Experimental thermal behavior of a power plant reheater

Energy 31 (2006) 665–676 www.elsevier.com/locate/energy

Experimental thermal behavior of a power plant reheater M. Manuela Prieto Gonza´leza,*, F. Javier Ferna´ndez Garci´aa, Ine´s Sua´rez Ramo´na, Hilario Sa´nchez Rocesb Departamento de Energi´a, Universidad de Oviedo, Campus de Viesques, 33204 Gijo´n, Asturias, Spain b Central Te´rmica de Soto de Ribera, Soto de Ribera, Asturias, Spain

a

Abstract The process conditions of power plant components subjected to high pressures and temperatures are essential to determine their remaining life, availability and efficiency. It is, therefore, expedient to pay special attention to critical components, such as superheater and reheater heat exchangers, headers, and main and reheated steam lines. In this paper, on-line and off-line variables of a power plant reheater that has presented problems of thickness losses and repetitive tube fissures are studied. The fissures are associated with the effect of a thermal–mechanical mechanism. Off-line measurements were taken of the following variables: pressure, temperature, velocity and composition of the gases. On-line instrumentation was completed by the installation of specific thermocouples to ascertain the temperatures in the tubes outlet. Various angles for the fuel inlet of the burners and variations in the number and location of the working burners were also assayed. As a consequence of this analysis, it can be deduced that there are important differences in the outlet temperature of the reheater tubes that decrease for lower powers. Finally, it is pointed that a non-uniform distribution of the steam flow in the reheater might be the cause of the problem. q 2005 Elsevier Ltd. All rights reserved.

1. Introduction Special attention is paid nowadays to the estimation of the performance and the remaining life of critical components of power plants [1,2]. This evaluation may be carried out during maintenance inspections or on-line by means of the analysis of operation variables [3,4]. Evaluation enables anticipating possible failures and planning actions before installation shut down [5,6,7]. Off-line methods, such as ultrasonic measurement of tube thickness, penetrating liquids or magnetic * Corresponding author. Fax: C34 985 18 21 43. E-mail address: [email protected] (M.M.P. Gonza´lez). 0360-5442/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.04.009

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particles, are also employed. Other complementary techniques include metallurgic tests that use spectroscopy, fluorescence with X-ray and in situ hardness measurement. Critical components as superheater and reheater heat exchangers, headers and main and reheated steam lines, which are submitted to high pressures and temperatures, are essential to determine the life, availability and efficiency of the installation. These components substantially affect production and environmental costs. Several factors that cause damage or a loss in performance of the heat exchanger tubes can be found in the literature [8]. The principal mechanisms that cause tube failure have been summarized. In order to classify the mechanisms, a distinction can be made between those that produce a decrease in thickness and even tube failures and others that diminish the heat transfer rate and increase tube temperature because of fouling formation. Among the former, we may cite: † External tubes surface damage: erosion, corrosion in the gas side and overheating. † Internal tubes surface damage: hydrogen effects, caustic corrosion and chemical reaction. † Fissures induced in welding and by thermal stress may also be subject to corrosion. This paper presents a study of a thermal power plant reheater that has suffered problems of thickness losses and repetitive tube fissures. The defects are located in both the straight and curved tubes. The fissures are associated with the effect of a thermal mechanism. The material of some tubes in the reheater has been changed in zones where the fissures were produced or damages were detected in maintenance operations. Nevertheless, the problem still remains, and so the present study was carried out in order to increase our knowledge of the operating conditions that influence the thermal state of the reheater. Experimental measurements of the tube outlet temperature in the reheater were taken to estimate the thermal conditions along the tubes. Off-line measurements of the following variables were also carried out: pressure, temperature, velocity and composition of the gas.

2. Installation description The study refers to a 370 MW controlled steam circulation coal-fired boiler with tangential burners. Fig. 1 shows an overall scheme of the boiler. The economizer (1), on the right-hand side has two tube bundles. The superheater has five heating stages: horizontal superheater (2), finishing superheater (3), roof sections and backpass walls (4), pendant assemblies (5) and division panels (6). The reheater is made up of three stages: radiant wall (7), primary reheater (8) and secondary reheater (9). The furnace has six burner rows with four directional burners in each level. Under nominal load the boiler has a steam generating capacity of 1120 T/h at a temperature and pressure of 540 8C and 170 kg/cm2, respectively, and a capacity for reheating steam of 980 T/h at 540 8C and 40 kg/cm2. Fig. 2(a) shows a detail of the primary and secondary reheaters, where the tube fissures have been detected. The movement of the steam and gases is indicated using dashed lines and broad arrows, respectively. The points where it is possible to install instrumentation for off-line gases measurements have also been marked. The primary reheater has 32 serpentines made up of 14 tubes each, while the secondary reheater has 62 serpentines of 7 tubes, 2 serpentines of 5 tubes and 1 serpentine of 4 tubes. Fig. 2(b) presents a scheme of section B–B 0 along a horizontal plane parallel to the boiler roof, in which the rows of tubes associated with each serpentine are represented as little rectangles. What will be denoted in the following as the east and west sides of the boiler are also indicated in this figure.

