- Email: [email protected]
Leakage quantification of compressed air using ultrasound and infrared thermography
Measurement 45 (2012) 1689–1694
Contents lists available at SciVerse ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Leakage quantification of compressed air using ultrasound and infrared thermography Slobodan Dudic´ a, Ivana Ignjatovic´ a,⇑, Dragan Šešlija a, Vladislav Blagojevic´ b, Miodrag Stojiljkovic´ b a b
University of Novi Sad, Faculty of Technical Sciences, Department of Industrial Engineering and Management, Trg Dositeja Obradovic´a 6, 21000 Novi Sad, Serbia University of Niš, Mechanical Faculty, Aleksandra Medvedeva 14, 18000 Niš, Serbia
a r t i c l e
i n f o
Article history: Received 20 January 2012 Received in revised form 24 March 2012 Accepted 25 April 2012 Available online 8 May 2012 Keywords: Non-destructive testing IR thermography Ultrasound Leak quantification Compressed air
a b s t r a c t With the leakage elimination in compressed air systems, it is possible to save up to 40% of energy. With appropriate inspection and maintenance of compressed air systems, leakage elimination should be a routine. This paper describes and compares two different noncontact methods for compressed air leakage quantification, ultrasound and infrared thermography. The potentials and limitations of these technologies are analyzed, as well as the reliability and accuracy of results thus obtained. From the results presented in this paper, it can be concluded that thermography offers good results for the leakage quantification from the orifices greater than 1.0 mm and ultrasound should be used for leakage detection for all the dimensions of orifices, but for the quantification purposes only for smaller leaks. As a support for leakage quantification, we proposed diagrams of a leak flow as a function of sound level and as a function of detected temperature change. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Beside electric energy, compressed air is nowadays the most universal energy medium used by a number of industries. Despite all its advantages, compressed air is an expensive energy resource. Energy costs contribute to 75% of the total costs for compressed air production. In the EU, compressed air production contributes to 10% of the total electric energy consumption in industry [1], in some industrial branches, such as the glass industry, even 30%. A significant share of compressed air is lost on various accounts. Compressed air losses constitute 25–30% of the total compressed air requirements. However, there are particular systems in which this percent amounting from 30% up to even 60% [2]. Leaks are the most visible and most significant contributors to compressed air losses. Leakage ⇑ Corresponding author. Tel.: +381 21 4852127; fax: +381 21 459536. E-mail addresses: [email protected] (S. Dudic´), ivanai@ uns.ac.rs (I. Ignjatovic´), [email protected] (D. Šešlija), vlada@masfak. ni.ac.rs (V. Blagojevic´), [email protected] (M. Stojiljkovic´). 0263-2241/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.measurement.2012.04.019
rate varies between 20% and 40% of the total air usage [3]. Beside economic effect, the reduction of compressed air losses is significant for the environment. The reduction of losses also reduces demands for production, which in itself reduces emission of CO2 and other harmful substances into atmosphere. In order to reduce leaks and increase energy efficiency it is necessary to detect leaking spots and remove causes of leaks. Clear idea that stands behind this paper is a role of ultrasound and infrared technology for compressed air leak detection and quantification. Due to their small size, leaks are often hard to locate by other methods, which necessitate the use of ultrasound, or infrared technology, depending on the certain situation. A combination of passive ultrasound method and infrared thermography allows detection of very small leaks. Leakage quantification is possible with several wellknown methods [4], but they cannot give the allocation of leakage on the distribution net. This paper compares mentioned methods, their potential and limitations of individual and combined application
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of ultrasound and IR technology on estimation of losses due to compressed air leaks. 2. Detection and quantification of compressed air leaks The problem of air and fluid leaks has been in the focus of scientific literature for years. Numerous authors have dealt with the leakage of technical gases (propylene, ethylene, etc.), natural gas, oil, water, and other fluids [5]. There is a relative scarcity of papers dealing with the detection of compressed air leaks. However, due to similar physical and exploitation properties of gases, the problem of compressed air leak detection can be dealt with equivalent to other technical gases. According to [6–8], all methods of leak detection for gases and liquids can be classified as hardware methods, biological or software methods. All this methods have their specific traits that define their limitations and area of application. Comparison and analysis of these methods and their potential application for leak detection was given by [9], based on seven criteria, but several methods are most popular in the practical detection of compressed air leaks [10]. In many cases, leaks could be detected quite easily. Heavy leaks are easy to hear. However, smaller leaks are harder to detect, and cannot be easily heard. In some cases, a single method is sufficient to detect leaks, while in other cases a combination of methods have to be used in order to detect their location. Once the magnitude of losses is established, the total value of compressed air losses could be calculated based on price of compressed air production per 1 m3. There exist various methods of compressed air leak quantification [4]. They are based, mostly, on the measurement of compressor operation time, although flow gauges, ultrasound detectors, and IR thermography are also gaining popularity [11]. 3. Theoretical background 3.1. Ultrasound technology Ultrasound is the sound with the frequency above the upper limit of human hearing. The audible frequency range in humans goes from 10 Hz to approximately 20 kHz. Ultrasound technology utilizes sound waves that are beyond human perception, and ranges between 20 kHz and 100 kHz. Ultrasound testing methods belong to the group of nondestructive testing methods (NDT). Currently, the ultrasound testing method is most popular in medicine. Noninvasive ultrasound diagnostics is mostly used for examination of internal organs [12]. Exposure of cells to ultrasound does not have harmful effects, although the ultrasound waves penetrate relatively deep into human body. Beside medicine, ultrasound has also been widely used in industrial applications. It has been increasingly used in the processing, chemical, petrochemical, food, metal industry, as well as in civil engineering and architecture. It is most often used for measurement of physical quantities, such as fluid flow, fluid level, material thickness,
