LIMITATIONS, RESTRICTIONS AND RECOMMENDATIONS IN DIAGNOSIS BY TERMOGHRAPHY OF DISTRICT HEATING SYSTEMS

Transcription

1 Nonconventional Technologies Review Romania, April, Romanian Association of Nonconventional Technologies LIMITATIONS, RESTRICTIONS AND RECOMMENDATIONS IN DIAGNOSIS BY TERMOGHRAPHY OF DISTRICT HEATING SYSTEMS Eng Ciobanca Adrian 1, Eng Lolea Marius Savu 2, Eng Gale Vespasian Sorin 3, Eng Creţ Petru 2 1 SC Electrocentrale Oradea SA, adrianciobanca@gmailcom 2 University of Oradea, Faculty of Energy Engineering and Industrial Management, Departement of Energy Engineering, mlolea@uoradearo, pcret@uoradearo 3 SC Compania de Apă Oradea SA, gvespasiansorin@yahoocom, ABSTRACT: In this paper the authors present some aspects regarding the correct application of thermographic diagnosis in assessing the integrity of District heating systems and installations for heat loss calculation on their route Also presents the factors on which the effective use of this type of investigation Thermographic imaging allows nondestructive diagnostics, remote different technical systems Principle based on infrared thermography is the detection of heat radiation and heat flow anomalies and producing a corresponding visual image This method uses the thermal imager and software dedicated to processing information However it appears many errors concerning the camera before use as setings and operator ability and accuracy of data acquisition or how to process them If central heating systems diagnosis results can lead to significant errors in the horse and heat losses in the final planning maintenance operations To limit these, the authors propose a statement of their main limitations restrictions and recommendations in the using of thermovision and thermography and also presenting an concrete Thermographic imaging for diagnostic applications belonging District heating systems of Oradea, on relationship: infrared camera - thermography - thermograms - interpretation of results The work comprises several parts, based on both theoretical aspects and practical analysis or calculation results obtained in several conditions KEYWORDS: thermography, energy efficiency, district heating, infrared radiation, District Heating Systems(DHS) 1 INTRODUCTION The operation of any technical system occur at different stages of life, some malfunctions and errors To ensure a long lifetime it is necessary to identify, promptly and correctly, all symptoms that can lead to incidents and failures One of the modern methods of identifying these symptoms is the diagnosis by thermography This method, however, can lead to erroneous conclusions if not properly applied The Thermography allows nondestructive diagnostics, at a distance, for various components of the DHS This can make quantitative measurements, such as determining the technical state of contactors, or qualitative, such as determining the surface temperature of thermal insulation which can be quantified based on heat loss This method uses thermal cameras or infrared cameras and dedicated software for data processing However, this method may occur due to errors: programming room before use, ability to use the camera operator, the accuracy of the data acquisition and the processing thereof In this context, the authors present the results of thermographic investigations, both in qualitative and in quantitative terms, made in DHS from City of Oradea Qualitative evaluation with errors that may occur, is exemplified by a case study performed that refers to the determination of heat loss through the insulation on a section of primary thermal network The chosen example is topical and important, given the importance of maintaining heat loss through the insulation to a minimum lower For existing equipments in the majority DHS from Romania, these losses are at a high level due to existing insulation technology used at the time of construction, adding insulation degradation over 20 years of operation Establishing the technical condition of the insulation and quantifying the degradation effect on the heat loss was and is an ongoing concern of specialists from this domain For their evaluation, in case of heating networks in which circulate hot water or boiling water, several models was developed [1, 2, 3, 4, 5, 7] An important parameter which characterizes the state of the thermal insulation of the installation is the temperature at the surface This paper presents two possible ways of determining the surface temperature insulation: by calculation, in which case the state is assumed 22