M.M.P. Gonza´lez et al. / Energy 31 (2006) 665–676 Primary Secondary Pendant Reheater (8) Reheater (9) Division Assemblies (5) Panels (6) Roof and Backpass Walls (4) Drum Finishing Superheater (3) Reheater Radiant Wall (7)

Horizontal Superheater (2) Economiser (1)

Water Walls Furnace Downcomer

Burners

Fig. 1. Schematic construction diagram of the boiler.

EAST B'

B

A

A'

Primary Reheater 32 Serpentines WEST Off-line measurement points

(a)

Secondary Reheater 65 Serpentines (inlet) Secondary Reheater 65 Serpentines (outlet) (b)

Fig. 2. Primary and secondary reheater and measurement points: (a) lateral view, (b) section B–B 0 .

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668

3. Analysis of the variables 3.1. Thermodynamic on-line variables The historical data of the variables controlled at the power plant that are automatically registered every 15 min were represented versus time in order to carry out the analysis. The variables chosen for the representation were those available that could be related with the thermodynamics of the steam and gases and the heat transfer at the primary and secondary reheaters. The times selected for a more in-dept analysis were obtained in the search for the unit steady state. To accomplish this, attention was focused on full electrical load and on low electrical load. The variables represented to define the times were the generated power load and the reheated steam mass flow rate, both having a great influence in the fluid dynamics of the reheater. The data from 5 weeks were considered, two sets of data being selected in each week: one for full load and other for low load. The aim of this selection was to have a sufficient amount of consecutive data (time instants). Table 1 summarizes the periods of time (starting and ending times), dates, the steady state considered and the average load taken into account. Fig. 3 represents the load and the reheated steam mass flow rate versus time from 9:00 on 12/5/2003 to 21:00 on 13/5/2003, comprising the data considered for full load and low load. The following additional variables were also represented for the selected periods: coal and combustion air mass flow rates, ashes in the flow of gases, excess of oxygen (two measurements: one situated on the east side and other on the west side of the boiler), differential pressures between the coal mills and the furnace, mean steam and reheated steam flows, steam pressure and temperature at the entrance of the medium pressure turbine and temperatures in two links that transport the steam from the reheater radiant wall (7) to the primary reheater. One of the links is situated on the east side of the boiler, whereas the other is situated on the west side. On checking the situation of the tubes that had presented fissures or in which the thickness was seriously diminished, it was realised that they are mainly situated in both reheaters on the west side of the boiler, and at the first and second tube rows relative to the direction of the gas flow. This fact could be associated with a non-uniform spatial distribution in the flow of gases or in the steam that feeds the serpentines. If so, it will produce differences in the temperature of the material of the tubes, and there could be tubes submitted to temperatures above the recommended thresholds for avoiding thermal Table 1 Description of the periods of time studied Data set

Period

Date

Starting time

Ending time

Load (MW)

1. From 9:00, 25/11/2002 to 9:00, 30/11/2002 2. From 10:00, 3/12/2002 to 12:00, 7/12/2002 3. From 9:00, 27/3/2003 to 22:00, 30/3/2003 4. From 10:00, 29/4/2003 to 21:00, 2/5/2003 5. From 9:00, 12/5/2003 to 21:00, 13/5/2003

Full load Low load Full load Low load Full load Low load Full load Low load Full load Low load

25/11/2002 30/11/2002 3/12/2002 6/12/2002 29/3/2003 30/3/2003 30/4/2003 1/5/2003 12/5/2003 13/5/2003

19:00 4:00 19:00 4:00 10:30 4:00 16:30 16:30 20:30 3:30

23:00 8:00 23:00 8:00 16:00 9:30 20:00 20:00 23:00 6:00

341.8 190.6 343.5 192.6 343.7 192.2 344.2 181.7 343.8 195.1

M.M.P. Gonza´lez et al. / Energy 31 (2006) 665–676 (a)

(b) Stream Mass Flow Rate (T/h)

410

350 Load (MW)

669

290

230

170

1000

800

600

400

12/05/200312/05/200312/05/200313/05/200313/05/2003 13/05/200313/05/2003 09:00:00 15:00:00 21:00:00 03:00:00 09:00:00 15:00:00 21:00:00

