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Thermal load deviation model for superheater and reheater of a utility boiler
Applied Thermal Engineering 20 (2000) 545±558
www.elsevier.com/locate/apthermeng
Thermal load deviation model for superheater and reheater of a utility boiler Lijun Xu a,*, Jamil A. Khan a, Zhihang Chen b a
Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA University of Shanghai for Science and Technology, 516 JunGong Road, Shanghai 200093, China
b
Received 18 September 1998; accepted 11 May 1999
Abstract The extreme steam temperature deviation experienced in the superheater and reheater of a utility boiler can seriously aect its economic and safe operation. This temperature deviation is one of the root causes of boiler tube failures (BTF), which causes about 40% of the forced power station outages. The steam temperature deviation is mainly due to the thermal load deviation in the lateral direction of the superheater and reheater. This variation is very dicult to measure in situ using direct experimental techniques. In this paper, we propose a thermal load model that is based on the power plant thermodynamic parameters, thermal deviation theory, and ¯ow rate deviation theory. It is found that the calculated results from our model agree well with the in situ experimental results. The predicted BTF positions are the same as that in the reheater of a 300 MW utility boiler at Wujing Power Plant. The proposed model has been used to improve the design of utility boiler in Boiler Works, predict the possible BTF in the design stage, and assess the existing designs. This model can also be applied to utility boilers of dierent manufactures, and has been successfully applied to the BTF prediction and prevention in the Power Station. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Utility boiler; Superheater; Reheater; Thermal load deviation; Thermal deviation; Flow rate deviation; Boiler tube failure
* Corresponding author. 1359-4311/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 9 9 ) 0 0 0 4 9 - 6
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Nomenclature d E0 F G k H l p q Q s1 s2 x xf a Di Dt z Z x t
tube outer diameter correction factor of middle tube area heat transfer area ¯ow rate resistance coecient maximum dynamic pressure of header inlet/outlet heated tube length pressure drop absorbed energy per unit area total absorbed energy tube longitudinal separation tube transverse separation location axis view factor total friction resistance enthalpy rise between the inlet and outlet of the tube or tube row temperature dierence thermal deviation coecient deviation coecient heat transfer deviation factor gas average penetration rate
Subscripts av average c collection header d distribution header ¯ ¯ow rate fr front gas room radiation i ith row tubes max maximum mc middle tubes bundle convection min minimum mr middle tubes bundle radiation rr rear gas room radiation st structure th thermal load tot total x location axis along header w width direction h height direction Superscripts a tube segment A
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1. Introduction The boiler tube failures (BTF) often occur in the superheaters and the reheaters of a utility boiler, after the boiler has operated for a period of time. This BTF seriously aects the economic and safe operation of the utility boiler. It has been observed that the BTF causes about 40% of the forced outage rate of power stations [1]. BTF continue to be the single largest source of the forced outages in the fossil-®red electricity generating stations. Even in the US, the BTF of the superheater and the reheater is the leading cause of forced outages at the fossil-®red utility boiler [2]. Eliminating BTF could save the electric power industry about $5 billion a year [3]. It has been observed that the BTF continues to occur on the same tube, same material or in the same boiler sections over and over again [4]. Signi®cant eort has been expended to predict the BTF and to determine the mechanisms responsible for the BTF by utility boiler companies, boiler manufactures, and by academic researchers [5±20]. The imperfection in the design method of the utility boiler and the thermal load deviation in the lateral direction are the leading causes for the BTF in most superheaters and reheaters. For example, Shanghai Boiler Works in China utilized the most recently patented technology of Combustion Engineering Company in USA (ABB-CE) to design the ®fth utility boiler with a steam capacity of 1025 ton/h at Shiheng Power Station, China. This boiler begun commercial operation on June 30, 1987. The ®rst BTF occurred on September 28, 1989 at the welding joints of the left-hand side of the 55th row for the boiler tubes of the ®nal stage reheater. The BTF location is shown in the steam ¯ow diagram of Fig. 1. The boiler tube failed again at the same position on April 1, 1990. In another case, the Harbin Boiler Co. of China designed and built its ®rst 600 MW assessment utility boiler for Pingwei Power Plant by utilizing the ABBCE's newest patented technology. This utility boiler experienced steam temperature deviations in the range of 20±408C at the boiler reheater tubes, detected from the left-hand side to the right-hand side (Fig. 1, Editor's note). As a result the steam temperature could not meet the design target, and aected the safe and economic operation of the plant. On this facility the BTF occurred on the individual tubes on August 10, 1992. In a third case, a 350 MW imported utility boiler from Japan experienced BTF at Baoshan Iron and Steel (Group) Corp. on September 23, 1988. The position of the BTF was the ®fth tube's lower welded joint on ®fth row of the right-hand side, and the cause of the BTF was long-term overheating [1]. It can be mentioned that most of the large capacity utility boilers use the four angletangential ®ring systems. The main advantage of this kind of the ®ring system is that it works eciently for a wide variety of coals. The inherent strengths of the tilting tangential ®ring system are its high combustion eciency, consistent thermal performance and low emissions. In this type of ®ring system, the rotating ¯ame envelope formed within the furnace delivers thermal energy uniformly to each of the furnace walls, independent of unit load or fuel input combinations. A feature unique to tangentially ®red boilers is the ability to regulate furnace heat absorption for steam temperature control, which is done by tilting the fuel and air nozzle assemblies up or down automatically. Superheat and reheat temperatures can, thus, be controlled with minimal plant heat rate impact. Global furnace aerodynamics provide eective furnace volume utilization for higher heat absorption and lower bulk gas temperatures compared to wall-®red boilers. Complete combustion is assured by the combination of maximum residence time and vortex turbulence. But, the vortex turbulence of the tilted
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Fig. 1. Steam ¯ow diagram for the reheater of the 300 MW utility boiler in Wujing Power Station.