2 known insulation (used in design) and measurement by thermography, when used existing insulation For the diagnosis of district heating systems the results can lead to significant errors in calculating heat losses and finally to planning maintenance operations To limit them, the authors propose a statement of the main limitations, restrictions and recommendations in the use of correct thermovision and presenting a concrete model of thermography for diagnosys applications in DHS of Oradea, by next relationship: infrared camera - thermography - thermogram - the interpretation of results 21 ENERGY PROCESSES INSIDE OF DISTRICT HEATING SYSTEMS(DHS) The District Heating Systems DHS - is a technologically and functionally unitary assembly consisting of the following main subsystems: one or more heat sources (HS), primary thermal networks(ptn), heat distribution points (HDP), secondary heating networks(shn), customer installation (CI), consumption water networks(cwn); elements that can be seen in Figure 1 SHN HDP 1 CI 1 CI 2 HS 1 HS 2 PTN HDP 2 CWN CI 3 CI 4 Figure 1 Principle scheme of District Heating System(DHS) 2 THERMOGRAPHY IN DIAGNOSIS AND MAINTENANCE OF POWER PLANTS Inside heat source, primary energy from fuels is converted in termal energy only or in heat and electricity (cogenerare) Fuels used in power plants can be of two tipes: fossil fuels (coal, oil, natural gas) and also renewable fuel type (biomass, geothermal water, etc) The thermal energy produced in the form of boiling water (with temperature up to 150 o C), is transmitted to the HDP by means of a water pump and PTN In HDP is produced the heat transfer between primary and secondary heat carrier inside the heat exchanger The secondary heat carrier (hot water with temperature up to 90 C) is the energy fluid which reaches, by pumping, from HDP through SHN into CI For DHS as a whole to be competitive and to ensure a high degree of continuity in the thermal power in terms of energy efficiency, as each subsystem component must meet particular requirements These requirements cover both the reliability or operational safety and energy efficiency of the subsystems mentioned An important component in achieving their preventive maintenance consists in applying one method which is the diagnosis by thermography 22 THERMOGRAPHIC INVESTIGATIONS IN DHS OF ORADEA As is well known, the transformation of primary energy from the fuel in the two forms of useful energy, thermal energy and electricity and its transport to the place of consumption, is accompanied by losses in the form of heat To locate and quantify these losses is one of thermographic investigations applicable solutions Such were investigated electrical installations and equipments(stations, connections, motors) and also thermo-mechanical equipments and installations Thermographic camera utilised is the model FLUKE Ti20 and wich is the property of dedicated laboratory of the Center for Research in Energy processes Management, belonging Energy 23

3 Engineering Department of the University of Oradea (figure 2) For downloading and processing data will be transferred to the computer using a dedicated software application named InsideIRThe operating system requirements are[11]: - Microsoft Windows 2000 or XP; - Version of Internet Explorer 501 or greater required to use MicrosoftNet framework; - Microsoft Net framework 11 and Microsoft Visual runtime components 11 (included on the InsideIR 30 CD - ROM) Hardware requirements[11]: - PC with a 1 GHz processor or faster; MB of RAM (or more depending on the number of termal images stored on the computer); MB of free hard disk space; - Super VGA monitor with the screen resolution set at 1024 x 768 or greater; small fonts setting and true color(32 bits); - CD ROM drive; - USB port (USB 20 High Speed recommended); - Mouse or pointing device To highlight the errors that may occur due to influence factors mentioned in paragraph 1, measurements were made in several different conditions, with different thermal imager settings used a b c Figure 2 Images with infrared camera that was used(fluke Ti20) a frontal view, b- side view, c dorsal view It is envisaged that the infrared radiation or heat radiation as it is known where the length is between 7-10 micrometers and frequency of about Hz Any material structure with ε τ ρ 24 temperature greater than 0 K emit thermal radiation due to molecular vibrations Like any electromagnetic wave propagation in the infrared may change as attenuation, reflection, refraction, etc, depending