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FLIR Technical SeriesApplication Note for Research & Science

Detection of Defects in Pipelines Using Transient Analysis 0f Thermal-Induced FluxG. Basile, C. Clienti, G.A. Fargione, A.L. Geraci, A. RisitanoDipartimento di Ingegneria Industriale e Meccanica, Facoltà di Ingegneria, Università di Catania, Italy

AbstractThrough a study of the current state-of-the-art techniques used to find hidden defects in steel elements (as well as in other metals including pipes made of titanium, inox steel, etc.) and on the basis of the application procedures of the different methodologies existing in literature for plane geometry, this experiment attempted to create a new methodology, simple and practicable on the field, for risk assessment, monitoring, and planning control of interconnected piping in oil refineries with the use of thermal infrared imagery. This methodology adds to the array of non-destructive testing methods, traditionally indicated as appropriate by rules of good technique and praxis and by sector-specific guidelines. The methodology, developed in the laboratory, elaborates on a technique used for safety verification of pipe in industrial plants. These pipes are subjected to corrosion phenomena with the subsequent risk of a break that could result in a spill of highly inflammable and polluting products. The technique allows an accurate scan of the critical pipe components, in spite of what the traditional thickness measurements do by sampling. Additionally, the proposed methodology enables checking warm pipe, while the traditional ones require the pipe to be at room temperature or within a certain range. This is based on the fact that heat deposited on a surface by a thermal impulse, very brief and of the order of few milliseconds, will disperse mostly along the plate thickness. Several trials, performed in order to find the most appropriate method to uniformly heat the pipe surface, pointed out the possibility of using thermography to locate defects artificially created on the pipe’s internal surface. The proposed technique consists of the generation of a warm or cold flux, inside or outside the pipe, and in the observation of the thermal pattern development on the external surface. The methodology leads to a quick diagnosis, highlighting areas that need to be controlled more accurately, once the critical points have been located.

IntroductionIn many sectors, and not only in the industrial ones, the control of the surface temperature map, as a reference to detect working anomalies, is of fundamental importance. In the end, all physical and chemical processes degrade in heat and temperature is the parameter that best represents this process. There are several industrial applications of thermography: control of temperature balance in remote heating plants, efficiency control of heat exchangers to locate partial or total obstructions in internal pipes, control of insulation in pipes conveying steam or other hot or cold fluids, control of valves to locate internal encrustations, inspection in conditioning plants, verification of steam traps working, inspection and monitoring of furnaces, management of loss of resistance to heat, verification of tank level, diagnosis of condenser tabs, etc. To this end, applied research has allowed the development of new investigation techniques. The first technique developed, passive thermography, relied on the principle of the

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energy conservation in which a certain amount of heat is released by any process. Subsequently, other new thermography techniques have extended available applications including the development of active thermography, pulsed thermography, lock-in thermography, step heating thermography, transient thermography, and vibro-thermography [1].

Lock-in Thermography (LT) is based on the use of thermal waves, generated inside the material under investigation, and in their remote detection. Wave generation is obtained though periodic heating (for example, a light modulated in a sinusoidal way), while the oscillating temperature field, in stationary condition, is detected remotely by the receiving of infrared radiation. Therefore, lock-in refers to the need to record the output signal as a function of the input one (the modulated heating). This can be done using a lock-in amplifier, in the case of point-by-point heating through a laser, or using a computer in the case of heating the entire field through a light, in order to produce images of the amplitude and phase of the temperature field. In Step Heating Thermography (SHT), increments of surface temperature are detected during the application of a thermal gradient of long duration (long impulse). SHT is used for thickness evaluation of coatings (including multilayer coatings composed of more than one material), inspection of the connection between a coating and a sub-layer or study of composite structures [1]. Transient Thermography (TT) can be considered as a combination of LT and SHT. The sample is heated with a long duration impulse. Heating and subsequent cooling are recorded to get information on phase and amplitude through Fourier analysis of local sequences of temporal data. Results are similar to LT, but with images for more than one frequency. [1]. VT is an active technique in which mechanical vibrations are induced in the structure and their effects are studied externally: heat is released from friction in the locations where defects, like fractures or detachments, exist (conversion of mechanical energy into thermal one) [1].

Therefore, thermography can be used to detect discontinuity conditions and the trigger points of corrosion phenomena in piping. It is necessary to define the external apparatus, allowing the transfer of typical laboratory analysis directly to the field. The extent of the defects involved, the number of factors directly or indirectly affecting the investigations, and the non-uniformity of the surfaces to be examined make this difficult. Laboratory and field trials had the aim, in this first phase, of analyzing through active thermography tracts of pipes, of differing diameters, thicknesses, and degrees of surface oxidation, that are usually employed in petrochemical industry. The purpose was to locate eventual thickness reductions caused by internal corrosion and to evaluate the main factors that directly affect thermal analysis when conducted in the field.