tangential ®ring system intensely aects the thermal load distribution in the convection horizontal gas passage, which increases the thermal load deviation. Development of high-capacity utility boilers, operating at high temperatures and pressures, results in increased thermal load deviation of the boiler in the lateral direction with horizontal gas passage. In spite of its many advantages, the thermal load deviation is inherent for the tangential ®ring system and cannot be eliminated completely. However, with the availability of improved models, like the one proposed here, design changes can be made to mitigate the severity of temperature deviations in the boiler tubes. It has been determined via in situ experimental measurements in many utility boilers that the thermal load deviation of the superheaters and reheaters is in the range of about 1.15±1.25 [1]. The distribution of the thermal load deviation for the tangential ®ring system in the utility boiler is almost the same, and is independent of the load and the number of burners in use. The magnitude of the structural deviation between the rows is small. The tee joint distributions of the headers aect the ¯ow rate deviation, and there is a vortex area in the vicinity of the tee joint [12±14]. Based on the ABB-CE calculation standard, the maximum thermal deviation is taken to be 1.12 along the gas passage lateral direction, which includes the ¯ow rate deviation between the rows [13,14].
L. Xu et al. / Applied Thermal Engineering 20 (2000) 545±558
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In this paper, we propose a thermal load deviation model that is based on the power plant in situ thermodynamic parameters, thermal deviation theory and ¯ow rate deviation theory. The proposed model has been used to improve the design of the utility boiler in Boiler Works, predict the possible failures in the design stage, and assess the existing designs. This model can also be applied to dierent types of utility boilers with dierent boiler manufactures, and has been successfully applied to the prediction and prevention of the BTF in the power station. In the design stages, the boiler can be assessed for BTF using the model and changes can be made to avoid this. The model can identify the location of maximum temperature deviation. It can be mentioned here that the ABB-CE calculation standard predicts only the maximum temperature deviation, but does not predict the location of this maximum deviation, and its maximum thermal deviation is less than the experimental measurements. Therefore, the proposed model is more robust. 2. Mathematical model The reheater and the superheater of the utility boiler are assembled by the manifold system such that the tube rows are parallel along the lateral direction of the boiler as shown in Figs. 1±3. The manifold systems are complicated and can vary depending on the types of utility boiler design. Wang and Chen [12,13] proposed the thermal deviation theory for the calculation of the heat transfer in the external surface of the complicated heated tubes. Several dierent modes of heat
Fig. 2. 300 MW utility boiler ¯ow diagram.
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transfer exist in the superheater and the reheater of the utility boiler, which include the front room gas radiation, the rear room gas radiation, and the tube bundle radiation and convection heat transfer as shown in Fig. 3. The heated tubes can be classi®ed into dierent groups; for example, the ®rst and the last tube in the ¯ow direction, the hanging tube, the contact tube, the middle tube, and dierent interval tube. In their model, each tube is divided into a number of segments based on the type of boiler tube. The ¯ow rate deviation theory was proposed by Chen [12] to solve the ¯ow rate distribution of the boiler tubes for the complicated manifold system. Fig. 2 shows the ¯ow diagram of a 300 MW utility boiler. The tube arrangements of one row for the plate and the ®nal reheaters are shown in more detail in Fig. 3, in which the ¯ow directions of the steam and the gas are also displayed. Fig. 1 is the schematic diagram of the direction of the steam ¯ow for the reheater of the 300 MW utility boiler of Wujing Power Station, China. Steam inlet and outlet ports for the tee joints along the distribution header and the collection headers are also shown. The steam from the wall reheater ¯ows into the distribution header through four tee joints. Two rows of the tubes from the distribution header are grouped into one tube row in the plate reheater as shown in Figs. 1 and 2. The plate reheater consists of 30 tube rows with 14 heated tubes (see Figs. 1 and 3). On the outlet of the plate reheater, one tube row of the plate reheater is divided into two tube rows. The ®nal reheater is assembled by 60 tube rows with 7 heated tubes as shown in Figs. 1 and 3. The steam ¯ow may vary due to the manifold system of the utility boiler, as a result the ¯ow may be signi®cantly lower in some tubes than in others. The poor cooling on the inside
Fig. 3. Tubes arrangement of tube row for plate and ®nal reheater.
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coupled with excessive heating on the outside may result in wall temperatures above the acceptable limits, leading to tube burn out. The ¯ow diagram of the calculation algorithm of the thermal load simulation of the proposed model is shown in Fig. 4. The thermal load deviation model includes three parts, namely the structural deviation model, the ¯ow rate deviation model, and the thermal deviation model. The structural deviation model is used to describe the actual structure of the reheater and the superheater in the utility boiler with the help of the database ®le, and is used to calculate the structural deviation. The ¯ow rate deviation model simulates the ¯ow rate and the heat absorbed by each boiler tube. The thermal deviation model is used to account for the absorbed heat on the external surface of the tubes. The thermal load deviation is obtained by an iterative procedure based on the three models and the in situ data. 2.1. Thermal deviation mathematical model Thermal deviation is a ratio of the enthalpy rise of the heated tube/row to the average enthalpy rise of that in the superheater and reheater of the utility boiler. The thermal deviation coecient, z, is expressed as a function of the thermal load deviation coecient Zth , the structural deviation coecient Zst , and the ¯ow rate deviation coecient Zfl as follows [1]: z
Di Q Gav q=qav F=Fav Z Zst th Diav Qav G G=Gav Zfl
Fig. 4. Thermal simulation calculation algorithm ¯ow diagram.