length, surface area, volume, speed, etc. In addition, it is frequently used for ultrasound cleaning, welding, process automation (ultrasound sensors) [13], analysis of material structure [14], detection of defective machine components and subassemblies, inspection of electrical installations and equipment, detection of foreign bodies in food [15], detection of fluid leaks [16], etc. Wide spectrum of sounds, ranging from audible to inaudible frequencies, also as ultrasound (also familiar as the white noise) is mostly generated by cavitation or turbulence of air molecules under the pressure, which flow out into the atmosphere through orifices, cracks, and seams. The application of ultrasound on leak detection shall be in the focus of the remaining part of this paper. According to [17], ultrasound leak detection can be classified on active, passive, and vibroacoustic. In this research, the passive ultrasound detection was used for quantification of compressed air leak. 3.2. Infrared thermography Infrared technology uses IR thermovision cameras to display and measure thermal energy radiated by an object. IR thermovision cameras generate images of infrared or thermal emission, allowing very accurate non-contact temperature measurement. In almost all compressed air systems, the occurrence of malfunction or air leak is accompanied by temperature change. Application of IR thermovision cameras to diagnose such changes can bring substantial savings. The goal of an infrared investigation is to transmit a packet of energy towards an object under examination, and observe its response to thermal excitation – the development in time of surface temperature distribution. Subsequent analysis can reveal the material structure beneath the surface, possible inclusions, cracks, or processes that occur beneath the surface [18]. The principles of infrared thermography are well presented in [19]. Infrared thermography is also used in agriculture, civil engineering, and architecture [20,21], electric power industry [22], automotive industry [23], medicine [24], manufacturing industry [25], environment protection, and protection of historic heritage [26,27,19], while it is also increasingly used for fluid leak detection [28,29]. The number of scientific papers that deal with the problem of thermographic detection and quantification of compressed air leaks is scanty. Kroll et al. [30] report on a novel method for automatic detection of compressed air leaks using software tools for thermogram analysis and pattern recognition. Lewis et al. [31] give a concise presentation of available methods for gas leak detection in landfills, with emphasis on IR thermography. They conclude that this method can be used exclusively for screening, and not for precise gas leak detection or mathematical modeling of gas emission in landfills. 4. Experimental investigations In order to establish prerequisites for compressed air leak quantification using ultrasound and IR technology,
S. Dudic´ et al. / Measurement 45 (2012) 1689–1694
the intensity of sound generated by air leak from a pneumatic hose was measured, as well as the change of air temperature at the orifice through which the air was released. These parameters were measured in laboratory conditions on a punctured 8 mm flexible hose, PUN 8 1.25 mm. Punctured orifices were of following diameters: 0.5, 0.7, 1.0, 1.3, 1.5, and 2.0 mm. In order to establish the real quantity of air that spout through the orifices, parallel to noise level, and temperature change, compressed air flow was measured at each of the orifices. All parameters (noise level, air temperature, and airflow) were measured at each orifice, for pressures of: 4, 5, 6, 7, and 8 bar. After the measurement, the obtained levels of noise and temperature change were compared to airflow in order to establish a correlation. The following section presents in detail the measurement procedure, related limitations and the results. 4.1. Flow measurement In these experiments, the flow was measured by FESTO Air Box portable device [32]. Air Box allows measurement of pressure, temperature, and flow, and enables user to perform air quality tests to establish humidity and oil content. Compressed air flows past a surface that is continuously heated. The flowing air absorbs heat energy from the warm surface. A thermal sensor quantifies the variation in temperature, which represents a specific airflow. The measuring frequency is 100 Hz, with accuracy ±3% and reproducibility ±0.3% for low flow sensor; and ±3% accuracy and ±0.8% reproducibility for high flow sensor. The obtained results indicate that compressed air flow gets higher with the increase of orifice diameter, and system pressure. For the purpose of experiment, measurement flow range was kept within the low range (10–200 l/min). The results obtained by measurement of flow at air exit locations on the punctured flexible pneumatic hose are given in Table 1. 4.2. Measurement of noise level Intensity of sound exiting the flexible pneumatic hose was measured by an ultrasound detector, Ultraprobe 100 [33,34], which accurately detects leaks and mechanical damage. The frequency response is 36–44 kHz and the response time 300 ms. Due to elasticity of material, the punctured orifices were of irregular shape, with rough walls. Hence, the
Table 1 Compressed air leakage (l/min) through orifices on a flexible pneumatic hose, as a function of pressure and orifice diameter. Pressure (bar)
Orifice diameter (mm) 0.5
4 5 6 7 8
2.1 2.6 3.2 3.7 4.2
0.7 4.3 5.3 6.5 7.9 8.8
1.0 8.4 10.7 12.3 14.3 15.6
1.3 18.7 23.3 29.0 34.8 40.0
1.5 39.9 37.0 45.1 51.9 56.2
intensity of ultrasound generated by flowing air was several times higher than it would have been if the orifices were round-shaped and smooth-walled. With the increase of pressure, airflow also rises, and the signal measured by the ultrasound detector becomes stronger. Noise level was measured at the loudest point of leak. Sound waves may need to be manipulated to a specific direction or angle to improve detectability [35]. Due to variable orifice shapes, rather than being perpendicular to orifice, the loudest point is most frequently at an angle of 30° [4]. The results of measurement of noise level generated by compressed air leak from the flexible pneumatic hose are shown in Table 2. Compressed air leaks are expressed as sound levels (dB), at various pressures. Due to relative imprecision of applied measuring instrument, the obtained values should only be used for orientation purpose in practice. As the orifices on the hose increase in size, the sound level changes at a slower rate with the change of pressure. The most prominent changes in sound level occur at orifices 0.5 mm in diameter, while at orifice diameters of 1.3 mm and larger, sound level becomes constant. Thus, it is not recommended to use sound level measurements to differentiate between orifice sizes above 1.5 mm. Obtained results match with the results reported in [4], where the experiments were performed for orifice diameters from 0.2 to 1.0 mm. Application of this method is recommended only for smaller orifices (0.5; 0.7 and 1.0 mm, and partially 1.3 mm) where it is possible to use noise level to clearly distinguish between the orifice sizes, and assess the related air loss. 4.3. Temperature measurement The measurement of temperature difference at the location of compressed air leak was performed by Fluke Ti20 camera with spectral range 7.5–14 lm, thermal sensitivity 200 mK, accuracy ±2 °C or 2% (whichever is greater) and repeatability ±1% or ±1 °C (±2 °F) whichever is greater. The field of view is rectangular and covers horizontal 20° and 15° vertical [36]. Due to inability to directly measure the temperature of compressed air, focus was placed on the measurement of temperature difference (DT), using Eq. (1), emitted by an object, in this case, the flexible hose (Tmaterial), and the temperature at the very orifice through which the compressed air is released (Torifice). Measurements were performed at Table 2 Noise level (dB) of compressed air leak from the flexible pneumatic hose, as the function of pressure and orifice diameter. Pressure (bar)
Orifice diameter (mm) 0.5
0.7
1.0
1.3
1.5
2.0
4 5 6 7 8
60 63 66 69 72
66 68 69 71 73
71 72 72 73 74
73 73 73 74 74
74 74 74 74 75
75 75 75 75 75
2.0 58.6 71.0 84.0 98.7 101.0
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Italic values: unclear area for ultrasound quantification of leakage.