First TrialsIn order to define the methods of thermographic analyses to be tested in the field, two pipe tracts were prepared in the laboratory. One was a smooth pipe, with a diameter of 219 mm (8 in) and a thickness of 5.54 mm with center welding. The other was a rough pipe, with a diameter of 273 mm (10 in) and a thickness of 7.11 mm. Notches and grindings with different depths and shapes were made on the internal surface of the tracts (ASTM A-106 Gr.B), to simulate the effects of an accentuated internal corrosion (Figure 1 and Figure 2). Infrared monitoring was performed with a FLIR thermal camera, the ThermaCAM SC 3000, which has the capability to read temperature variations as low as 0.02 °C.

Heat deposited on the whole plane surface by the thermal impulse, very brief and of the order of few milliseconds, disperses mostly along the plate thickness, as seen in Figure 3.

Figure 1. Artificial defects made on the internal surface of the pipes.

Figure 2. Artificial defects made on the internal surface of the pipes.

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This phenomenon is described by the equation:

Q=ρQ+αQ+τQ

where ρQ is the reflected component, αQ is the absorbed one, and τQ the transmitted one. Obviously, the velocity of heat transmission and the resulting thermal flux will depend, in preponderant way, on the material itself, or more specifically on its thermal conductivity k [Wm-1K-1], that represents the ability of the material to conduct heat. After heat is deposited in a uniform way on the whole surface, it diffuses in the material zones that are at a lower temperature than the surface layers. Initially, this will generate nearly linear heat diffusion across the thickness with a homogeneous temperature decrease over the whole monitored surface. When the heat diffusion will reaches the top of each defect, the surface heat reduction of the affected zone slows, since the heat no longer has layers of material below in which to diffuse. This will involve the appearance, for a very short time, of hot spots on the analyzed surface, observable through thermographic analysis that corresponds to the various defects. The greater the residual surface temperature is, the greater the depth of the located defect will be. Uniform heating of the surface to be observed is a fundamental condition for homogeneous heat diffusion across the thickness.

Experimental ApproachThe first factor to be considered in field analysis is the surrounding environment. It is not easy to deposit a certain amount of heat on a surface, ideally in a uniform way, so as to trigger in the material a diffusive phenomenon which can then produce the mentioned distributive anomalies of the thermal profile that are in correspondence to the found defect. This is especially true in a petrochemical environment which is at a high risk of relevant accidents. Indeed, the environmental thermal exchange phenomena necessary to achieve the rate of thermal flux needed for observation and quantification would require a powerful heat generator. To this end, chambers with reduced capacity for thermal exchange with the surrounding environment were made, so that the thermal flux would directly hit the surface under investigation. This tactic fulfilled the twofold aim of reducing the magnitude of the thermal source needed to trigger the diffusive phenomenon and of isolating the zone under investigation from direct external influences, such as reflections, incidental direct and/or indirect thermal sources, and incoming solar radiation. Some examples of these chambers, used to contain the thermal flux generated and for the thermographic acquisition of transient temperatures exhibited during the heating and subsequent cooling of the surface under investigation, are shown in Figure 4.

The results obtained, using active thermography (acquisition of thermal transients) with generation of thermal fluxes (streams of warm water, incandescent lamps) external to the surface under investigation, allowed the detection of surface defects (Figure 5). However, they did not significantly aid in the location of possible defects within the material thickness. Therefore, other investigation techniques, such as LT, SHT, TT, need to be investigated. The generation of thermal flux, inside the pipe examined, allowed the location of artificially-created defects within the pipe. Using the flux configuration shown in Figure 6 and Figure 7 (involving the whole pipe tract), the results reported in Figure 8 and Figure 9 were obtained.

Reducing the tract exposed to the thermal flux by using insulated walls (Figure 10), it was possible to highlight the defects the tract under investigation even better (Figure 11).

Figure 3. Diagram of heat transmission through a wall.

Figure 5. Example of surface defect detection (pitting corrosion, paint detachment, etc,).

Figure 4. Example of adiabatic chamber used for the experimental trials.

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Varying the flux configuration so that the pipe welding is involved (Figure 12), it was possible to highlight defects inside the weld (Figure 13).

ConclusionsExperimental studies, designed to transfer laboratory thermographic investigations directly to the field, were conducted. The laboratory analyses concerned pipes tracts, some painted and some not, typical of petrochemical environments. The lab work allowed us to lay the foundations for the application of techniques, such as LT, SHT; TT in the field, taking into account not only environmental conditions, but also the highly dangerous application ambit. The preliminary study on the use of warm air streams and the acquisition of thermal transients inside the pipe under investigation allowed the detection of defects, artificially made inside the pipe itself. Vice versa, thermal transients, generated with external warm air fluxes, demonstrated relative effectiveness. Therefore, on the basis of trials carried out, it is possible to assert that, using the warm or cold process fluids carried inside the pipes as thermal flywheel, it is possible to scan the whole surface. This can be done, utilizing a thermal contrast source (for example a cloth moistened with warm or cold water) as external impulse to start the thermal transient.