1
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Based on the thermal deviation theory each boiler tube is divided into a number of segments. An energy balance is made for each of the tube segments. For example, the total absorbed energy Qa by the tube segment A is calculated via an energy balance by adding the front gas room absorbed radiation energy Qafr , the rear gas room absorbed radiation energy Qarr , the tube bundle radiation energy Qamr , and the tubes bundle convection energy Qamc [13,14]. Qa Qafr Qarr Qamr Qamc
2
The front gas room absorbed radiation energy by the tube segment A, Qafr , is given as: Qafr qafr l s1 xffr tfr qafr xafr F a
3
The rear room absorbed radiation energy by the tube segment A, Qarr , is de®ned as: Qarr qarr l s1 xfrr trr qarr xarr F a
4
The tubes bundle radiation energy by the tube segment A, Qamr , is expressed as, Qamr qamr l s2 2xf qamr xamr F a
5
The energy of the tubes bundle convection heat transfer by the tube segment A, Qamc , is as follows: Qamc qamc p d l qamc xamc F a
6
The total energy Qa absorbed by the tube segment A, may then be expressed by combining Eqs. (2)±(6). ÿ Qa Qafr Qarr Qamr Qamc F a qafr xafr qarr xarr qamr xamr qamc xamc 7 In view of the thermal load deviation in the horizontal and vertical directions, the energy Qawh absorbed by the tube segment A is determined by using thermal load deviation coecient Zth,w in the horizontal (width) direction and thermal load deviation coecient Zth,h in the vertical (height) direction as follows: Qawh Zth,w Zth,h Qa ÿ Zth,w Zth,h F a qafr xafr qarr xarr qamr xamr qamc xamc
8
Taking into account the calculated tube segment temperature dierence Dt and the average temperature dierence Dtav , the energy absorbed by the segment A, Qatot , can be rewritten as follows: Qatot Qawh
Dt Zth,w Zth,h Dtav
ÿ Dt F a qafr xafr qarr xarr qamr xamr qamc xamc Dtav The total absorbed energy for each of the tube is expressed as follows:
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Qtot Qatot Qbtot Qctot Qdtot
n X i1
553
Qitot
10
where a, b, c, . . . , etc. are the dierent segments that the tube is divided into. 2.2. Heat transfer deviation factors Table 1 shows the radiation deviation factors of heat transfer for the tube bundle, the front gas room and the rear gas room, and the deviation factors of the convection heat transfer for the superheater and the reheater [1,13,14]. The correction factor for the middle tube area, E0, is determined as [13,14] 9 8 " 2 #0:5 = < s2 d d d ÿ 1ÿ E0 2 :1 cosÿ1 11 ; s2 s2 s2 d 2.3. Flow rate deviation mathematical model According to the ¯ow rate deviation theory, the static pressures dierence, pd,x , at the distribution header with the Z type arrangement, at x location, is expressed as [12]: 1 2 3 12 pd,x Hd 0:76 1 ÿ 1 ÿ x ÿ ad 1 ÿ 1 ÿ x 3 The static pressure distribution, pc,x , for the collection header with the Z type arrangement at any x location, is de®ned as [12]: 1 2 3 pc,x Hc 2 1 ÿ 1 ÿ x ac 1 ÿ 1 ÿ x 13 3 With the assumption that there are n tube rows for the complicated header connection system, the governing equations for the ¯ow rate deviation calculation of the superheater and the reheater is as follows: Table 1 Deviation factors of radiation and convection heat transfer Radiation deviation factor for heat transfer of the front gas room Radiation deviation factor for heat transfer of the rear gas room Radiation heat transfer deviation factor for the middle tube Radiation heat transfer deviation factor for the ®rst and the last tube Radiation heat transfer deviation factor of hanging tube Radiation heat transfer deviation factor of contact tube Radiation heat transfer deviation factor of dierence interval tube Convection heat transfer deviation factor for ®rst and the last tube Convection heat transfer deviation factor for all other tubes
xafr s1 =d xffr =E0 tfr xarr s1 =d xfrr =E0 trr xmr 1:0 xmr 0:5 0:25 p=E0 xmr p=E0 xmr p ÿ 0:5 p x=E0 xmr x2mr x3mr =2 xmc xmr 0:25 p=E0 xmc xmr
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pd,1 ÿ pc,1 k1 G 21 .. . pd,i ÿ pc,i ki G 2i .. . pd,n ÿ pc,n kn G 2n n X G Gi
14
i1
where pd,i ÿ pc,i is the ith row tubes pressure dierence between the distribution header and the collection header, and ki G 2i is ith row tube's pressure drop. 2.4. Thermal load distribution Based on the literature survey, the traditional distributions of the thermal load along the utility boiler lateral direction can be summarized as follows [1]. The thermal load deviation Zth is a polynomial of the order of four for the boiler width axis: Zth a0 a1 x a2 x 2 a3 x 3 a4 x 4
15
where a0, a1, a2, a3 and a4 can be decided by the two minimum values and a maximum value of thermal load deviation, and their axis. Sixth power of the boiler width for the analysis of ABB-CE utility boiler ®eld is: Zth a0 a1 x a2 x 2 a3 x 3 a4 x 4 a5 x 5 a6 x 6
16
where a0, a1, a2, a3, a4, a5 and a6 can be determined by a minimum and two maximum thermal load deviations, and their axis. Assuming the maximum thermal load deviation at the center of the boiler width: Zth Zth min Ajx ÿ 0:5j1:5 Bjx ÿ 0:5j2:5
17
A 3:353b7 ÿ 5 Zth min ÿ2 Zth max c B 9:898b3 Zth min 2 Zth max ÿ5c Assuming a parabolic distribution of the thermal load: 2
Zth A ÿ B x ÿ C
18
where A, B and C are the coecients relative to the maximum thermal load deviation and its axis.
