission (AE) is defined as a phenomenon, where one or more local sources in materials, which are under stress, are emitting energy and producing temporary elastic waves. AE covers a broad range in material science, construction and process development. The largest events which can be analyzed by AE are seismic occurrences, the smallest are dislocations occurring in metals by load. Between these two, there is a broad range of detailed research work and industrial application [1]. One of those application area is the inspection of buried LPG tanks of industrial plants and some domestic buildings. AE Testing (AET) has become a recognized NDT method commonly used to detect and locate faults in mechanically loaded structures and components. AE can provide comprehensive information on the origination of a discontinuity (flaw) in a stressed component and also provides information pertaining to the development of this flaw as the component is subjected to continuous or repetitive stress [2]. Huge quantities of LPG tanks have been installed in Turkey during the last decade. For the vast majority of those tanks are located under the ground because of the safety considerations. Since inspection is obligated by national legislations of occupational health and safety, after ten years of operation huge numbers of tanks are now set for inspection. Traditional methods of inspection require that the tank is unearthed, which means that they are cumbersome, slow and expensive, i.e. very cost-ineffective [2]. Thus, NDT techniques of AE are preferred for periodic inspection and proof testing of those tanks. This procedure is currently being validated via experimental tests on a large number of LPG tanks by comparing the results with those obtained by conventional NDT techniques. Initial results appear to confirm the effectiveness of the technique and encourage further research in this field [3]. The European Standards TS EN 12817 [4] (comprised of LPG Tanks up to 13 m3), TS EN 12819 [5] (comprised of LPG Tanks greater than 13 m3) and TS EN 14584 [6] allows AE-based techniques to be used in periodic inspection and the requalification of underground LPG tanks. The main objective of this study was to present general information *Corresponding authour Email: (A. Çetin) § This paper was presented at the IMSEC-2016 about AE technology and its applications, to provide comprehensive analysis of AE testing techniques of underground LPG tanks during service and to review the results and the outcoming data obtained from a performed AE inspection of an underground LPG tank belonging to a domestic building with a capacity of 5 m3. 2. OVERWIEV ON AE TESTING TECHNIQUE 2.1 Principles of AE Testing AE testing refers to a technique of testing, recording and analyzing AE signals using apparatus as well as speculating on the status of an AE source as normal or not based on AE signals. The elastic waves sent from the AE source are transmitted to the material surface via a transmission media and converted to electric signals by sensors before being magnified, processed and recorded. Through the analysis and processing of acquired signals, any defects inside the material could be detected [7] as illustrated in Figure 1. Figure 1: Principle of AET. [8] The diagram illustrates various parameter defined below: • Count: The number of times a peak in the wave lies above a set threshold frequency. • Hit/Event: A collective term for a group of AE counts that lie above the threshold amplitude. A hit is also defined as a signal that triggers the system channel to accumulate data. • Rise time: The time between a wave triggering above the threshold amplitude and the time of the peak amplitude of that wave. The rise time is related to the source-time function and can describe the type of fracture or eliminate noise signals. • Duration: The time between an AE waveform triggering above the threshold and its disappearance below that threshold. The duration is related to the source magnitude and noise filtering. • Amplitude: The peak voltage of a waveform. It is closely related to the magnitude of the source event [8]. • MARSE (Measured Area Under the Rectified Signal Envelope): It is derived from the rectified voltage signal over the duration of the AE waveform with voltage-time units and it is strongly sensitive to amplitude and duration [9]. The technology involves the use of ultrasonic sensors (20 Khz-1 Mhz) that listen for the sounds of material and structural failure. AE frequencies are usually in the range of 150–300 kHz, which is above the frequency of audible sound. Crack growth due to hydrogen embrittlement, fatigue, stress corrosion, and creep can be detected and located with the use of this technology. High-pressure leaks can also be detected and isolated [2]. When considering detecting an AE waveform, one must decide on the type of sensor, pre-amplification and band-pass filters. Typical sensors used in AET are piezoelectric in nature, which convert mechanical strain of the piezo element into an electric signal [10]. Another important consideration is how to attach the sensor to the material as well as the location of multi (...truncated)