Leakage detection in hydraulic and pneumatic systems through infrared thermography and CO2 as tracer gas

Transcription

1 Leakage detection in hydraulic and pneumatic systems through infrared thermography and CO2 as tracer gas Borja Rodríguez 1, Rosario Fernández 1, Mario Lahera 2, Fernando Lasagni 1 1 Materials & Processes division, CATEC - Advanced Center for Aerospace Technologies, C/Wilbur y Orville Wright 19, E La Rinconada (Seville), Spain, e- mail: rfernandez@catec.aero; flasagni@catec.aero 2 Pneumatic, Fuel & Power Plant System, AIRBUS DEFENCE & SPACE, San Pablo Sur Ctra. A-8010, km 4, E Seville, Spain, Mario.Lahera@airbus.com Abstract Every aircraft is plenty of fluid systems, both hydraulic and pneumatic. These systems are necessary for tasks such as fuel distribution and storage, mechanical actuators and carrying air to cabin interior between others. Manufacturing and assembly of these systems require the performance of leak testing for quality assurance. Traditionally, leakage detection testing is performed through system pressure decay monitoring once the system is filled with an inert gas or working fluid. If a pressure decrease is detected, leakages must be located usually by the application of soapy liquid in each junction point of system, which is a time-consuming task. In this work, new advances in leakage detection are presented by active infrared thermography and using carbon dioxide (CO 2 ) as tracer gas. The application of postprocessing algorithms, developed within this work, allows the automatic detection of the leakages, as well as a qualitative approach for its sizing. These algorithms have been tested on different scenarios that simulate aircraft systems in an industrial environment. 1. Introduction Infrared Thermography (IRT) is a Non-Destructive Technique (NDT) which enables the measurement and visualization of the temperature of object surfaces in an accurate and contactless way. The IRT provides thermal images of the infrared emission (with wavelength between 2-13 µm) of a body according to its thermal condition. Leakages localization requires the introduction of a trace gas inside of the analysed installation, like CO 2 or SF 6. Heteronuclear molecules have different vibration modes that provoke the absorption, emission or scattering of electromagnetic energy at a certain wavelength. In the case of CO 2, one of the characteristic wavelengths is 4.3 µm (1, 2). The vibration mode of CO 2 can be visualized by means of medium-wave infrared (MWIR) cameras (3-5 µm), using a spectral filter which delimits the signal around 4.3 µm (3). During the inspection it is necessary the thermal excitation of the background which provides energy to the leaked trace gas molecules (1). Creative Commons CC-BY-NC licence

2 2. Experimental method Inspections have been performed using the infrared camera FLIR SC7000, whose characteristics are: mid-wave Indium Antimonide (InSb) sensor, spectral response 3-5 µm or 8-12 µm, resolution 640 x 512 pixels, lens 5 mm F/2 22o x 17o and thermal sensitivity < 18 mk. The camera has a spectral CO 2 filter (4.25 µm) which enables the detection of the trace gas. The camera is connected with the Altair software (developed by FLIR). The acquisition is done by means of it. Trace gas is provided by a CO 2 bottle with a 100% concentration in percent volume. The background is excited using a quartz lamp (1). The measurement setup is presented in Figure 1. Figure 1. Setup of the leakage detection tests. The analysis of the thermographic sequences is carried out using software DETEC CO 2 developed by CATEC, which includes the post-processing algorithms used for the automatic analysis of infrared images, determination of leakages position and assessment of main leak parameters. Three different post-processing algorithms are available: static normalisation, dynamic normalisation and temporary derivative (3). 3. Leakage detection in aircraft systems Results obtained from leakages detection tests for two different devices are presented following: fuel tank specimen and general purpose specimen. These elements are representative from real aircraft systems. For each test, three thermographic sequences are presented in parallel: (i) the infrared sequence extracted from the inspection without any post-processing treatment, (ii) the post-processed sequence including the segmentation of CO 2 areas coming from the leakage points and (iii) the binarized sequence, in which leakages regions are highlighted in red color (3). The need for post-processing methods is demonstrated through the analysis of the raw sequence, where the leakage locations are identified with 2

3 difficulty only in cases of large leak size. The results presented below correspond to specific instants of the thermographic sequences, in which the better visualization of the leakage is achieved. 3.1 Fuel Tank Specimen The tested specimen is an element manufactured ad-hoc for this application (test tube), made of the characteristic material of the fuel tanks (Figure 2 a). Several artificial defects have been introduced to the sample, which will produce leakages during the tests. Figure 2. (a) Fuel tank specimen and test results (b) thermographic image, (c) segmented image and (d) binarized image. The results obtained from the inspections are shown in Figure 2 b-d, for an inner pressure of 0.15 bar. The last is a critical parameter for leakage detection, being necessary an ad-hoc analysis for each specific problem. The validity of post-processing algorithms is verified through these results, where leakages locations cannot be identified from the raw thermographic images. At these images, false positives caused by spurious reflections have been automatically removed, by means of the postprocessing algorithms. 3

4 3.2 General Purpose Specimen The second device consists of a manifold made of steel AISI-316 with three calibrated orifices, whose main purpose is to transfer fluids. The diameter of calibrated orifices is 25.4 µm (orifice 07), 12.7 µm (orifice 06) and 7.62 µm (orifice 05). The inner pressure was set at 4 bar. From Figure 3 a-c it can be observed the possibility of identifying three simultaneous leaks. Figure 3. Test results (a) thermographic image, (b) segmented image, (c) binarized image and (d) centroids of segmented areas leaking through the orifices (Areas in pixels) Figure 3 d presents the centroid of every segmented area, plotting the X and Y coordinates (in pixels). The centroids appear in three groups, corresponding each group to one orifice. The areas have been divided in three categories depending on their sizes: blue if the area is lower than 500 pixels, green if the area is between 500 and 1000 pixels and red if the area is greater than 1000 pixels. This representation enables to obtain a qualitative and quantitative estimation about the leakage size: it can be notice how the number of centroids is increased and the size of areas grows as well (the number of red circles is increased). 4. Conclusions Leakage detection with Infrared Thermography using CO 2 as trace gas has demonstrated to show a great potential for its application on aircraft systems. This procedure consists on a non destructive and fast technique, which provides in an 4

5 accurate way the leak position. For this, the background has to be thermally excited and post-processing activities have to be applied to the infrared sequence. In one shot, large areas can be inspected, without using any pollutant liquid or gas (which reduces cleaning works after test). In addition, an inspection record is obtained from each test. Although this technique is a method for leakage location, it is possible to evaluate the leakage size, analysing the number of CO 2 regions coming from the inspected element in a specific time and measuring their size. References 1. M. Vollmer and K. Möllmann, IR imaging of CO2 : basics, experiments, and potential industrial application, Proceedings IRS², M. Volker and K. Möllmann, IR imaging of gases: potential applications for CO2 cameras, Inframation, Vol 10, pp , B. Rodríguez, M. L. Santamaría, C. Galleguillos, A. Bollo, F. Lozano, L. Girela, and F. Lasagni, Detección de fugas en sistemas hidráulicos y neumáticos por termografía infrarroja y llenado de CO 2, Proceedings 13er Congreso Nacional de Ensayos No Destructivos, pp. 1 7,