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Fig. 5. Outside wall metal temperature of the ®nal reheater outlet tubes for the 300 MW utility boiler.
3. Simulation calculation and discussion As mentioned earlier, Fig. 4 shows the ¯ow diagram for the calculation algorithm used to get a converged solution of the thermal load simulation model. The calculations were performed by using experimentally measured utility boiler outer tubes' wall metal temperatures, the average inlet and outlet gas temperatures, the thermal deviation theory and the ¯ow rate deviation theory. All the experimental results were from in situ measurements made at the 300 MW utility boiler at Wujing Power Station, which uses the four angles reverse-tangential ®ring systems. Flow diagrams and tube arrangements are shown in Figs. 1±3. The calculated results based on the model are presented in Figs. 5±7. Fig. 5 compares the experimental value of the reheater outlet tube wall metal temperatures for the 300 MW utility boiler reheater of Wujing Power Station, with the values calculated from the simulation. The maximum temperature dierence between the calculated and the experimental results is found to be about 5.58C as shown in Fig. 5. The relative errors in the temperature of the model calculation results are within 1% of the in situ experimental results. The calculated results agree very well with the experimental results. Fig. 6 presents thermal load deviation coecient as a function of the tube row number. It can be seen from the ®gure that the maximum deviation of the thermal load between the superheater and reheater rows is about 215%. This predicted result is greater than the ABBCE design calculation standard. Fig. 7 presents the calculated thermal deviation results. This ®gure also presents the structural and ¯ow rate deviation values. As seen in the ®gure, the
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Fig. 6. Simulation calculation results of thermal load deviation coecient for the 300 MW utility boiler's ®nal reheater.
maximum ¯ow rate deviation between the reheater rows is about 212%. It is also observed that the ¯ow rate deviation below the normal ¯ow rate, i.e., lower ¯ow rate results in larger temperature dierence. Therefore, it may be concluded that the key contributing factors for thermal deviation is the ¯ow rate deviation and the thermal load deviation between rows. Thus, by minimizing the thermal deviation the key cause of the BTF may be eliminated. The maximum thermal deviation between the rows is about 29% at the tubes of the sixth row shown in Fig. 7. It can also be mentioned that the locations of the actual BTF indeed coincided with the location of the maximum thermal deviation. The in situ BTF occurred at the sixth row tubes of the reheater from left side, as shown in Fig. 1, in the 300 MW utility boiler of Wujing Power Station, in February 1995. The predicted results of the BTF by using the proposed model were the same as the in situ location of the BTF. 4. Conclusions In this paper, we propose a thermal load model that is based on the power plant in situ thermodynamic parameters, thermal deviation theory and ¯ow rate deviation theory. The model makes a systematic ¯ow analysis and makes an energy balance on each segment of the boiler tubes. Solution of the resulting governing equations provides the thermal load for each segment of the tubes and the ¯ow rate of each tube. Following this approach, the proposed
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Fig. 7. Simulation calculation results of thermal deviation for 300 MW utility boiler ®nal reheater.
model has been used to improve the design of the utility boiler in Boiler Works. It has also been used to predict the possible failures in the design stage, and to assess the existing designs. This model can also be applied to dierent type of utility boilers built by dierent boiler manufactures. This model has actually been used to predict and prevent the BTF in the power station. The predicted position of BTF is the same as the reheater positions of BTF at a 300 MW utility boiler of Wujing Power Plant. The relative errors in temperatures of the model calculation results are within 1% of the in situ experimental results. In general, the maximum deviation of the thermal load between the superheater and reheater rows is about 215%, this is more than the ABB-CE design calculation standard. This is the BTF root cause for the large capacity utility boiler superheater and reheater.
References [1] L.J. Xu, Z.H. Chen, The simulation research of thermal load distribution for utility boiler, Power Engineering (Supplement) (1996) 402±406. [2] K. Clark, C. Lee, K. Hara, Redistribute steam to extend boiler-tube life, Power 134 (1990) 53±55. [3] P. Mayer, Boiler tube failure mechanism recognition Ð an expert system, CIM-Bulletin 83 (1990) 92±95. [4] R.W. Smock, EPRI declares war on power station boiler tube failures, Power Engineering 94 (1990) 10. [5] B. Dooley, Don't let those boiler tubes fail again, Power Engineering 56 (1997) 56±61.