on the environment that meets the strong absorption of certain substances It is related to operator skill knowledge propagation phenomenology and physics of materials handling as well as the degree of thermal imager (knowledge of its characteristics and settings required for thermographic measurements) the accuracy of thermography process Thus, the radiation recorded by thermographic camera is composed of infrared waves emitted, reflected and transmitted object in its field of vision The measurement can trace in Figure 3 From the physical manifestation of radiation propagation and as reflected in the specialized technical literature study [8], we can write: ρ + τ + ε = 1 but as transmissivity has a less important role in practice, may be neglected and the previous relation becomes: ρ + ε = 1 in which: ρ Reflection coefficient (reflectivity) ε Emission coefficient (emissivity) For thermography this means: with how the emissivity is the lower, the greater is the infrared radiation reflected, the more difficult to measure the precise temperature and even more important is that the value of the thermal imager to be set correctly For example, measurements were made on some pipes in the Combined Heat and Power Plant CET I from City of Oradea Figure 4 shows images taken with a digital photographic camera and infrared camera at two pipes, one with DN = 800 mm through which primary fluid is passing and which was during the measurement with temperature of 83 C and another pipe with DN = 200 mm, which the time of measurement was not running DN is the nominal diameter of the pipes Figure 3 Thermographic measurements principle and the explanation of radiation propagation ε emissivity, τ transmissivity, ρ - reflectivity

4 Pipes analyzed are mounted in an open channel along the other pipelines e (fig4, b) Measurements were performed on a portion of the network that includes the isolated area and an area where the insulation was removed to eliminate a leak The atmospheric conditions during the measurements were 24 o C outdoor temperature, wind speed - about 02 m / s, relative humidity - 53% Pipe surface temperature determined by thermography for different coefficients of emissivity set to the infrared camera, are summarized in Table 1 As will be noted, for the same area and the same measurement conditions (weather, angle-of-sight, distance), the temperature indicated by the device increases with decreasing emissivity set, the differences are even greater as the temperature is higher For emissivity of 09 and lower pipe surface temperature is higher than the fluid flowing through the pipe, which of course is an error, denoting either that the machine was not properly set or there are other circumstances disturbance mentioned above If the review finds relatively large differences between the temperatures recorded on the same type of surface If in isolated areas of the pipeline DN800, these differences may be partly due to uneven insulation and different emissivity protective sheet (partially oxidized), where bare surface comes another influencing factor It is given by the immediate pipeline, which is protected with new galvanized plate, highly reflective In this case, both the phenomenon of radiation occurs, the noninsulated pipe and the reflection of the pipe insulated steel sheet These phenomena are shown in the thermograms in Figure 4, through the high temperature, with the linear strip, registered with galvanized pipe, which ends with the uninsulated portion of the pipeline DN800 Note that the average temperature on the surface of the insulated conduits is relatively close, old pipe insulation having an average temperature of 14 o C higher than new insulated pipe is not running Figure 5 presents a screenshot of the temperature profile recorded in the analyzed lines on the two axes On the X axis is relatively constant temperature recorded in the noninsulated area and reflects the state of insulation and surface emissivity for the isolated area The Y-axis there is a change in temperature, on the noninsulated pipe DN 800, having a minimum (809 o C),at approx one-third from the bottom of the pipeline The lower part has a temperature slightly increases up to 813 o C, while at the top of pipeline, the increase is more pronounced to 845 o C (higher than the temperature of the water in the pipeline) It is also observed a peak temperature of 707 o C in the area of isolated new pipe And in this case highlights the interaction of radiation and reflection between the