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Fig. 1. Measurement location – comparison of photography and thermogram.
Fig. 2. Analysis of temperature profile for the 0.7 mm orifice at 4 bar pressure.
orifices of various diameters, at various pressures, and constant room temperature. Based on the mean value of temperatures measured in particular zones (see Fig. 1), the temperature difference was calculated according to Eq. (2).
DT ¼ T material T orifice
ð1Þ
T material ¼ ðT 1 þ T 2 þ T 3 Þ=3
ð2Þ
where T1 is the temperature of flexible hose before the orifice (°C), T2 is the temperature of flexible hose after the orifice (°C), and T3 is the temperature of flexible hose below the orifice (°C). Fig. 2 presents the analysis of temperature profile of the 0.7 mm orifice, at 4 bar. There is a sudden change of temperature at the orifice location. Analysis of all images leads to conclusion that the increase of pressure in the system causes the drop in temperature of compressed air exiting through the orifice, which, in turn, increases DT. The re-
sults obtained by measurement of temperature differences at orifice locations on the flexible pneumatic hose are shown in Table 3. Analysis of measurement results shows that the temperature change at orifice locations is the function of the orifice size. As the size of orifice increases, regardless of pressure, there is a visible drop in temperature at air exit points. Table 3 Temperature change DT (°C) as a function of pressure and orifice diameter on the flexible pneumatic hose. Pressure (bar)
4 5 6 7 8
Orifice diameter (mm) 0.5
0.7
1.0
1.3
1.5
2.0
0.5 0.6 0.8 0.9 1.0
1.4 1.5 1.6 1.7 1.9
2.2 2.7 3.0 3.2 3.3
2.9 3.4 3.7 4.1 4.6
3.6 4.1 4.4 5.1 5.5
4.3 4.7 4.9 5.4 5.9
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Fig. 3. Leak flow as the function of detected ultrasound level.
Fig. 4. Leak flow as the function of detected temperature change DT (°C).
5. Discussion
6. Conclusion
In this investigation, we have started from the known values of leak diameters and appropriate leak flow. Nevertheless, in reality, maintenance personal is facing the problem of unknown leak area and, adequately, unknown leak flow that they should, in some way, quantify. Based on these experiments, leak flow can be found as the function of measured ultrasound or temperature change. In Fig. 3 is given the diagram with the fitted curve of leak flow as the function of detected ultrasound level for each examined pressure. It can be noticed that these dependence is very strong for small leak diameters, especially in area of up to 1.0 mm leak diameters. When leak diameters are from 1.0 to 1.3 mm, correlation is weakening and with diameters of 1.5 mm and above, it becomes almost useless. However, from the other hand, there are significant changes in leak flow (e.g. more than 80%, at 6 bar pressure) between 1.5 mm orifice and 2.0 mm orifice and that change could hardly be identified with ultrasound detector. However, IR technology can clearly identify this change of leak flow due to the increase in leak diameter what can be seen in Fig. 4. From the presented results, it can be concluded that ultrasound should be used for leakage detection for all the dimensions of orifices, but for the quantification purposes only for smaller leaks, precisely, for the leaking with the sound level up to the 74 dB. If the sound level is higher than 74 dB, IR camera should be used for leakage quantification and it will give a reliable estimation of leakage. Therefore, quantification of bigger losses due to the leakage through the orifices 1.3–2.0 mm is not able with ultrasound detector but it is steel possible with the IR camera.
This paper shows the possibility for quantifying losses due to leakage using IR thermography. While for the ultrasound methods are already published some results concerning leakage quantifications, we have shown the comparison of those two methods. Nevertheless relatively imprecise measurement equipment used in our experiments, we have obtained clear view of areas in which each of those methods are appropriate. Besides, availability of ultrasound and infrared equipment in bigger factories is increasing, so with procedures defined in this paper the efficiency of leakage quantification can be significantly improved. Numerous factors affect the flow, the size, shape, and configuration of leak orifice, temperature and humidity of expelled air, etc. Ultrasound method has the limitation by the background noise, which is generated within the system and its environment. Advantage of the ultrasound leak detection lies in its versatility, speed of detection, ease of use, ability to perform on shop floor, and ability to detect various types of leaks. Scanning of the tested area reveals leak locations quickly and easily. Applying infrared thermography for the leakage quantification purposes requires a thermal contrast at leak locations. Those contrasts are imperceptible for small leaks, so infrared technology is not an optimal choice in those situations, but it offers good results for the leakages from the orifices greater than 1.0 mm. However, it requires well-differentiated temperatures, and is hampered by sources with weak radiation, and conditions of extreme lighting. Developed approach of leakage quantification in this paper can be applied concerning leakage of other fluids, e.g. LPG in pipelines and technical gasses (oxygen,
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nitrogen, etc.) in some factories. For that purposes, in order to get valid results, it is necessary to make measurements with appropriate measuring equipment, based on procedures defined in this paper.