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[6] M.H. Wang, The protection measure and the temperature deviation for the superheater and reheater of the high capacity utility boiler, Boiler Technology 3 (1992) 1±11. [7] R. Abbott, G.P. Singh, G.A. Lamping, Applying latest technology helps prevent boiler-tube failures, Power 136 (1992) 37±38. [8] R.C. Abbott, R. Tiley, S. Gehl, New software directs the ®ght against boiler tube failure, Power Engineering 96 (1992) 59±62. [9] D. Lopez-Lopez, A. Wong-Noreno, L. Martinez, Unusual superheater tube wastage associated with carburization, Materials-Performance 33 (1994) 45±48. [10] S. Collins, Computer calculation provides early warning of boiler tube leaks, Power 137 (1993) 72. [11] B. Dooley, A vision for reducing boiler tube failures, Power Engineering 96 (1992) 33±37. [12] Z.H. Chen, The calculation and analysis of the ¯ow distribution and the thermal variation of the single-phase ¯uid ¯owing through the manifold systems in boilers, Journal of University of Shanghai for Science and Technology 19 (1997) 160±178. [13] M.H. Wang, Thermal deviation of the same row in the convective superheater and reheater of utility boiler, Power Engineering 2 (1984) 11±17. [14] M.H. Wang, The calculation of thermal deviation for plate superheater, Power Engineering 2 (1986) 7±14. [15] W.J. Liu, The improved measure and cause analysis of partial overheat for 1025 ton/h utility boiler reheater, Boiler Technology 7 (1993) 4±12. [16] R.G. Yang, The improved measure and diagnosis of boiler tube failure for the reheater of 1160 ton/h utility boiler, Boiler Technology 2 (1991) 5±8. [17] R.G. Yang, BTF cause and diagnosis for the reheater of 1160 ton/h utility boiler at Baoshan Iron and Steel Group Corp, Boiler Technology 8 (1989) 1±8. [18] Q.L. Bian, The experimental study of the boiler tube failure for the high temperature superheater of 1000 ton/h utility boiler at Wangting Power Station, Boiler Technology 4 (1987) 1±9. [19] L.J. Xu, G. Qu, D. Liu, Solution method and mechanism of failure diagnosis for boiler pipe, Journal of Engineering for Thermal Energy and Power 9 (4) (1994) 226±229. [20] L.J. Xu, Research and analysis of boiler tubes failure diagnosis for superheater and reheater, Journal of Energy Research and Utilization 33 (3) (1994) 34±38.
www.elsevier.com/locate/apthermeng
Thermal load deviation model for superheater and reheater of a utility boiler Lijun Xu a,*, Jamil A. Khan a, Zhihang Chen b a
Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA University of Shanghai for Science and Technology, 516 JunGong Road, Shanghai 200093, China
b
Received 18 September 1998; accepted 11 May 1999
Abstract The extreme steam temperature deviation experienced in the superheater and reheater of a utility boiler can seriously aect its economic and safe operation. This temperature deviation is one of the root causes of boiler tube failures (BTF), which causes about 40% of the forced power station outages. The steam temperature deviation is mainly due to the thermal load deviation in the lateral direction of the superheater and reheater. This variation is very dicult to measure in situ using direct experimental techniques. In this paper, we propose a thermal load model that is based on the power plant thermodynamic parameters, thermal deviation theory, and ¯ow rate deviation theory. It is found that the calculated results from our model agree well with the in situ experimental results. The predicted BTF positions are the same as that in the reheater of a 300 MW utility boiler at Wujing Power Plant. The proposed model has been used to improve the design of utility boiler in Boiler Works, predict the possible BTF in the design stage, and assess the existing designs. This model can also be applied to utility boilers of dierent manufactures, and has been successfully applied to the BTF prediction and prevention in the Power Station. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Utility boiler; Superheater; Reheater; Thermal load deviation; Thermal deviation; Flow rate deviation; Boiler tube failure
* Corresponding author. 1359-4311/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 9 9 ) 0 0 0 4 9 - 6
546
L. Xu et al. / Applied Thermal Engineering 20 (2000) 545±558
Nomenclature d E0 F G k H l p q Q s1 s2 x xf a Di Dt z Z x t
tube outer diameter correction factor of middle tube area heat transfer area ¯ow rate resistance coecient maximum dynamic pressure of header inlet/outlet heated tube length pressure drop absorbed energy per unit area total absorbed energy tube longitudinal separation tube transverse separation location axis view factor total friction resistance enthalpy rise between the inlet and outlet of the tube or tube row temperature dierence thermal deviation coecient deviation coecient heat transfer deviation factor gas average penetration rate
Subscripts av average c collection header d distribution header ¯ ¯ow rate fr front gas room radiation i ith row tubes max maximum mc middle tubes bundle convection min minimum mr middle tubes bundle radiation rr rear gas room radiation st structure th thermal load tot total x location axis along header w width direction h height direction Superscripts a tube segment A
L. Xu et al. / Applied Thermal Engineering 20 (2000) 545±558
547
1. Introduction The boiler tube failures (BTF) often occur in the superheaters and the reheaters of a utility boiler, after the boiler has operated for a period of time. This BTF seriously aects the economic and safe operation of the utility boiler. It has been observed that the BTF causes about 40% of the forced outage rate of power stations [1]. BTF continue to be the single largest source of the forced outages in the fossil-®red electricity generating stations. Even in the US, the BTF of the superheater and the reheater is the leading cause of forced outages at the fossil-®red utility boiler [2]. Eliminating BTF could save the electric power industry about $5 billion a year [3]. It has been observed that the BTF continues to occur on the same tube, same material or in the same boiler sections over and over again [4]. Signi®cant eort has been expended to predict the BTF and to determine the mechanisms responsible for the BTF by utility boiler companies, boiler manufactures, and by academic researchers [5±20]. The imperfection in the design method of the utility boiler and the thermal load deviation in the lateral direction are the leading causes for the BTF in most superheaters and reheaters. For example, Shanghai Boiler Works in China utilized the most recently patented technology of Combustion Engineering Company in USA (ABB-CE) to design the ®fth utility boiler with a steam capacity of 1025 ton/h at Shiheng Power Station, China. This boiler begun commercial operation on June 30, 1987. The ®rst BTF occurred on September 28, 1989 at the welding joints of the left-hand side of the 55th row for the boiler tubes of the ®nal stage reheater. The BTF location is shown in the steam ¯ow diagram of Fig. 1. The boiler tube failed again at the same position on April 1, 1990. In another case, the Harbin Boiler Co. of China designed and built its ®rst 600 MW