two pipes and a little uncle of sight In the example shown, that for determining surface temperature of pipes by thermography with as much accuracy as knowledge needed technical characteristics of the equipment used and the plants on which determinations are made For a correct interpretation of the results obtained by thermography also be known phenomena and processes occurring in them Otherwise, any distraction occurs, unnoticed and interpreted incorrectly, can lead to wrong decisions with negative economic and technical implications For example, below is the calculation of heat loss for a pipe insulated with mineral wool in order to assess its quality = 0,70 Figure 4/a Section of pipe which was investigated by thermography (photo and thermogram nr 1) 25

5 = 0,90 = 0,98 Figure 4/b Section of pipe which was investigated by thermography (thermogram nr 2 and 3) Table 1 The temperature at pipelienes surface registered by thermography Pipeline DN800 Pipeline DN200 Isolated area Uninsolated area Affected area Unaffected area e min e max e med e min e max e med e min e max e med e min e max e med % C C C C C C C C C C C C 70 32,0 46,8 36,0 95,8 101,0 97,7 65,2 88,6 78,9 33,0 38,9 35, ,7 39,6 31,1 79,4 85,3 82,9 49,6 75,7 64,6 27,9 33,5 30, ,2 38,8 30,8 77,1 81,4 78,7 49,4 71,4 62,1 27,7 32,4 29,4 Temperature differences at insulation surface for minimal and maximal value of emissivity C 4,8 8,0 5,2 18,7 19,6 19,0 15,8 17,2 16,8 5,3 6,5 5,6 % 17,6 20,6 16,9 24,3 24,1 24,1 32,0 24,1 27,1 19,1 20,1 19,0 Figure 5 Data analysis with dedicated software 3 EVALUATION OF HEAT DEPERDITIONS THROUGH THE INSULATION 31 MATHEMATICAL MODEL Notations used: temperature of heat carrier [ C]; temperature of external air [ C]; i temperature at internal surface of insulation [ C]; 26 e temperature at external surface of insulation [ C]; a heat transfer coefficient by convection, of the fluid to the inner surface of the pipeline ; a heat transfer coefficient by convection and radiation, from insulated pipe at the envinronment ; λ thermal conductivity of the pipe [W/m K]; thermal conductivity of insulation layer no:,,j [W/m K];

6 q iz the heat loss per unit of insulated pipe [W/m]; From [ 1,2,3], to quantify the heat loss and the temperature drop for an area of overhead thermal network is further establishes an algorithm using the notation of Figure 6, as defined above For the calculation of heat losses in accordance with [1, 2, 3], can be applied to the relationship (1): (1) dc diz de di λ t i Figure 6 Heat transfer through the pipes i e In the case of insulated piping, the heat transition coefficient of the fluid from the external environment, related to a meter of pipe k includes the resistance of the n-layer of insulation and is defined by the next formula: (2) q i i q i t e Perete Izolaţie Strat protector From the equality of equation (4) to (7), with the notation of (5) and (6), we obtain a relation for calculating the surface temperature of insulation: (8) For the calculation of the coefficient of heat transfer by convection and radiation, when the pipe is mounted outside, can be used as [3], the relationship: (9) The value of coefficient of thermal conductivity j is specific to each insulating material used as specified by the pipe manufacturer company, in the technical datasheet For networks built before 1990, the insulation was done mainly with mineral wool or glass wool with outer protective paperboard and fill plate The thermal conductivity, for mineral wool in the form of mats according to its average temperature can be calculated [3] by the relation : (10); Neglecting the resistance of the inner surface of the pipe,, thermal resistance of the pipe wall,, thermal resistance of protection plate, given the fact that the wall thickness of steel tubes and sheet metal protection is low and with raised and i >> e Under these circumstances equation (2) becomes: (3) Then, the heat loss through insulated pipes can write: In relation (4), we note by: (4) (5) the thermal resistance of the insulation, and with: (6) the resistance to heat transfer from the outer surface of the last layer of insulation to the outside air If known, the temperature at the surface of the insulation, the specific heat loss can be determined with next relation [3]: (7) 27 And how is the arithmetic mean of and the following calculation is performed by successive iterations Are given of, to compute and, until value assigned is a deviation from the value calculated with (8) formula, in the limit imposed of minimal