References [1] P. Radgen, E. Blaustein, Compressed Air Systems in the European Union, Energy, Emissions, Saving Potentials and Policy Actions, LOGX Verlag GmbH, Stuttgart, 2001. [2] Office of Energy Efficiency and Renewable Energy, US Department of Energy, Assessment of the Market for Compressed Air Efficiency Services, Burlington, Xenergy Inc., 2001. [3] D. Šešlija, I. Ignjatovic´, S. Dudic´, B. Lagod, Potential energy savings in compressed air systems in Serbia, Afr. J. Bus. Manag. 5 (14) (2011) 5637–5645. [4] SDT Ultrasound Solution, Compressed Air-Leak Surveyor Handbook, third ed., SDT North America Ltd., Cobourg, 2009. [5] R.S. Asatryan, S.R. Asatryan, L.A. Sukoyan, N.R. Khachatryan, A spectroradiometer for remote ecological testing of gas main pipelines, Russ. J. Nondestr. Test. 46 (8) (2010) 598–602. [6] J.R. Bose, M.K. Olson, TAPS’s leak detection seeks greater precision, Oil Gas J. 91 (14) (1993) 43–47. [7] B.N. Carlson, Selection and use of pipeline leak detection methods for liability management into the 21st century, Pipeline Infrastructure II, in: Proceedings of the International Conference of the American Society of Chemical Engineers ASCE, 1993, pp. 369–383. [8] N.C. Turner, Hardware and software techniques for pipeline integrity and leak detection monitoring, Proc. Offshore Eur. 91 (1991) 139– 149. [9] J. Zhang, Designing a cost effective and reliable pipeline leak detection system, in: Proceedings of Pipeline Reliability Conference, 1996. [10] A.Y. Rozinov, Physical evaluation of the efficiency of applying liquid indicators during leakproofness inspection, Russ. J. Nondestr. Test. 42 (8) (2006) 551–557. [11] A.E. Rozlosnik, Infrared thermography and ultrasound both test analyzing valves, Proc. Termosense 3361 (1998) 137–175. [12] N.D. Jong, P.J. Frinking, A. Bouakaz, F.J. Ten Cate, Detection procedures of ultrasound contrast agents, Ultrasonics 38 (1–8) (2000) 87–92. [13] A. Püttmer, New applications for ultrasonic sensors in process industries, Ultrasonics 44 (1) (2006) e1379–e1383. [14] J. Zhang, B.W. Drinkwater, P.D. Wilcox, A.J. Hunter, Defect detection using ultrasonic arrays: the multi-mode total focusing method, NDT&E Int. 43 (4) (2010) 123–133. [15] E. Hæggström, M. Luukkala, Ultrasound detection and identification of foreign bodies in food products, Food Control 12 (1) (2001) 37–45. [16] S. Shuh-Haw, C. Hual-Te, C.R. Apostolos, Ultrasonic techniques for detecting helium leaks, Sens. Actuat. B: Chem. 71 (3) (2001) 197– 202.
[17] C. Moon, W.C. Brown, S. Mellen, E. Frenz, D.J. Pickering, Ultrasound techniques for leak detection, in: Proceedings of SAE 2009 Noise and Vibration Conference and Exhibition Ultrasound Techniques for Leak Detection, 2009. [18] T.J. Kulp, P.E. Powers, R.B. Kennedy, Remote imaging of controlled gas releases using active and passive infrared imaging systems, Proc. Infrared Technol. Appl. XXIII 3061 (1997) 269–278. [19] C. Meola, G.M. Carlomagno, Recent advances in the use of infrared thermography, Meas. Sci. Technol. 15 (9) (2004) R27–R58. [20] C.A. Balaras, A.A. Argiriou, Infrared thermography for building diagnostics, Energy Build. 34 (2) (2002) 171–183. [21] A. Moropoulou, N.P. Avdelidis, M. Koui, I. Tzevelekos, Determination of emissivity for building materials using infrared thermography, J. Thermol. Int. 10 (3) (2000) 115–118. [22] A.R. Al-Kassir, J. Fernandez, F.V. Tinaut, F. Castro, Thermographic study of energetic installations, Appl. Therm. Eng. 25 (2–3) (2005) 183–190. [23] M.Ö. Korukçu, M. Kilic, The usage of IR thermography for the temperature measurements inside an automobile cabin, Int. Commun. Heat Mass Transf. 36 (8) (2009) 872–877. [24] E.F.J. Ring, K. Ammer, The technique of infrared imaging in medicine, Therm. Int. 10 (1) (2000) 7–14. [25] C. Meola, A new approach for estimation of defects detection with infrared thermography, Mater. Lett. 61 (3) (2007) 747–750. [26] G.M. Carlomagno, C. Meola, Comparison between thermographic techniques for frescoes NDT, NDT&E Int. 35 (8) (2002) 559–565. [27] A. Tavukcuoglu, A. Duzgunes, E.N. Caner-Saltik, S. Demirci, Use of IR thermography for the assessment of surface-water drainage problems in a historical building, Ag˘zıkarahan (Aksaray), Turkey, NDT&E Int. 38 (5) (2005) 402–410. [28] E. Brodetsky, M. Savic, Leak monitoring system for gas pipelines, in: Proceedings of IEEE International Conference Acoustics, Speech, and Signal Processing, vol. 3, 1993, pp. 17–20. [29] G.J. Weil, Non contact, remote sensing of buried water pipeline leaks using infrared thermography, in: Proceedings of Water Resources Planning and Management and Urban, Water Resources, 1993, pp. 404–407. [30] A. Kroll, W. Baetz, D. Peretzki. On autonomous detection of pressured air and gas leaks using passive IR-thermography for mobile robot application, in: Proceedings of IEEE International Conference on Robotics and Automation, 2009, pp. 291–296. [31] A.W. Lewis, S.T.S. Yuen, A.J.R. Smith, Detection of gas leakage from landfills using infrared thermography – applicability and limitations, Waste Manag. Res. 21 (5) (2003) 436–447. [32] FESTO, Air Box Type GHDA-FQ-M-FDMJ-A – Operating Instructions, Festo AG&Co. KG. Esslingen, 2006. [33] UE Systems Inc., Ultraprobe 100 – User Manual, Elmsford, 2005. [34] UE Systems Inc., Compressed Air, Ultrasonic Leak Detection Guide, Elmsford, 2005. [35] S. Kleven, M. Blair, Limitations on the detection of casting discontinuities using ultrasonic and radiography, Mater. Eval. 10 (2003). [36] Fluke, Ti20 Thermal Imager – Users Manual, Everett; 2006.