12/05/200312/05/200312/05/2003 13/05/200313/05/200313/05/2003 13/05/2003 09:00:00 15:00:00 21:00:00 03:00:00 09:00:00 15:00:00 21:00:00

Fig. 3. (a) Load of the unit, (b) reheated steam mass flow rate.

stress. This was why special attention was paid to the on-line control variables with measurements on both sides of the boiler (east–west). The available measurements were: excess of oxygen in the flow of gas (oxygen content) and steam temperature at the reheater radiant wall outlet (links placed on the east and on the west sides of the boiler). These variables are represented in the Fig. 4 from 9:00 on 12/5/ 2003 to 21:00 on 13/5/2003. The steam temperature in both links is quite different and given that the links feed the primary reheater with steam, this could be a cause of having tubes with higher temperatures. Different values for oxygen content in both sides are also appreciated. These values depend on the unit load, as can be noted in Fig. 4. In order to confirm these conclusions in the rest of the data sets, Tables 2 and 3 present the mean and standard deviation of the radiant wall outlet steam temperature and of the oxygen content, respectively. These results were calculated for the east and west sides and for the selected periods in every data set. Respect to Table 2, the steam temperature at the outlet of the radiant wall is higher for the east, both for full or low loads. The standard deviations are lower for low loads, indicating a minor dispersion of the data for these powers. The oxygen contents, presented in Table 3, are lower on the east side in the majority of periods, and are always higher for low loads. Therefore, the idea of different thermal conditions on both sides of the boiler was confirmed.

(a)

(b)

6

Temperature (K)

O2 (%)

5

4

3

660

640

620

Oxygen in Economiser (E Side) Oxygen in Economiser (W Side)

2

Radiant Wall Outlet Temperature (E Side) Radiant Wall Outlet Temperature (W Side)

600

12/05/2003 12/05/2003 12/05/2003 13/05/2003 13/05/2003 13/05/2003 13/05/2003 09:00:00 15:00:00 21:00:00 03:00:00 09:00:00 15:00:00 21:00:00

12/05/200312/05/2003 12/05/2003 13/05/200313/05/2003 13/05/200313/05/2003 09:00:00 15:00:00 21:00:00 03:00:00 09:00:00 15:00:00 21:00:00

Fig. 4. (a) Excess of oxygen in gases, (b) steam temperature in the radiant wall outlet.

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670

Table 2 Steam temperature at the outlet of radiant wall Data set

Full load

Low load

East

1 2 3 4 5

West

East

West

Mean

Standard deviation

Mean

Standard deviation

Mean

Standard deviation

Mean

Standard deviation

370.1 365.9 376.9 379.6 375.3

1.643 3.629 3.882 3.750 2.872

361.5 357.4 365.7 368.4 366.4

1.449 3.556 3.765 2.386 3.197

343.2 349.3 353.3 355.9 348.5

0.091 0.888 1.513 1.232 0.659

331.7 338.2 341.6 342.5 340.0

0.228 1.382 1.606 1.462 0.487

3.1.1. Measurements of temperature of the tubes surface Contact thermocouples were installed to ascertain the temperature of the tube surface at the outlet of the secondary reheater, and these temperatures were automatically stored on the plant computer. The primary and secondary reheaters are made up of 448 tubes situated at the outlet of the secondary reheater as shown in Fig. 5. In this figure, the serpentines are represented with numbers (1–65) and the tubes forming every serpentine with letters (A–G). There are thermocouples situated in 52 tubes; 16 of them are placed in the E tubes, in serpentines numbers 2, 6, 10, 14, 18, 22, 26, 30, 36, 40, 44, 48, 52, 56, 60 and 64. The representation of the temperatures measured in these 16 tubes will be called east–west. The serpentines numbers 10, 22, 26, 36, 52 and 64 have thermocouples in all their seven tubes (A–G). For each data set in Table 1 an instant in the middle of each selected period was chosen to represent the temperatures at that instant in all the east–west tubes. Fig. 6 shows the temperatures for full load and low load for the set of data from 9:00 on 12/5/2003 to 21:00 on 13/5/2003. The temperatures in most of the thermocouples are higher under full load, reaching 840 K. In the serpentines situated on the west side, the temperatures are higher for the low load. The temperature differences between tubes are higher for nominal power. In Fig. 6, three serpentines have been indicated with letters A, B and C to check the temperature differences between full and low loads for all the periods. Table 4 summarizes the most important information obtained from the study of temperature profiles in tubes E for the five data sets studied. For each period the following values are indicated: Table 3 Oxygen content in gases Data set