assessment utility boiler for Pingwei Power Plant by utilizing the ABBCE's newest patented technology. This utility boiler experienced steam temperature deviations in the range of 20±408C at the boiler reheater tubes, detected from the left-hand side to the right-hand side (Fig. 1, Editor's note). As a result the steam temperature could not meet the design target, and aected the safe and economic operation of the plant. On this facility the BTF occurred on the individual tubes on August 10, 1992. In a third case, a 350 MW imported utility boiler from Japan experienced BTF at Baoshan Iron and Steel (Group) Corp. on September 23, 1988. The position of the BTF was the ®fth tube's lower welded joint on ®fth row of the right-hand side, and the cause of the BTF was long-term overheating [1]. It can be mentioned that most of the large capacity utility boilers use the four angletangential ®ring systems. The main advantage of this kind of the ®ring system is that it works eciently for a wide variety of coals. The inherent strengths of the tilting tangential ®ring system are its high combustion eciency, consistent thermal performance and low emissions. In this type of ®ring system, the rotating ¯ame envelope formed within the furnace delivers thermal energy uniformly to each of the furnace walls, independent of unit load or fuel input combinations. A feature unique to tangentially ®red boilers is the ability to regulate furnace heat absorption for steam temperature control, which is done by tilting the fuel and air nozzle assemblies up or down automatically. Superheat and reheat temperatures can, thus, be controlled with minimal plant heat rate impact. Global furnace aerodynamics provide eective furnace volume utilization for higher heat absorption and lower bulk gas temperatures compared to wall-®red boilers. Complete combustion is assured by the combination of maximum residence time and vortex turbulence. But, the vortex turbulence of the tilted
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Fig. 1. Steam ¯ow diagram for the reheater of the 300 MW utility boiler in Wujing Power Station.
tangential ®ring system intensely aects the thermal load distribution in the convection horizontal gas passage, which increases the thermal load deviation. Development of high-capacity utility boilers, operating at high temperatures and pressures, results in increased thermal load deviation of the boiler in the lateral direction with horizontal gas passage. In spite of its many advantages, the thermal load deviation is inherent for the tangential ®ring system and cannot be eliminated completely. However, with the availability of improved models, like the one proposed here, design changes can be made to mitigate the severity of temperature deviations in the boiler tubes. It has been determined via in situ experimental measurements in many utility boilers that the thermal load deviation of the superheaters and reheaters is in the range of about 1.15±1.25 [1]. The distribution of the thermal load deviation for the tangential ®ring system in the utility boiler is almost the same, and is independent of the load and the number of burners in use. The magnitude of the structural deviation between the rows is small. The tee joint distributions of the headers aect the ¯ow rate deviation, and there is a vortex area in the vicinity of the tee joint [12±14]. Based on the ABB-CE calculation standard, the maximum thermal deviation is taken to be 1.12 along the gas passage lateral direction, which includes the ¯ow rate deviation between the rows [13,14].
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In this paper, we propose a thermal load deviation model that is based on the power plant in situ thermodynamic parameters, thermal deviation theory and ¯ow rate deviation theory. The proposed model has been used to improve the design of the utility boiler in Boiler Works, predict the possible failures in the design stage, and assess the existing designs. This model can also be applied to dierent types of utility boilers with dierent boiler manufactures, and has been successfully applied to the prediction and prevention of the BTF in the power station. In the design stages, the boiler can be assessed for BTF using the model and changes can be made to avoid this. The model can identify the location of maximum temperature deviation. It can be mentioned here that the ABB-CE calculation standard predicts only the maximum temperature deviation, but does not predict the location of this maximum deviation, and its maximum thermal deviation is less than the experimental measurements. Therefore, the proposed model is more robust. 2. Mathematical model The reheater and the superheater of the utility boiler are assembled by the manifold system such that the tube rows are parallel along the lateral direction of the boiler as shown in Figs. 1±3. The manifold systems are complicated and can vary depending on the types of utility boiler design. Wang and Chen [12,13] proposed the thermal deviation theory for the calculation of the heat transfer in the external surface of the complicated heated tubes. Several dierent modes of heat
Fig. 2. 300 MW utility boiler ¯ow diagram.
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transfer exist in the superheater and the reheater of the utility boiler, which include the front room gas radiation, the rear room gas radiation, and the tube bundle radiation and convection heat transfer as shown in Fig. 3. The heated tubes can be classi®ed into dierent groups; for example, the ®rst and the last tube in the ¯ow direction, the hanging tube, the contact tube, the middle tube, and dierent interval tube. In their model, each tube is divided into a number of segments based on the type of boiler tube. The ¯ow rate deviation theory was proposed by Chen [12] to solve the ¯ow rate distribution of the boiler tubes for the complicated manifold system. Fig. 2 shows the ¯ow diagram of a 300 MW utility boiler. The tube arrangements of one row for the plate and the ®nal reheaters are shown in more detail in Fig. 3, in which the ¯ow directions of the steam and the gas are also displayed. Fig. 1 is the schematic diagram of the direction of the steam ¯ow for the reheater of the 300 MW utility boiler of Wujing Power Station, China. Steam inlet and outlet ports for the tee joints along the distribution header and the collection headers are also shown. The steam from the wall reheater ¯ows into the distribution header through four tee joints. Two rows of the tubes from the distribution header are grouped into one tube row in the plate reheater as shown in Figs. 1 and 2. The plate reheater consists of 30 tube rows with 14 heated tubes (see Figs. 1 and 3). On the outlet of the plate reheater, one tube row of the plate reheater is divided into two tube rows. The ®nal reheater is assembled by 60 tube rows with 7 heated tubes as shown in Figs. 1 and 3. The steam ¯ow may vary due to the manifold system of the utility boiler, as a result the ¯ow may be signi®cantly lower in some tubes than in others. The poor cooling on the inside
Fig. 3. Tubes arrangement of tube row for plate and ®nal reheater.
