Corrosion Science 51 (2009) Contents lists available at ScienceDirect. Corrosion Science. journal homepage:

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1 Corrosion Science 51 (2009) Contents lists available at ScienceDirect Corrosion Science journal homepage: Prediction of oxide scale growth in superheater and reheater tubes J. Purbolaksono a, *, A. Khinani a, A.Z. Rashid a, A.A. Ali a, N.F. Nordin b a Department of Mechanical Engineering, Universiti Tenaga Nasional, Km 7 Jalan Kajang-Puchong, Kajang 43009, Selangor, Malaysia b TNB Research Sdn Bhd, No. 1 Lorong Air Hitam, Kajang 43000, Selangor, Malaysia article info abstract Article history: Received 20 August 2008 Accepted 24 February 2009 Available online 6 March 2009 Keywords: A. Steel A. Steam B. Modeling studies B. Heat transfer C. Oxidation In this paper a procedure on how to estimate the oxide scale growth in superheater and reheater tube utilizing the empirical formulae and the finite element modeling is proposed. An iterative procedure consisting of empirical formulae and numerical simulation is used to determine scale thickness as both temperature and time increase. Results of the scale thickness over period of time for two different design temperatures of the steam and different heat transfer parameters are presented. The procedures may provide better estimation on the oxide scale growth, provided that all the heat transfer parameters are well specified. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Failures resulting from long-term overheating occur in steamcooled tubes such as superheaters and reheaters. As reported by Port and Herro [1], almost 90% of failures caused by long-term overheating occur in superheaters, reheaters and wall tubes. Tubes that are especially subjected to overheating often contain significant deposits. The deposits will reduce coolant flow, and the tubes experience excessive fire-side heat input. They also described that heat transfer is markedly influenced by a thin gas film that normally exists on external surfaces. A temperature drop commonly occurs across this film. Scales and other materials on external surfaces will slightly reduce metals temperatures. The thermal resistance of the tube wall may cause a very slight drop in temperature across the wall. When heat transfer through the steam-side surface is considered, the effect of deposits is reversed. Steam layers and scales insulate the metal from the cooling effects of the steam, resulting in reduced heat transfer into the steam and increased metal temperatures. When the tube metal is in contact with the steam over period of time, the oxidation process may begin to form a layer of magnetite (Fe 3 O 4 ) scale. In the prolonged exposure this phenomenon will worsen situation that leads to potential creep rupture problems. Scales inside the superheater and reheater steam tubes have also been found to be one of the major contributors to the tube failure. Heat transfer rate across the tube also decreases due to the accumulated scales inside the tube. * Corresponding author. Tel.: ; fax: addresses: judha@uniten.edu.my, j.purbolaksono@gmail.com (J. Purbolaksono). A further effect of growing scales is that the tube will have higher temperatures than those as originally specified. Such exposure may cause degradation of the tube alloy, and this eventually will lead to tube rupture. It is estimated that 10% of all power-plant breakdowns are caused by creep fractures of boiler tubes due to the scales formation [2]. Clark et al. [3], who were working for Aptech Engineering Services Inc., provided a validated procedure in the form of computer code for predicting the remaining useful life of SA213-T22 superheater and reheater tubes. One of the important tasks they performed is acquisition and compilation of oxide growth information for 2.25%Cr-1Mo steel. The procedure was to be based on steam-side oxide scale measurements by ultrasonic technique, tube geometry measurements and other readily available operating parameters. Viswanathan et al. [4,5] reported a methodology developed by Electric Power Research Institute (EPRI) and its contractors to help utilities make more informed run/replace decisions for tubes by judiciously combining calculation, nondestructive, and destructive evaluations. In the methodology, the tubes/tube assemblies at risk are identified by ultrasonically measuring the thickest steam-side oxide scale and thinnest wall thickness in the tubes. The research has further refined the methodology by validating the ultrasonic technique for scale measurement, identifying the appropriate stress formula and oxide growth laws. Babcock & Wilcox Company, USA, has designed and built the portable, ultrasonic Nondestructive Oxide Thickness Inspection System (NOTIS Ò ) for measuring oxide scale on the inner surface of tubes [6]. The application of NOTIS Ò makes it possible to nondestructively assess a large number of tubes within a superheater X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:116/j.corsci