Contents lists available at SciVerse ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Leakage quantification of compressed air using ultrasound and infrared thermography Slobodan Dudic´ a, Ivana Ignjatovic´ a,⇑, Dragan Šešlija a, Vladislav Blagojevic´ b, Miodrag Stojiljkovic´ b a b
University of Novi Sad, Faculty of Technical Sciences, Department of Industrial Engineering and Management, Trg Dositeja Obradovic´a 6, 21000 Novi Sad, Serbia University of Niš, Mechanical Faculty, Aleksandra Medvedeva 14, 18000 Niš, Serbia
a r t i c l e
i n f o
Article history: Received 20 January 2012 Received in revised form 24 March 2012 Accepted 25 April 2012 Available online 8 May 2012 Keywords: Non-destructive testing IR thermography Ultrasound Leak quantification Compressed air
a b s t r a c t With the leakage elimination in compressed air systems, it is possible to save up to 40% of energy. With appropriate inspection and maintenance of compressed air systems, leakage elimination should be a routine. This paper describes and compares two different noncontact methods for compressed air leakage quantification, ultrasound and infrared thermography. The potentials and limitations of these technologies are analyzed, as well as the reliability and accuracy of results thus obtained. From the results presented in this paper, it can be concluded that thermography offers good results for the leakage quantification from the orifices greater than 1.0 mm and ultrasound should be used for leakage detection for all the dimensions of orifices, but for the quantification purposes only for smaller leaks. As a support for leakage quantification, we proposed diagrams of a leak flow as a function of sound level and as a function of detected temperature change. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Beside electric energy, compressed air is nowadays the most universal energy medium used by a number of industries. Despite all its advantages, compressed air is an expensive energy resource. Energy costs contribute to 75% of the total costs for compressed air production. In the EU, compressed air production contributes to 10% of the total electric energy consumption in industry [1], in some industrial branches, such as the glass industry, even 30%. A significant share of compressed air is lost on various accounts. Compressed air losses constitute 25–30% of the total compressed air requirements. However, there are particular systems in which this percent amounting from 30% up to even 60% [2]. Leaks are the most visible and most significant contributors to compressed air losses. Leakage ⇑ Corresponding author. Tel.: +381 21 4852127; fax: +381 21 459536. E-mail addresses: [email protected] (S. Dudic´), ivanai@ uns.ac.rs (I. Ignjatovic´), [email protected] (D. Šešlija), vlada@masfak. ni.ac.rs (V. Blagojevic´), [email protected] (M. Stojiljkovic´). 0263-2241/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.measurement.2012.04.019
rate varies between 20% and 40% of the total air usage [3]. Beside economic effect, the reduction of compressed air losses is significant for the environment. The reduction of losses also reduces demands for production, which in itself reduces emission of CO2 and other harmful substances into atmosphere. In order to reduce leaks and increase energy efficiency it is necessary to detect leaking spots and remove causes of leaks. Clear idea that stands behind this paper is a role of ultrasound and infrared technology for compressed air leak detection and quantification. Due to their small size, leaks are often hard to locate by other methods, which necessitate the use of ultrasound, or infrared technology, depending on the certain situation. A combination of passive ultrasound method and infrared thermography allows detection of very small leaks. Leakage quantification is possible with several wellknown methods [4], but they cannot give the allocation of leakage on the distribution net. This paper compares mentioned methods, their potential and limitations of individual and combined application
1690
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of ultrasound and IR technology on estimation of losses due to compressed air leaks. 2. Detection and quantification of compressed air leaks The problem of air and fluid leaks has been in the focus of scientific literature for years. Numerous authors have dealt with the leakage of technical gases (propylene, ethylene, etc.), natural gas, oil, water, and other fluids [5]. There is a relative scarcity of papers dealing with the detection of compressed air leaks. However, due to similar physical and exploitation properties of gases, the problem of compressed air leak detection can be dealt with equivalent to other technical gases. According to [6–8], all methods of leak detection for gases and liquids can be classified as hardware methods, biological or software methods. All this methods have their specific traits that define their limitations and area of application. Comparison and analysis of these methods and their potential application for leak detection was given by [9], based on seven criteria, but several methods are most popular in the practical detection of compressed air leaks [10]. In many cases, leaks could be detected quite easily. Heavy leaks are easy to hear. However, smaller leaks are harder to detect, and cannot be easily heard. In some cases, a single method is sufficient to detect leaks, while in other cases a combination of methods have to be used in order to detect their location. Once the magnitude of losses is established, the total value of compressed air losses could be calculated based on price of compressed air production per 1 m3. There exist various methods of compressed air leak quantification [4]. They are based, mostly, on the measurement of compressor operation time, although flow gauges, ultrasound detectors, and IR thermography are also gaining popularity [11]. 3. Theoretical background 3.1. Ultrasound technology Ultrasound is the sound with the frequency above the upper limit of human hearing. The audible frequency range in humans goes from 10 Hz to approximately 20 kHz. Ultrasound technology utilizes sound waves that are beyond human perception, and ranges between 20 kHz and 100 kHz. Ultrasound testing methods belong to the group of nondestructive testing methods (NDT). Currently, the ultrasound testing method is most popular in medicine. Noninvasive ultrasound diagnostics is mostly used for examination of internal organs [12]. Exposure of cells to ultrasound does not have harmful effects, although the ultrasound waves penetrate relatively deep into human body. Beside medicine, ultrasound has also been widely used in industrial applications. It has been increasingly used in the processing, chemical, petrochemical, food, metal industry, as well as in civil engineering and architecture. It is most often used for measurement of physical quantities, such as fluid flow, fluid level, material thickness,