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coupled with excessive heating on the outside may result in wall temperatures above the acceptable limits, leading to tube burn out. The ¯ow diagram of the calculation algorithm of the thermal load simulation of the proposed model is shown in Fig. 4. The thermal load deviation model includes three parts, namely the structural deviation model, the ¯ow rate deviation model, and the thermal deviation model. The structural deviation model is used to describe the actual structure of the reheater and the superheater in the utility boiler with the help of the database ®le, and is used to calculate the structural deviation. The ¯ow rate deviation model simulates the ¯ow rate and the heat absorbed by each boiler tube. The thermal deviation model is used to account for the absorbed heat on the external surface of the tubes. The thermal load deviation is obtained by an iterative procedure based on the three models and the in situ data. 2.1. Thermal deviation mathematical model Thermal deviation is a ratio of the enthalpy rise of the heated tube/row to the average enthalpy rise of that in the superheater and reheater of the utility boiler. The thermal deviation coecient, z, is expressed as a function of the thermal load deviation coecient Zth , the structural deviation coecient Zst , and the ¯ow rate deviation coecient Zfl as follows [1]: z
Di Q Gav q=qav F=Fav Z Zst th Diav Qav G G=Gav Zfl
Fig. 4. Thermal simulation calculation algorithm ¯ow diagram.
1
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Based on the thermal deviation theory each boiler tube is divided into a number of segments. An energy balance is made for each of the tube segments. For example, the total absorbed energy Qa by the tube segment A is calculated via an energy balance by adding the front gas room absorbed radiation energy Qafr , the rear gas room absorbed radiation energy Qarr , the tube bundle radiation energy Qamr , and the tubes bundle convection energy Qamc [13,14]. Qa Qafr Qarr Qamr Qamc
2
The front gas room absorbed radiation energy by the tube segment A, Qafr , is given as: Qafr qafr l s1 xffr tfr qafr xafr F a
3
The rear room absorbed radiation energy by the tube segment A, Qarr , is de®ned as: Qarr qarr l s1 xfrr trr qarr xarr F a
4
The tubes bundle radiation energy by the tube segment A, Qamr , is expressed as, Qamr qamr l s2 2xf qamr xamr F a
5
The energy of the tubes bundle convection heat transfer by the tube segment A, Qamc , is as follows: Qamc qamc p d l qamc xamc F a
6
The total energy Qa absorbed by the tube segment A, may then be expressed by combining Eqs. (2)±(6). ÿ Qa Qafr Qarr Qamr Qamc F a qafr xafr qarr xarr qamr xamr qamc xamc 7 In view of the thermal load deviation in the horizontal and vertical directions, the energy Qawh absorbed by the tube segment A is determined by using thermal load deviation coecient Zth,w in the horizontal (width) direction and thermal load deviation coecient Zth,h in the vertical (height) direction as follows: Qawh Zth,w Zth,h Qa ÿ Zth,w Zth,h F a qafr xafr qarr xarr qamr xamr qamc xamc
8
Taking into account the calculated tube segment temperature dierence Dt and the average temperature dierence Dtav , the energy absorbed by the segment A, Qatot , can be rewritten as follows: Qatot Qawh
Dt Zth,w Zth,h Dtav
ÿ Dt F a qafr xafr qarr xarr qamr xamr qamc xamc Dtav The total absorbed energy for each of the tube is expressed as follows:
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Qtot Qatot Qbtot Qctot Qdtot
n X i1
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Qitot
10
where a, b, c, . . . , etc. are the dierent segments that the tube is divided into. 2.2. Heat transfer deviation factors Table 1 shows the radiation deviation factors of heat transfer for the tube bundle, the front gas room and the rear gas room, and the deviation factors of the convection heat transfer for the superheater and the reheater [1,13,14]. The correction factor for the middle tube area, E0, is determined as [13,14] 9 8 " 2 #0:5 = < s2 d d d ÿ 1ÿ E0 2 :1 cosÿ1 11 ; s2 s2 s2 d 2.3. Flow rate deviation mathematical model According to the ¯ow rate deviation theory, the static pressures dierence, pd,x , at the distribution header with the Z type arrangement, at x location, is expressed as [12]: 1 2 3 12 pd,x Hd 0:76 1 ÿ 1 ÿ x ÿ ad 1 ÿ 1 ÿ x 3 The static pressure distribution, pc,x , for the collection header with the Z type arrangement at any x location, is de®ned as [12]: 1 2 3 pc,x Hc 2 1 ÿ 1 ÿ x ac 1 ÿ 1 ÿ x 13 3 With the assumption that there are n tube rows for the complicated header connection system, the governing equations for the ¯ow rate deviation calculation of the superheater and the reheater is as follows: Table 1 Deviation factors of radiation and convection heat transfer Radiation deviation factor for heat transfer of the front gas room Radiation deviation factor for heat transfer of the rear gas room Radiation heat transfer deviation factor for the middle tube Radiation heat transfer deviation factor for the ®rst and the last tube Radiation heat transfer deviation factor of hanging tube Radiation heat transfer deviation factor of contact tube Radiation heat transfer deviation factor of dierence interval tube Convection heat transfer deviation factor for ®rst and the last tube Convection heat transfer deviation factor for all other tubes
xafr s1 =d xffr =E0 tfr xarr s1 =d xfrr =E0 trr xmr 1:0 xmr 0:5 0:25 p=E0 xmr p=E0 xmr p ÿ 0:5 p x=E0 xmr x2mr x3mr =2 xmc xmr 0:25 p=E0 xmc xmr
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pd,1 ÿ pc,1 k1 G 21 .. . pd,i ÿ pc,i ki G 2i .. . pd,n ÿ pc,n kn G 2n n X G Gi
14
i1