2 J. Purbolaksono et al. / Corrosion Science 51 (2009) An accurate prediction of the scale growth on the inner surfaces of the superheater and reheater tubes will aid the power plant inspectors or engineers in order to evaluate the remaining life of the boiler tubes. French [7] described the procedures to estimate the growth of scale thickness on the inner surface of the tubes using empirical formula correlating scale thickness with Larsen- Miller parameter [8] and approximated formulae of the temperature increase for limited cases. Ennis and Quadakkers [9] discussed the significance of the formation of thick oxide scales during exposure of Cr steels in steamcontaining environments on the service life of components. The quantitative effects of the loss of load-bearing cross-section on the creep rupture life are presented. Potentially much more damaging is, however, the thermal insulation effect of the porous, thick oxide scales, which leads to overheating of heat transfer tubes. The higher metal temperatures will then lead to early failure by creep. The scale present after 10,000 h exposure in steam may be sufficient to cause a temperature increase which will reduce the rupture time for a constant stress. They also reported several consequences of the formation of thick oxide scales for the service behaviour of components. Starr et al. [10] proposed an expert system for identifying the root causes of the failures in superheater tubing made of the P91 and P92 martensitic alloys. The system may encapsulate current knowledge about superheater problems in the form of If-Then rules. Root causes of creep failures include furnace design and operation, overestimation of alloy creep properties, inadequate heat treatment and a non-optimum content of strengthening elements. A characteristic of the P91 and P92 martensitic alloys is that oxidation on the steam side of the tubing can induce premature failures due to the insulating effect of the oxide scales raising tube temperatures. In addition, scale spallation could also increase tube temperatures, as spallation debris may collect in the bottom of tubes, blocking steam flow. Attention is drawn to a potential runaway affect in which the tube temperature and rate of oxidation increase with time as the oxide builds up. The root cause of this could either be excessive rates of heat transfer or could be due to inadequate oxidation resistance caused by low levels of protective elements. With the respect to the concerns stated in the previous works [1 7,9,10], the present study confined the analysis in the absence of oxide scale developed on the external surface of the boiler tubes. The procedure on how to estimate oxide scale growth in reheater and superheater tubes utilizing the empirical formula correlating scale thickness with Larsen-Miller parameter [8] and the finite element modeling is proposed in this paper. Finite element models for heat transfer analyses, that involve forced convections on the inner surface due to the turbulent flow of steam and on the outer surface due to cross flow of the hot flue gas over bare tubes, are carried out in order to obtain temperature distribution in the tube. An iterative procedure is used to determine scale thickness as both temperature and time increase. The scale thickness over period of time for two different design temperatures for steam and different heat transfer parameters is presented. The finite element analysis is carried out using software package of ANSYS. 2. Numerical models In modeling of the steady state heat transfer for the problem using ANSYS, the area of the model is divided into two regions, i.e. scale region and tube region (see Fig. 1). The steam region is taken into account in determining the convection coefficient of steam film for fully developed turbulent flow in circular tube. Model of the tube section used is 100 mm in length. Three different geometries of the tube as shown in Table 1 are used. Steam flows through the internal of tube with two different inlet temperatures 100 mm Steam Hollow Radius of 540 and 605 C, and the detailed other heat transfer parameters are tabulated in Tables 2 4. Heat transfer along the external surface between the flue gas and the tube wall is considered as forced convection heat transfer due to cross