error 32 RESULTS AND DISCUSSION Calculation model presented is applied to calculate the specific heat loss through the insulation to the pipe DN 800, the following assumptions: - the pipe is aerial type(air mounted); - outside temperature is the same as when thermography; - thermal insulation is made of mineral wool mats with a thickness of 90 mm; - other influencing factors, it neglects Table 2 shows the results obtained in both cases of determination of the temperature e : - Based on project data with relation (4); - Based on measurements using the relation (7), for = 0,7, = 0,9, = 0,98, By calculation algorithm presented, its evaluated the temperature at insulation surface of air ducts under ideal conditions, by project, in

7 which the insulation build is fully complies with the requirements of its prescribed properties In these conditions its obtained a temperature at surface of insulation of 26 o C and a specific loss of heat of 1078 W / m On the basis of measurement of the outside temperature of insulation for several selected factors of emissivity, the heat losses are between 1752 W / m and 1248 W / m The value range is determined on the one hand, the different of transmission factors selected and on the other hand, by the heterogeneity of the surface in terms of emissivity Physical parameter Mean Maximal Minimal Table 2 The temperature at insulation surfaces and specific heat losses determined by calculation and by measurements Measured of e, calculated of for e measured, Unit of and deviations from calculated ( ) Calculated measurement 0,, 0,98 [UM] e ; q iz [%] e ; q iz [%] e ; q iz [%] e C 26,0 36,0 38,5 31,1 19,6 30,8 18,5 W/m 107,8 657,0 509,5 388,7 260,6 372,3 245,4 e C 26,0 46,8 80,0 39,6 52,3 38,8 49,2 W/m 107,8 1248,0 1057,7 854,1 692,3 810,3 651,7 e C 26,0 32,0 23,1 27,7 6,5 27,2 4,6 W/m 107,8 437,9 306,2 202,5 87,8 175,2 62,5 4 CONCLUSIONS The thermal imager can identify a number of qualitative deficiencies in facilities, location of certain defects which are manifested by different emissivity The results of applying this method is materialized in measures to prevent serious defects of equipment and facilities In case of DHS, the processes of thermovision, can be used for both fault location in power plants and in the thermo-mechanical instalations Can also locate the most important areas where heat loss heating networks, which allows prioritization of maintenance work to reduce them In using this method, however, errors may occur due to: room schedule before use, ability to use the camera operator, the accuracy of the data acquisition and the processing thereof In addition, when measuring temperature of insulation surface for quantitative evaluation of heat loss through the insulation, the temperature obtained can be affected by other factors: weather, neighboring elements, inhomogeneous emission factors of surface insulation, etc In the present case heat loss from using thermography, revealed large errors that may occurtemperature field does not always reflect the actual temperature recorded at the surface of the pipe Unlike some areas with reduced or degraded insulation, where the temperature is high at registration, there are cases when the recorded of high temperature may be caused by different emissivity factors of surface insulation, solar radiation or the thermal radiation of nearby pipes 5 REFERENCES 1 Athanasovici V şa Tratat de inginerie termică Alimentări cu căldură Cogenerare, Ed AGIR Bucureşti, 2010; 2 Drăghici, N - Conducte pentru transportul fluidelor, Ed Tehnică, Bucureşti, 1971; 3 Ilina, M şa - Manual de instalaţii, volum - Instalaţii de încălzire, Ed ARTECNO, Bucureşti, 2002; 4 Iordache, F - Comportamentul dinamic al echipamentelor şi sistemelor termice, Ed Matrix Rom, Bucureşti, 2008; 5 Iordache, F,şa - Randamentul unui sistem districtual de încălzire centrală, Revista Română de Inginerie Civilă, Volumul 2, Nr 2, 2011; 6 Mihai, A, -Termografia în infraroşu Fundamente, Ed Tehnică, Bucureşti, 2005; 7 Keçebaş, A - Determination of insulation thickness by means of exergyanalysis in pipe insulation, Energy Conversionand Management, nr 58, pag 76 83, 2012; 8 Leca, A, Mladin, C E, Stan, M - Transfer de caldură şi masă Bucureşti, Editura Tehnică, 1998; 9 *** Normativ pentru proiectarea şi executarea instalaţiilor de încălzire centrală, Indicativ I13 02; 10 ***Metodologie privind determinările termografice în constructii, indicativ MP wwwtestequipmentdepot/fluke/pdf/ti20_ manupdf, Fluke Ti20 ThermalImagerUsersManual; 12 wwwflukecom 28