SummaryA field methodology was developed for risk assessment, monitoring, and check planning of piping in petrochemical plants. It includes the use of thermal infrared, assisted by traditional NDT (Non-Destructive Testing) methods suggested by the pertinent codes, guidelines, and good technique. The thermographic methods suggested by literature are not suitable for field investigation because of equipment involved and procedures required for use in an industrial plant. The proposed methodology focuses on the peculiarities of the pipelines, with or without coating paint, characterized by a large extension and a number of factors affecting the investigation. The methodology emphasizes the concept of verification and safety analysis of pipes which are subjected to corrosion phenomena with risk of spillage of highly inflammable and polluting substances. Heat deposited on a surface by a very short thermal impulse is mostly dispersed along the element thickness. Heat transmission velocity, and so thermal flux, will depend significantly on the material. Subsequently, heat will diffuse in the material parts at lower temperature that are beneath the surface. When diffusion reaches the top of each defect, heat reduction slows down, since material is not available below. As a consequence, hot spots observable by thermography at the defects will appear for a very short time. The proposed technique consists in the generation of a warm or cold fluid flux, inside or outside the pipe, and in the thermographic observation of the thermal transient condition of the external surface. ReferencesCarosena Meola, “A New Approach for Estimation of Defects Detection with Infrared Thermography”, Materials Letters, 2006.

Clienti, C., D. Corallo, A.L. Geraci, and A. Risitano, “Location of Defects in Pipelines Using Thermal Infrared Imagery”, Proceedings of the Infrared Camera Application (InfraMation) Conference, Reno, Nevada, USA, 2008.

Geraci, A.L., G. La Rosa, and A. Risitano, “Applications of Remote Sensing Techniques to Industrial Plants”, International Conference on Monitoring, Surveillance, and Predictive Maintenance of Plants Structures, Associazione Italiana Prove non Distruttive, Taormina - Giardini Naxos, Italy, 1989.

Geraci, A.L., G. La Rosa, and A. Risitano, “A Thermographic Method for the Localization of Leaks in- Service Buried Piping Systems”, Second

Figure 6. Experimental configuration for internal flux generation (φ 219 mm smooth pipe with center welding).

Figure 7. Flux direction inside the pipe tract.

Figure 8. Location of internal defects (Figure 6 configuration, smooth pipe, φ 219 mm, center welding).

Figure 9. Location of internal defects (Figure 7 configuration, rough pipe, φ 273 mm, 7.11 mm thickness).

Figure 10. Experimental configuration for internal flux generation (φ 219 mm smooth pipe with center welding).

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International Congress on Water Ecology and Technology, International Water Supply Association, Moscow, Russia, 1996.

Geraci, A.L., A. Risitano, and M. Dicembre, “Innovative Methodology to Monitor the Condition of Aged Pipelines in Petrochemical Plants”, Proceedings of the Infrared Camera Application (InfraMation) Conference, Las Vegas, Nevada, USA, 75-78, 2007.

Grinzato, E., P.G. Bison, S. Marinetti, and V. Vavilov, “Hidden Corrosion Detection in Thick Metallic Components by Transient IR Thermography”, Infrared Physics & Technology, 49, 234-238, 2007.

Maldague, X.P., “Theory and Practice of Infrared Technology for Non-Destructive Testing”, Wiley & Sons, Inc., New York, USA, 2001.

Maldague, X.P., H. D. Benitez, C. Ibarra Castenado, A. Benada, H. Laiza, and E. Caicedo, “Definition of a New Thermal Contrast and Pulse Correction for Defect Quantification in Pulsed Thermography”, Infrared Physics & Technology, 51, 160-167, 2008.

Sagakami, T., and S. Kubo, “Application of Pulse Heating Thermography and Lock-In Termography to Quantitative Non-Destructive Evaluations”, Infrared Physics & Technology, 43, 211-218, 2002.

About the AuthorsA.L. Geraci and A. Risitano are Professors at the University of Catania, Italy, and have been using thermal infrared imagery for more than 30 years in different applications. G.A. Fargione is an Assistant Professor at the same University with great experience in thermographic applications. C. Clienti and G. Basile are researchers at the same institution.

Figure 11. Location of internal defects (Figure 9 configuration, smooth pipe, φ 219 mm, center welding).

Figure 12. Experimental configuration for internal flux generation (φ 219 mm smooth pipe with center welding).

Figure 13. Location of internal defects (Figure 9 configuration, smooth pipe, φ 219 mm, center welding).