flow of the hot flue gas over bare tubes. The material of the seamless ferritic low-alloy steel tube used in this work is SA213-T22 (see Table 2 for its thermal conductivity). The chemical composition of the material is listed in Table 5. Ferritic low-alloy material such as SA213-T22 generally cannot withstand highly oxidizing environment for a long period of time. The use of the material is usually limited to locations where the temperatures are relatively lower. The steam-side scale is usually reported to be duplex (inner spinel layer and outer magnetite layer) or triplex (inner spinel layer, middle magnetite layer and outer hematite layer). In this study material of the scale is treated to be all magnetite. Phenomenon of heat transfer inside the boiler tube is considered as forced convection with turbulent flow. Correlation for fully developed turbulent flow in tube is expressed as [11]: Nu s ¼ 0:023ðRe s Þ 0:8 ðpr s Þ 0:4 where Re s is Reynolds number that may be expressed as Re s ¼ 4 m o s pdl s Oxide Scale X Steel Remaining metal thickness Initial tube thickness Hot gases Fig. 1. Model of the superheater and reheater tubes with scale on the inner surface. Table 1 Geometries of the tubes. Tube Inner radius (m) Outer radius (m) Table 2 Properties of steam and solid materials. ð1þ ð2þ Temperature, 540 C Temperature, 605 C Inlet steam properties [11] Thermal conductivity W/m C W/m C Specific heat 2161 J/kg C 2205 J/kg C Density kg/m kg/m 3 Dynamic viscosity e 05 N s/m e 05 N s/m 2 Water wall properties [7] Tube material SA213-T22 Thermal conductivity W/m C Fe 3 O 4 iron oxide (magnetite) [7] Thermal conductivity W/m C

3 1024 J. Purbolaksono et al. / Corrosion Science 51 (2009) Table 3 Combinations of the studied models. Model o in which m s is mass flow rate of the steam; D is the inner diameter of the tube; l s is steam viscosity, and Pr s is its Prandtl number that is defined as Pr s ¼ l s Cp s k s Steam temperature ( C) Mass flow rate (kg/h) Steam pressure (MPa) Flue gas temperature ( C) in which Cp s and k s are specific heat and thermal conductivity of the steam, respectively. Eq. (1) must comply with the following conditions: Fluid properties are evaluated at the mean bulk temperature. Re s > 10; :7 < Pr s < 100. L/D P 10; L is the length of the tube. Tube Table 4 Properties of the flue gas and convection coefficients at different temperatures [11]. Temperature ( C) Dynamics viscosity l g (N s/m 2 ) Specific heat Cp g (J/kg C) Thermal conductivity k g (W/m C) Table 5 Chemical composition of SA213-T22 [12]. Code C Si Mn P (max) SA213- T S (max) Convection coefficient of steam film for fully developed turbulent flow in circular tube is expressed as [11]: h s ¼ 0:023 k s D ðre sþ 0:8 ðpr s Þ 0:4 ð4þ where k s is steam conductivity. The convection coefficients h s on the internal surface of the boiler tube are obtained from Eq. (4) using parameters given in Tables 2 and 3. The coefficient values are presented in Table 6. Cr Table 6 The convection coefficients h s on internal surface of the boiler tube. Model Mo ð3þ h s (W/m 2 C) Heat transfer outside the boiler tube is considered as forced convection due to cross flow of the hot flue gas over bare tubes. A conservative estimated convection coefficient of flue gas h g on outer surface of bare tube in inline and staggered arrangements (see Fig. 4) is given by [13] h g ¼ 0:33 12k g d 0 ðre g Þ 0:6 ðpr g Þ 0:33 ð5þ where k g is flue gas conductivity; d 0 is outer diameter of the tube; Pr g is defined as Pr g ¼ l g Cp g k g in which Cp g and k g are specific heat and thermal conductivity of the flue gas, respectively. The corresponding Reynolds number Re g may be expressed as Re g ¼ Gd 0 12l g where G is gas mass velocity and may be defined as W g G ¼ 12 N w LðS t d 0 Þ in which W g is gas flow; N w is number of tube wide; S t is transverse pitch (see Fig. 4), and L is the tube length. In this study parameters used to determine gas mass velocity are given in Table 7. Compositions of flue gas at 15% excess air as shown in Table 8 is used in this study. The convection coefficients h g on external surface of the boiler tube are obtained from Eq. (5) using parameters given in Tables 4 and 7. The coefficient values are presented in Table 9. Superheater and reheater tubes operate at a continually increasing temperature, and a prediction must be made of scale thickness as a function of time and temperature. In this work in order to perform a scale growth prediction, steam-side scale formation for ferritic steel of 1 3% chromium correlated with the Larsen-Miller parameter as reported by Rehn et al. [8] is utilized (see Fig. 2). The data of Fig. 2 may be