length, surface area, volume, speed, etc. In addition, it is frequently used for ultrasound cleaning, welding, process automation (ultrasound sensors) [13], analysis of material structure [14], detection of defective machine components and subassemblies, inspection of electrical installations and equipment, detection of foreign bodies in food [15], detection of fluid leaks [16], etc. Wide spectrum of sounds, ranging from audible to inaudible frequencies, also as ultrasound (also familiar as the white noise) is mostly generated by cavitation or turbulence of air molecules under the pressure, which flow out into the atmosphere through orifices, cracks, and seams. The application of ultrasound on leak detection shall be in the focus of the remaining part of this paper. According to [17], ultrasound leak detection can be classified on active, passive, and vibroacoustic. In this research, the passive ultrasound detection was used for quantification of compressed air leak. 3.2. Infrared thermography Infrared technology uses IR thermovision cameras to display and measure thermal energy radiated by an object. IR thermovision cameras generate images of infrared or thermal emission, allowing very accurate non-contact temperature measurement. In almost all compressed air systems, the occurrence of malfunction or air leak is accompanied by temperature change. Application of IR thermovision cameras to diagnose such changes can bring substantial savings. The goal of an infrared investigation is to transmit a packet of energy towards an object under examination, and observe its response to thermal excitation – the development in time of surface temperature distribution. Subsequent analysis can reveal the material structure beneath the surface, possible inclusions, cracks, or processes that occur beneath the surface [18]. The principles of infrared thermography are well presented in [19]. Infrared thermography is also used in agriculture, civil engineering, and architecture [20,21], electric power industry [22], automotive industry [23], medicine [24], manufacturing industry [25], environment protection, and protection of historic heritage [26,27,19], while it is also increasingly used for fluid leak detection [28,29]. The number of scientific papers that deal with the problem of thermographic detection and quantification of compressed air leaks is scanty. Kroll et al. [30] report on a novel method for automatic detection of compressed air leaks using software tools for thermogram analysis and pattern recognition. Lewis et al. [31] give a concise presentation of available methods for gas leak detection in landfills, with emphasis on IR thermography. They conclude that this method can be used exclusively for screening, and not for precise gas leak detection or mathematical modeling of gas emission in landfills. 4. Experimental investigations In order to establish prerequisites for compressed air leak quantification using ultrasound and IR technology,
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the intensity of sound generated by air leak from a pneumatic hose was measured, as well as the change of air temperature at the orifice through which the air was released. These parameters were measured in laboratory conditions on a punctured 8 mm flexible hose, PUN 8 1.25 mm. Punctured orifices were of following diameters: 0.5, 0.7, 1.0, 1.3, 1.5, and 2.0 mm. In order to establish the real quantity of air that spout through the orifices, parallel to noise level, and temperature change, compressed air flow was measured at each of the orifices. All parameters (noise level, air temperature, and airflow) were measured at each orifice, for pressures of: 4, 5, 6, 7, and 8 bar. After the measurement, the obtained levels of noise and temperature change were compared to airflow in order to establish a correlation. The following section presents in detail the measurement procedure, related limitations and the results. 4.1. Flow measurement In these experiments, the flow was measured by FESTO Air Box portable device [32]. Air Box allows measurement of pressure, temperature, and flow, and enables user to perform air quality tests to establish humidity and oil content. Compressed air flows past a surface that is continuously heated. The flowing air absorbs heat energy from the warm surface. A thermal sensor quantifies the variation in temperature, which represents a specific airflow. The measuring frequency is 100 Hz, with accuracy ±3% and reproducibility ±0.3% for low flow sensor; and ±3% accuracy and ±0.8% reproducibility for high flow sensor. The obtained results indicate that compressed air flow gets higher with the increase of orifice diameter, and system pressure. For the purpose of experiment, measurement flow range was kept within the low range (10–200 l/min). The results obtained by measurement of flow at air exit locations on the punctured flexible pneumatic hose are given in Table 1. 4.2. Measurement of noise level Intensity of sound exiting the flexible pneumatic hose was measured by an ultrasound detector, Ultraprobe 100 [33,34], which accurately detects leaks and mechanical damage. The frequency response is 36–44 kHz and the response time 300 ms. Due to elasticity of material, the punctured orifices were of irregular shape, with rough walls. Hence, the
Table 1 Compressed air leakage (l/min) through orifices on a flexible pneumatic hose, as a function of pressure and orifice diameter. Pressure (bar)
Orifice diameter (mm) 0.5
4 5 6 7 8
2.1 2.6 3.2 3.7 4.2
0.7 4.3 5.3 6.5 7.9 8.8
1.0 8.4 10.7 12.3 14.3 15.6
1.3 18.7 23.3 29.0 34.8 40.0
1.5 39.9 37.0 45.1 51.9 56.2
intensity of ultrasound generated by flowing air was several times higher than it would have been if the orifices were round-shaped and smooth-walled. With the increase of pressure, airflow also rises, and the signal measured by the ultrasound detector becomes stronger. Noise level was measured at the loudest point of leak. Sound waves may need to be manipulated to a specific direction or angle to improve detectability [35]. Due to variable orifice shapes, rather than being perpendicular to orifice, the loudest point is most frequently at an angle of 30° [4]. The results of measurement of noise level generated by compressed air leak from the flexible pneumatic hose are shown in Table 2. Compressed air leaks are expressed as sound levels (dB), at various pressures. Due to relative imprecision of applied measuring instrument, the obtained values should only be used for orientation purpose in practice. As the orifices on the hose increase in size, the sound level changes at a slower rate with the change of pressure. The most prominent changes in sound level occur at orifices 0.5 mm in diameter, while at orifice diameters of 1.3 mm and larger, sound level becomes constant. Thus, it is not recommended to use sound level measurements to differentiate between orifice sizes above 1.5 mm. Obtained results match with the results reported in [4], where the experiments were performed for orifice diameters from 0.2 to 1.0 mm. Application of this method is recommended only for smaller orifices (0.5; 0.7 and 1.0 mm, and partially 1.3 mm) where it is possible to use noise level to clearly distinguish between the orifice sizes, and assess the related air loss. 4.3. Temperature measurement The measurement of temperature difference at the location of compressed air leak was performed by Fluke Ti20 camera with spectral range 7.5–14 lm, thermal sensitivity 200 mK, accuracy ±2 °C or 2% (whichever is greater) and repeatability ±1% or ±1 °C (±2 °F) whichever is greater. The field of view is rectangular and covers horizontal 20° and 15° vertical [36]. Due to inability to directly measure the temperature of compressed air, focus was placed on the measurement of temperature difference (DT), using Eq. (1), emitted by an object, in this case, the flexible hose (Tmaterial), and the temperature at the very orifice through which the compressed air is released (Torifice). Measurements were performed at Table 2 Noise level (dB) of compressed air leak from the flexible pneumatic hose, as the function of pressure and orifice diameter. Pressure (bar)
Orifice diameter (mm) 0.5
0.7
1.0
1.3
1.5
2.0
4 5 6 7 8
60 63 66 69 72
66 68 69 71 73
71 72 72 73 74
73 73 73 74 74
74 74 74 74 75
75 75 75 75 75
2.0 58.6 71.0 84.0 98.7 101.0
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Italic values: unclear area for ultrasound quantification of leakage.