where pd,i ÿ pc,i is the ith row tubes pressure dierence between the distribution header and the collection header, and ki G 2i is ith row tube's pressure drop. 2.4. Thermal load distribution Based on the literature survey, the traditional distributions of the thermal load along the utility boiler lateral direction can be summarized as follows [1]. The thermal load deviation Zth is a polynomial of the order of four for the boiler width axis: Zth a0 a1 x a2 x 2 a3 x 3 a4 x 4
15
where a0, a1, a2, a3 and a4 can be decided by the two minimum values and a maximum value of thermal load deviation, and their axis. Sixth power of the boiler width for the analysis of ABB-CE utility boiler ®eld is: Zth a0 a1 x a2 x 2 a3 x 3 a4 x 4 a5 x 5 a6 x 6
16
where a0, a1, a2, a3, a4, a5 and a6 can be determined by a minimum and two maximum thermal load deviations, and their axis. Assuming the maximum thermal load deviation at the center of the boiler width: Zth Zth min Ajx ÿ 0:5j1:5 Bjx ÿ 0:5j2:5
17
A 3:353b7 ÿ 5 Zth min ÿ2 Zth max c B 9:898b3 Zth min 2 Zth max ÿ5c Assuming a parabolic distribution of the thermal load: 2
Zth A ÿ B x ÿ C
18
where A, B and C are the coecients relative to the maximum thermal load deviation and its axis.
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Fig. 5. Outside wall metal temperature of the ®nal reheater outlet tubes for the 300 MW utility boiler.
3. Simulation calculation and discussion As mentioned earlier, Fig. 4 shows the ¯ow diagram for the calculation algorithm used to get a converged solution of the thermal load simulation model. The calculations were performed by using experimentally measured utility boiler outer tubes' wall metal temperatures, the average inlet and outlet gas temperatures, the thermal deviation theory and the ¯ow rate deviation theory. All the experimental results were from in situ measurements made at the 300 MW utility boiler at Wujing Power Station, which uses the four angles reverse-tangential ®ring systems. Flow diagrams and tube arrangements are shown in Figs. 1±3. The calculated results based on the model are presented in Figs. 5±7. Fig. 5 compares the experimental value of the reheater outlet tube wall metal temperatures for the 300 MW utility boiler reheater of Wujing Power Station, with the values calculated from the simulation. The maximum temperature dierence between the calculated and the experimental results is found to be about 5.58C as shown in Fig. 5. The relative errors in the temperature of the model calculation results are within 1% of the in situ experimental results. The calculated results agree very well with the experimental results. Fig. 6 presents thermal load deviation coecient as a function of the tube row number. It can be seen from the ®gure that the maximum deviation of the thermal load between the superheater and reheater rows is about 215%. This predicted result is greater than the ABBCE design calculation standard. Fig. 7 presents the calculated thermal deviation results. This ®gure also presents the structural and ¯ow rate deviation values. As seen in the ®gure, the
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Fig. 6. Simulation calculation results of thermal load deviation coecient for the 300 MW utility boiler's ®nal reheater.
maximum ¯ow rate deviation between the reheater rows is about 212%. It is also observed that the ¯ow rate deviation below the normal ¯ow rate, i.e., lower ¯ow rate results in larger temperature dierence. Therefore, it may be concluded that the key contributing factors for thermal deviation is the ¯ow rate deviation and the thermal load deviation between rows. Thus, by minimizing the thermal deviation the key cause of the BTF may be eliminated. The maximum thermal deviation between the rows is about 29% at the tubes of the sixth row shown in Fig. 7. It can also be mentioned that the locations of the actual BTF indeed coincided with the location of the maximum thermal deviation. The in situ BTF occurred at the sixth row tubes of the reheater from left side, as shown in Fig. 1, in the 300 MW utility boiler of Wujing Power Station, in February 1995. The predicted results of the BTF by using the proposed model were the same as the in situ location of the BTF. 4. Conclusions In this paper, we propose a thermal load model that is based on the power plant in situ thermodynamic parameters, thermal deviation theory and ¯ow rate deviation theory. The model makes a systematic ¯ow analysis and makes an energy balance on each segment of the boiler tubes. Solution of the resulting governing equations provides the thermal load for each segment of the tubes and the ¯ow rate of each tube. Following this approach, the proposed
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Fig. 7. Simulation calculation results of thermal deviation for 300 MW utility boiler ®nal reheater.
model has been used to improve the design of the utility boiler in Boiler Works. It has also been used to predict the possible failures in the design stage, and to assess the existing designs. This model can also be applied to dierent type of utility boilers built by dierent boiler manufactures. This model has actually been used to predict and prevent the BTF in the power station. The predicted position of BTF is the same as the reheater positions of BTF at a 300 MW utility boiler of Wujing Power Plant. The relative errors in temperatures of the model calculation results are within 1% of the in situ experimental results. In general, the maximum deviation of the thermal load between the superheater and reheater rows is about 215%, this is more than the ABB-CE design calculation standard. This is the BTF root cause for the large capacity utility boiler superheater and reheater.
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