approximated as log X 0:0254 ¼ 0:00022P 7:25 where X is scale thickness in mm. In the Larsen-Miller method, time and temperature are related by the following equation: 9 5 T þ 492 ðc þ log tþ ¼P ð6þ ð7þ ð8þ ð9þ ð10þ where P is the Larsen-Miller parameter; T is the temperature in degree Celsius; t is the service time in hours; C is a constant equal to 20. The increasing of metal temperature DT for the reheater or superheater tube (SA213-T22) may be obtained from the Table 7 Parameters used to determine gas mass velocity G. Gas flow (kg/h) 400,000 Number of tube wide 32 Transverse pitch (m) 16 Tube length (m) 10 Table 8 Compositions of flue gas at 15% excess air [13]. Nitrogen (mole %) Oxygen (mole %) 2.46 Carbon dioxide (mole %) 8.29 Water (mole %) 18.17

4 J. Purbolaksono et al. / Corrosion Science 51 (2009) Table 9 The convection coefficients h g on external surface of the boiler tube. Temperature ( C) Outer diameter of tube (m) Convection coefficient h g (W/m 2 C) Scale thickness, µm Field observations year s exposure 538 0C - 5 yr 538 0C - 1 yr 621 0C - 5 yr 538 0C - 100,000 hr Scale thickness Penetration Laboratory data X 5380C/ 2500 hr Y 6210C/ 2500 hr Z 649 0C/ 2500 hr Parameter 34,000 36,000 38,000 40,000 42,000 P = (9/5 T + 492) (20 + log t) Fig. 2. Steam-side scale formation for ferritic steels of 1 3% chromium correlated with the Larsen-Miller parameter [8]. numerical simulation corresponding to the given running hours and scale thickness. In this work, the simulations performed for the predictions are made up to the maximum of 160,000 h with an increment of time as shown in Table 10. The iterative procedures used to determine scale thickness as a function of time and temperature are as follows: Step 1. The design temperature for the steam is set to T s at the inlet of reheater or superheater tube. From the numerical simulation in the absence of scale (X 0 ), the average temperature of T ave1 is the temperature on the inner surface of the tube. Eqs. (9) and Table 10 Total time after the steps of time used in the iterative procedure. Step , , , , , , , , ,000 h (10) are used to calculate the scale thickness of X 1a for the service hours of 1 h and the scale thickness of for the service hours of 250 h (see Table 10) using the average temperature of T ave1. Next, by subtracting one from the other, the scale increase of DX 1 (= X 1b X 1a ) is determined and a new scale thickness of X 1 (= X 0 + DX 1 ) is obtained. Step 2. The average temperature of T ave2 is then determined from the numerical modeling with the new scale thickness on the inner surface. The average temperature of T ave2 obtained from the average of the inner surface and the scale/metal interface temperatures is then used to calculate the incremental scale thickness from 250 to 500 h using Eqs. (9) and (10). For service hours of 500 h, P is calculated using Eq. (10) and X 2b is found from Eq. (9). For service hours of 250 h, P is calculated using Eq. (10) and X 2a is found from Eq. (9). Subtracting one from the other (X 2b X 2a ) produces the incremental scale formation from 250 to 500 h, which is added to X 1 to give a new scale thickness of X 2. Repeat Step 2 for further predictions up to the maximum of 160,000 h with the steps of time shown in Table 10. Since the initial increment of time determines the further estimation results, it is proposed to use the steps of time as shown in Table 10. A smaller increment of time might provide a better estimation, whereas a bigger increment of time for initial iteration may be resulting in inaccuracy estimation or less conservative prediction. For estimations starting from the service hours of 20,000 h, an increment of time may be proposed to be taken at every 20,000 h. There is general agreement that the growth of thick scales on iron follows a parabolic-rate law [7] and [14]. However, in the extreme conditions with very high steam and flue gas temperatures the scale growth will initially follow a parabolic-rate and then tends to follow an exponential-rate [3]. Finite element models are generated according to the geometry of the tube, the scale thickness and heat transfer parameters governing the problem. It is important to note that all the geometrical units used for modeling are in m. Hence, the meshing size control of 01 is used to generate the 2D solid triangular elements in order to allow the model having appropriate size of elements. The properties of the elements are then defined as 2Daxisymmetric solid elements. As stated in Step 1, the finite element model is produced in the absence of oxide scale. The bulk temperature and convection coefficient h g of the flue gas are applied