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Fig. 1. Measurement location – comparison of photography and thermogram.
Fig. 2. Analysis of temperature profile for the 0.7 mm orifice at 4 bar pressure.
orifices of various diameters, at various pressures, and constant room temperature. Based on the mean value of temperatures measured in particular zones (see Fig. 1), the temperature difference was calculated according to Eq. (2).
DT ¼ T material T orifice
ð1Þ
T material ¼ ðT 1 þ T 2 þ T 3 Þ=3
ð2Þ
where T1 is the temperature of flexible hose before the orifice (°C), T2 is the temperature of flexible hose after the orifice (°C), and T3 is the temperature of flexible hose below the orifice (°C). Fig. 2 presents the analysis of temperature profile of the 0.7 mm orifice, at 4 bar. There is a sudden change of temperature at the orifice location. Analysis of all images leads to conclusion that the increase of pressure in the system causes the drop in temperature of compressed air exiting through the orifice, which, in turn, increases DT. The re-
sults obtained by measurement of temperature differences at orifice locations on the flexible pneumatic hose are shown in Table 3. Analysis of measurement results shows that the temperature change at orifice locations is the function of the orifice size. As the size of orifice increases, regardless of pressure, there is a visible drop in temperature at air exit points. Table 3 Temperature change DT (°C) as a function of pressure and orifice diameter on the flexible pneumatic hose. Pressure (bar)
4 5 6 7 8
Orifice diameter (mm) 0.5
0.7
1.0
1.3
1.5
2.0
0.5 0.6 0.8 0.9 1.0
1.4 1.5 1.6 1.7 1.9
2.2 2.7 3.0 3.2 3.3
2.9 3.4 3.7 4.1 4.6
3.6 4.1 4.4 5.1 5.5
4.3 4.7 4.9 5.4 5.9
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Fig. 3. Leak flow as the function of detected ultrasound level.
Fig. 4. Leak flow as the function of detected temperature change DT (°C).
5. Discussion
6. Conclusion
In this investigation, we have started from the known values of leak diameters and appropriate leak flow. Nevertheless, in reality, maintenance personal is facing the problem of unknown leak area and, adequately, unknown leak flow that they should, in some way, quantify. Based on these experiments, leak flow can be found as the function of measured ultrasound or temperature change. In Fig. 3 is given the diagram with the fitted curve of leak flow as the function of detected ultrasound level for each examined pressure. It can be noticed that these dependence is very strong for small leak diameters, especially in area of up to 1.0 mm leak diameters. When leak diameters are from 1.0 to 1.3 mm, correlation is weakening and with diameters of 1.5 mm and above, it becomes almost useless. However, from the other hand, there are significant changes in leak flow (e.g. more than 80%, at 6 bar pressure) between 1.5 mm orifice and 2.0 mm orifice and that change could hardly be identified with ultrasound detector. However, IR technology can clearly identify this change of leak flow due to the increase in leak diameter what can be seen in Fig. 4. From the presented results, it can be concluded that ultrasound should be used for leakage detection for all the dimensions of orifices, but for the quantification purposes only for smaller leaks, precisely, for the leaking with the sound level up to the 74 dB. If the sound level is higher than 74 dB, IR camera should be used for leakage quantification and it will give a reliable estimation of leakage. Therefore, quantification of bigger losses due to the leakage through the orifices 1.3–2.0 mm is not able with ultrasound detector but it is steel possible with the IR camera.
This paper shows the possibility for quantifying losses due to leakage using IR thermography. While for the ultrasound methods are already published some results concerning leakage quantifications, we have shown the comparison of those two methods. Nevertheless relatively imprecise measurement equipment used in our experiments, we have obtained clear view of areas in which each of those methods are appropriate. Besides, availability of ultrasound and infrared equipment in bigger factories is increasing, so with procedures defined in this paper the efficiency of leakage quantification can be significantly improved. Numerous factors affect the flow, the size, shape, and configuration of leak orifice, temperature and humidity of expelled air, etc. Ultrasound method has the limitation by the background noise, which is generated within the system and its environment. Advantage of the ultrasound leak detection lies in its versatility, speed of detection, ease of use, ability to perform on shop floor, and ability to detect various types of leaks. Scanning of the tested area reveals leak locations quickly and easily. Applying infrared thermography for the leakage quantification purposes requires a thermal contrast at leak locations. Those contrasts are imperceptible for small leaks, so infrared technology is not an optimal choice in those situations, but it offers good results for the leakages from the orifices greater than 1.0 mm. However, it requires well-differentiated temperatures, and is hampered by sources with weak radiation, and conditions of extreme lighting. Developed approach of leakage quantification in this paper can be applied concerning leakage of other fluids, e.g. LPG in pipelines and technical gasses (oxygen,
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nitrogen, etc.) in some factories. For that purposes, in order to get valid results, it is necessary to make measurements with appropriate measuring equipment, based on procedures defined in this paper.