on the right edge of the model (see Fig. 1). Next, the bulk temperature and convection coefficient h s of the steam are applied on the left edge of the model. In the presence of the oxide scale as stated in Step 2 the model will have two domain areas, i.e. scale and tube metal. In order to make connectivity of the domain areas at scale/metal interface, a merge-size control of 001 is used. The merge-size control should considerably be smaller than the meshing size control. The bulk temperature and convection coefficient h g of the flue gas are applied on the right edge of the tube metal region and the bulk temperature and convection coefficient h s of the steam are applied on the left edge of the oxide scale region. Fig. 3 shows the isometric view of the 3=4 expansion model from 2D-axisymmetric model and the estimated temperature distribution in Tube 1 with the steam temperature of 540 C and the flue temperature of 900 C for the service hours of 60,000 h. 3. Results and discussion Results of scale thickness predictions over the increasing of service hours obtained using the iterative procedures with different heat transfer parameters as specified in Tables 2 4 are plotted in

5 1026 J. Purbolaksono et al. / Corrosion Science 51 (2009) Model 3, Tube 1 Model 5, Tube 2 Model 6, Tube 3 3/4 expansion 2D axisymmetric Fig. 3. Temperature distribution in Tube 1 with the steam temperature of 540 C and the flue temperature of 900 C for the service hours of 60,000 h. Fig. 5. Estimated scale thickness as a function of time with the steam temperature of 540 C for different tube geometries (see Table 3). Figs The corresponding temperatures at scale/metal interface are presented in Table 11. Some features may be deduced according to the specified parameters as follows: Geometries of the tubes. Three different sizes of tube as shown in Table 1 are used in this study. It can be seen from Fig. 5, the largest estimated scale thickness at every service hours is found in Tube 2. According to the geometry of the tubes shown in Table 1, inner radius of Tube 2 is bigger than the radius of Tube 3; the tube thickness of Tube 2 is larger than the thickness of Tube 1. It is clear that the geometry of tube influences the growth of oxide scale. The thinner tube has less incremental scale formation. It means that the metal temperature is also less increasing (see Table 11). However, the thinner tube causes higher operational hoop stress. Conversely, the thicker tubes have the greater growth of scale as a result of higher temperature increase. It may result in changes of microstructure of the tube metal and cause material degradation. Mass flow rate of steam. Mass flow rate is taken into account in determining coefficient of the forced convection. The lower mass flow rate of steam at a design temperature will increase the growth of oxide scale on the inner surface. It may indicate that the poor or impaired mass flow rate of the steam, e.g. blocking steam flow, will cause significant increase of the scale growth as a result of higher temperature developed in scale/tube metal interface (see Table 11). The effect of different mass flow rate of steam on scale growth is shown in Fig. 6. Steam temperature. For design temperature of 605 C shown in Fig. 7, the curve tends to have higher increasing of scale thickness at the same of service hours. This feature is related to the reduced oxidation resistance if the tube metal has higher temperature. The increasing of scale thickness consequently becomes larger, and it will result in the greater temperature increase in the tube metal (see Table 11). Convection coefficient on the outer surface of tube and flue gas temperature. It can be deduced from Fig. 8, the higher convection coefficient and higher flue gas temperature lead to a larger increase of scale thickness. In order to estimate the growing of scale in a certain tube, the convection coefficient and flue gas temperature used in finite element modeling may be specified according to the particular location of the tube, for instance, the tube situated facing directly to the furnace section has different convection coefficient and flue gas temperature. Increments of time. Fig. 9 shows that a bigger increments results in less conservative estimation. Initial increments are particularly important to be treated since they will significantly affect further estimation during the iterations. In order to validate the reference data used shown in Fig. 2 and the results obtained, comparison of the data for a long-term exposure needs to be made. However, the heat transfer parameters and Fig. 4. Inline and staggered arrangements of the bare tubes.

6 J. Purbolaksono et al. / Corrosion Science 51 (2009) Model 1, 3600 kg/h Model 2, 720 kg/h 0.50 Model 1, C Model 3, C Fig. 6. Estimated scale thickness as a function of time with the steam temperature of 540 C for different mass flow rates of steam (see Table 3) Model 1, C Model 7, C Fig. 7. Estimated scale thickness as a function of time for different steam temperatures (see Table 3). geometry of the specimens for the reference data (Fig. 2) are not given. It might give the comparison to be compromised. Scale thickness for service time of 100,000 h at 538 C is used for comparison with the estimated results obtained using the proposed technique. Model 4, C Fig. 8. Estimated scale thickness as a function of time for different flue gas temperatures (see Table 3). The corresponding Larsen-Miller parameter P for service time of 100,000 h at 538 C is 36,500. It can be obtained from Fig. 2 that the corresponding scale thickness is around lm ( mm). It can be seen from Figs. 5 8 that the estimated results for the models with steam temperature of 540 and mass flow rate of 3600 kg/h are ranging from 0.25 to 0.47 mm and showing higher than those shown in Fig. 2. The differences might be due to different parameters used. According to Figs. 5 8, parameters governing the problem such as higher mass flow rate of steam, lower steam and flue gas temperature and smaller inner diameter and thickness of the tube may reduce the scale growth rate. Further, it is important to compare the estimated values with the actual data of the available reports [15] and [16] which were obtained from dimensional measurements for the scale thickness of the as-received reheater tube samples taken from Kapar Power Station Malaysia. Two different cases with different tube diameters from two different locations are used. The detailed samples used are shown in Table 12. Sample for Case 1 was located at the first row facing to the burner while the sample for Case 2 was located a relatively further from the burner. Operating steam temperature of both tubes is 576 C. The flue gas temperatures were reported ranging from C. Parameters used to determine gas mass velocity G and the estimated convection coefficients h s and h g for the internal and external surfaces are shown in Tables 13 and 14, respectively. The estimated scale thickness and the actual data are plotted in Fig. 10. It can be seen that the scale thickness for the actual data of Case 1 is relatively close to the estimated scale Table 11 Temperatures ( C) at the scale/metal interface of the tubes. Service time (h) Model 1 Model 2 Model 3 Model 4 Model 5 Model 6 Model , , , , , , , , ,

7 1028 J. Purbolaksono et al. / Corrosion Science 51 (2009) Model 1 with steps of time as specified in Table 10 Model1 with increment of time at every 20,000 h Fig. 9. Estimated scale thickness as a function of time for different increment of time Estimated Case 1, C Estimated Case 1, C Actual Case 1 [15] Estimated Case 2, C Estimated Case 2, C Actual Case 2 [16] Table 12 Oxide scale thickness and geometry of the as-received tubes. Fig. 10. The estimated scale thickness and the actual data. Case Inner radius (m) Tube thickness (mm) Service time (h) Year of failure , , Scale thickness, mm Table 13 Parameters used to determine gas mass velocity G for validation of actual data. Gas flow (kg/h) 500,000 Number of tube wide 50 Transverse pitch (m) 16 Tube length (m) Model 1 B = 0.5 B = 1.0 B = 2.0 Table 14 The estimated convection coefficients h s and h g for internal and external surfaces, respectively. Case h s (W/m 2 C) h g (W/m 2 C) thickness for the flue gas temperature of 900 C. It is apparently shown to be in good agreement with data with respect to the location of the tube since the higher temperature of the flue gas will accelerate scale growth over period of time. Bigger inner diameter and thickness of the tube may also accelerate scale growth. The scale thickness for Case 2 also fairly agrees with the estimated scale thickness for the flue gas temperature of 800 C. It is essential to make appropriate monitoring of the heat transfer parameters which may govern the problem from time to time. In order to show that the growth of thick scales on iron generally follows a parabolic-rate law as stated by French [7] and Uhlig [14], a temperature increase DT as a function of a scale thickness increase DX over a period of time may be used as DT ¼ B DX ð11þ where B is a constant. DT in Eq. (11) may be introduced by adding it with the temperature on the inner surface stated in Step 1 to replace the average temperature in Step 2. Model 1 is used to examine this procedure. It can be seen from Fig. 11 that the oxide scale Fig. 11. The oxide scale growths for different constants B for the approximated increases of temperature. growths for different B tend to follow parabolic-rate law. For instance, if B is set on trial to be 1 for Model 1, the estimations for the scale thickness using the given DT over period of time are shown to be in very good agreement with those obtained using procedures given in Steps 1 and 2. It means that the approximated value for B can be well determined for future predictions after estimations of scale thickness over period of time for the corresponding condition using Steps 1 and 2 have been made. The oxide scale thickness developed on the inner surface of reheater and superheater tubes over period of time can be estimated by utilizing the empirical formula correlating scale thickness with Larsen-Miller parameter and the finite element modeling. In the finite element simulations, the data for tube geometries and all the heat transfer parameters that might govern the problem are taken into account in order to determine the temperature distribution in tube metal. The temperature increase is an important influence on the scale growth rates. Data for heat transfer parameters according to variations in the operating conditions in the system may be introduced into the iteration procedure. Better estimation of the scale growth could be obtained, provided that all the heat transfer parameters used are well specified.