References [1] P. Radgen, E. Blaustein, Compressed Air Systems in the European Union, Energy, Emissions, Saving Potentials and Policy Actions, LOGX Verlag GmbH, Stuttgart, 2001. [2] Office of Energy Efficiency and Renewable Energy, US Department of Energy, Assessment of the Market for Compressed Air Efficiency Services, Burlington, Xenergy Inc., 2001. [3] D. Šešlija, I. Ignjatovic´, S. Dudic´, B. Lagod, Potential energy savings in compressed air systems in Serbia, Afr. J. Bus. Manag. 5 (14) (2011) 5637–5645. [4] SDT Ultrasound Solution, Compressed Air-Leak Surveyor Handbook, third ed., SDT North America Ltd., Cobourg, 2009. [5] R.S. Asatryan, S.R. Asatryan, L.A. Sukoyan, N.R. Khachatryan, A spectroradiometer for remote ecological testing of gas main pipelines, Russ. J. Nondestr. Test. 46 (8) (2010) 598–602. [6] J.R. Bose, M.K. Olson, TAPS’s leak detection seeks greater precision, Oil Gas J. 91 (14) (1993) 43–47. [7] B.N. Carlson, Selection and use of pipeline leak detection methods for liability management into the 21st century, Pipeline Infrastructure II, in: Proceedings of the International Conference of the American Society of Chemical Engineers ASCE, 1993, pp. 369–383. [8] N.C. Turner, Hardware and software techniques for pipeline integrity and leak detection monitoring, Proc. Offshore Eur. 91 (1991) 139– 149. [9] J. Zhang, Designing a cost effective and reliable pipeline leak detection system, in: Proceedings of Pipeline Reliability Conference, 1996. [10] A.Y. Rozinov, Physical evaluation of the efficiency of applying liquid indicators during leakproofness inspection, Russ. J. Nondestr. Test. 42 (8) (2006) 551–557. [11] A.E. Rozlosnik, Infrared thermography and ultrasound both test analyzing valves, Proc. Termosense 3361 (1998) 137–175. [12] N.D. Jong, P.J. Frinking, A. Bouakaz, F.J. Ten Cate, Detection procedures of ultrasound contrast agents, Ultrasonics 38 (1–8) (2000) 87–92. [13] A. Püttmer, New applications for ultrasonic sensors in process industries, Ultrasonics 44 (1) (2006) e1379–e1383. [14] J. Zhang, B.W. Drinkwater, P.D. Wilcox, A.J. Hunter, Defect detection using ultrasonic arrays: the multi-mode total focusing method, NDT&E Int. 43 (4) (2010) 123–133. [15] E. Hæggström, M. Luukkala, Ultrasound detection and identification of foreign bodies in food products, Food Control 12 (1) (2001) 37–45. [16] S. Shuh-Haw, C. Hual-Te, C.R. Apostolos, Ultrasonic techniques for detecting helium leaks, Sens. Actuat. B: Chem. 71 (3) (2001) 197– 202.
[17] C. Moon, W.C. Brown, S. Mellen, E. Frenz, D.J. Pickering, Ultrasound techniques for leak detection, in: Proceedings of SAE 2009 Noise and Vibration Conference and Exhibition Ultrasound Techniques for Leak Detection, 2009. [18] T.J. Kulp, P.E. Powers, R.B. Kennedy, Remote imaging of controlled gas releases using active and passive infrared imaging systems, Proc. Infrared Technol. Appl. XXIII 3061 (1997) 269–278. [19] C. Meola, G.M. Carlomagno, Recent advances in the use of infrared thermography, Meas. Sci. Technol. 15 (9) (2004) R27–R58. [20] C.A. Balaras, A.A. Argiriou, Infrared thermography for building diagnostics, Energy Build. 34 (2) (2002) 171–183. [21] A. Moropoulou, N.P. Avdelidis, M. Koui, I. Tzevelekos, Determination of emissivity for building materials using infrared thermography, J. Thermol. Int. 10 (3) (2000) 115–118. [22] A.R. Al-Kassir, J. Fernandez, F.V. Tinaut, F. Castro, Thermographic study of energetic installations, Appl. Therm. Eng. 25 (2–3) (2005) 183–190. [23] M.Ö. Korukçu, M. Kilic, The usage of IR thermography for the temperature measurements inside an automobile cabin, Int. Commun. Heat Mass Transf. 36 (8) (2009) 872–877. [24] E.F.J. Ring, K. Ammer, The technique of infrared imaging in medicine, Therm. Int. 10 (1) (2000) 7–14. [25] C. Meola, A new approach for estimation of defects detection with infrared thermography, Mater. Lett. 61 (3) (2007) 747–750. [26] G.M. Carlomagno, C. Meola, Comparison between thermographic techniques for frescoes NDT, NDT&E Int. 35 (8) (2002) 559–565. [27] A. Tavukcuoglu, A. Duzgunes, E.N. Caner-Saltik, S. Demirci, Use of IR thermography for the assessment of surface-water drainage problems in a historical building, Ag˘zıkarahan (Aksaray), Turkey, NDT&E Int. 38 (5) (2005) 402–410. [28] E. Brodetsky, M. Savic, Leak monitoring system for gas pipelines, in: Proceedings of IEEE International Conference Acoustics, Speech, and Signal Processing, vol. 3, 1993, pp. 17–20. [29] G.J. Weil, Non contact, remote sensing of buried water pipeline leaks using infrared thermography, in: Proceedings of Water Resources Planning and Management and Urban, Water Resources, 1993, pp. 404–407. [30] A. Kroll, W. Baetz, D. Peretzki. On autonomous detection of pressured air and gas leaks using passive IR-thermography for mobile robot application, in: Proceedings of IEEE International Conference on Robotics and Automation, 2009, pp. 291–296. [31] A.W. Lewis, S.T.S. Yuen, A.J.R. Smith, Detection of gas leakage from landfills using infrared thermography – applicability and limitations, Waste Manag. Res. 21 (5) (2003) 436–447. [32] FESTO, Air Box Type GHDA-FQ-M-FDMJ-A – Operating Instructions, Festo AG&Co. KG. Esslingen, 2006. [33] UE Systems Inc., Ultraprobe 100 – User Manual, Elmsford, 2005. [34] UE Systems Inc., Compressed Air, Ultrasonic Leak Detection Guide, Elmsford, 2005. [35] S. Kleven, M. Blair, Limitations on the detection of casting discontinuities using ultrasonic and radiography, Mater. Eval. 10 (2003). [36] Fluke, Ti20 Thermal Imager – Users Manual, Everett; 2006.