8 J. Purbolaksono et al. / Corrosion Science 51 (2009) Conclusions Estimation on the oxide scale growth in superheater and reheater tube utilizing the empirical formulae and the finite element modeling was proposed. An iterative procedure was used to determine scale thickness as both temperature and time increase. The scale thickness and the corresponding temperatures at scale/metal interface over period of time for two different design temperatures for steam and different heat transfer parameters were presented. The scale growths were influenced by the tube geometry and heat transfer parameters such as steam temperatures, mass flow rates of steam, flue gas temperatures and convection coefficients on the outer surface of tube. The procedures may provide better estimation on the oxide scale growth, provided that all the heat transfer parameters are well specified. Acknowledgements This work is supported by the Ministry of Science Technology and Innovation, Malaysia through the research projects of IRPA EA001 and Sciencefund SF0003. The authors wish to thank Universiti Tenaga Nasional, Kapar Energy Ventures Sdn Bhd and TNB Research Sdn. Bhd Malaysia for permission of utilizing all the facilities and resources during this study. References [1] R.D. Port, H.M. Herro, The NALCO Guide to Boiler Failure Analysis, Nalco Chemical Company, McGraw-Hill Inc., [2] D.R.H. Jones, Creep failures of overheated boiler. Superheater and reformer tubes, Engineering Failure Analysis 11 (2004) [3] K.J. Clark, S.R. Paterson, T.W. Rettig, Remaining Life Assessment of Superheater and Reheater Tubes, Aptech Engineering Services Inc., Sunnyvale, California, [4] R. Viswanathan, S.R. Paterson, H. Grunloh, S. Gehl, Life assessment of superheater reheater tubes in fossil boilers, Journal of Pressure Vessel Technology, Transactions of the ASME 116 (1) (1994) [5] R. Viswanathan, H. Grunloh, S.R. Paterson, S. Gehl, Life assessment of superheater/reheater tubes, American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP 240 (1992) [6] T.J. Wardle, Creep-rupture assessment of superheater tubes using nondestructive oxide thickness measurements, in: International Conference on Life Management and Life Extension of Power Plant, China, May [7] D.N. French, Metallurgical Failures in Fossil Fired Boilers, A Wiley-Interscience Publication, John Wiley and Sons Inc., New York, [8] I.M. Rehn, W.R. Apblett Jr., J. Stringer, Controlling steamside oxide exfoliation in utility boiler superheaters and reheaters, Mater Performance (1981) [9] P.J. Ennis, W.J. Quadakkers, Implications of steam oxidation for the service life of high-strength martensitic steel components in high-temperature plant, International Journal of Pressure Vessels and Piping 84 (2007) [10] F. Starr, J. Castle, R. Walker, Potential problems in the identification of the root cause of superheater tube failures in 9Cr martensitic alloys, Materials at High Temperatures 21 (3) (2004) [11] F.P. Incropera, D.P. DeWitt, Introduction to Heat Transfer, third ed., John Wiley, [12] J.E. Bringas, Handbook of Comparative World Steel Standards ASTM DS67A, second ed., [13] V. Ganapathy, Industrial Boilers and Heat Recovery Steam Generators: Design, Applications, and Calculations, Marcel Dekker, New York, [14] H.H. Uhlig, Corrosion and Corrosion Control, second ed., Wiley, New York, [15] J. Ahmad, Technical memorandum of Kapar Power Station Sdn Bhd Malaysia: Remaining Life Assessment of Reheater Tube, TNB Generation Sdn Bhd, Mei [16] J. Ahmad, Technical Memorandum of Kapar Power Station Sdn Bhd Malaysia: Report of Reheater Tube Failure, TNB Generation Sdn Bhd, January 2004.