Full load

Low load

East

1 2 3 4 5

West

East

West

Mean

Standard deviation

Mean

Standard deviation

Mean

Standard deviation

Mean

Standard deviation

4.362 3.921 3.921 3.833 3.669

0.076 0.117 0.028 0.074 0.041

3.599 3.869 3.817 4.378 4.109

0.024 0.121 0.035 0.098 0.030

5.885 5.787 5.586 6.010 5.676

0.018 0.026 0.046 0.057 0.028

5.618 5.626 5.798 6.224 5.684

0.031 0.029 0.040 0.051 0.066

M.M.P. Gonza´lez et al. / Energy 31 (2006) 665–676

671

EAST A B C D E F G 1 2 3

East-West Temperature Representations (Tubes E)

44

64 65

WEST

Fig. 5. Situation scheme of the thermocouples at the secondary reheater outlet.

the temperature differences between full and low loads for the serpentines number 10 (DTA), 44 (DTB) y 56 (DTC); and the mean and the standard deviation for full and low loads. The differences are positive in serpentines 10 and 44 and negative in serpentine 56 for all the periods, higher temperatures, therefore, being reached in full load periods. Other results obtained from the analysis of the table are that the maximum temperature differences are about 40 K and the mean temperatures and the standard deviations are higher for full load. Therefore, there are lower tube temperatures and less dispersion for low load. The temperature differences for full load are very important and the zone with the highest temperature coincides with the location where the majority of the tube failures occurred. The conclusion is that decreasing the temperature differences in tubes E of the serpentines could improve the reheater behavior and decrease the number of failures and hence the unit maintenance costs. To do so, it is necessary to find the origin of the difference in temperatures. Thus, a study looking into the effects of burner orientation and of the placement of active burner rows was B

Temperature (K)

840 A

C

820

800

780

Low Load Full Load

760 2

6

10 14 18 22 26 30 36 40 44 48 52 56 60 64 Serpentine (number)

Fig. 6. Temperature profile in tubes E for full and low loads.

M.M.P. Gonza´lez et al. / Energy 31 (2006) 665–676

672

Table 4 Temperature difference in three thermocouples, mean and standard deviation for tubes E Data set

DTA

DTB

DTC

Tube temperature (RH outlet) Full load

1 2 3 4 5

18.3 15.6 24.0 30.9 22.6

32.1 21.4 27.4 34.0 26.9

K15.9 K20.5 K16.4 K5.0 K25.7

Low load

Mean

Standard deviation

Mean

Standard deviation

808.7 822.6 824.0 819.1 823.6

16.1 15.0 19.5 14.0 19.6

801.2 817.2 811.1 801.6 809.9

11.3 7.6 10.8 10.1 9.2

carried out, due to their presumable influence in the thermodynamics of the gas flow and consequently in the temperature of the tubes. 3.1.2. Effect of burners on the temperature of the tubes The study was performed for full load owing the higher tube temperature dispersion and thermal level. There are six rows of burners in this boiler, the upper level being the number six, and the orientation of the burners can change with respect to the correspondent boiler horizontal plane from K30 to 308. This part of the study was carried out with data corresponding to a week when the unit was constantly maintained at full load, and hence there was no influence of load variations. The orientation of the burners is not an on-line registered control variable; accordingly its value was taken manually. The orientation is varied simultaneously in all the burner levels during the unit operation. 3.1.2.1. Orientation of the burners. There were five periods of 1 h when the orientation of the burners was varied. Table 5 shows a description of the periods, the change in the orientation of the burners, the maximum and minimum temperature difference in tubes E (with the number of the serpentine where it occurred), and the mean difference in temperature. The summarized results were obtained from the measured temperatures for the thermocouples of 16 serpentines, which were registered every 15 min. The maximum differences in the thermocouple measurements were obtained comparing all the instants of every period, the greatest of these differences being 11.02 K, which is reached in serpentine 64 and for the data from 14 to 15 h on 26/03/2003. The mean difference taking into consideration data from all the instants of every period and the measurements of 16 thermocouples was less than 8 K. On the other hand, a substantial difference in the temperature distribution profile is not expected, as the mean differences are Table 5 Maximum, minimum and mean temperature difference in tubes E for changes in burner orientation Period

Orientation change

Maximum difference

Minimum difference

Mean difference

13–14 h—25/03/2003 14–15 h—26/03/2003 16–17 h—26/03/2003 8–9 h—27/03/2003 18–19 h—27/03/2003

from from from from from

9.71 (S64) 11.02 (S64) 3.45 (S64) 10.56 (S64) 6.91 (S22)

2.32 (S18) 5.60 (S40) 0.33 (S10) 2.33 (S36) 4.71 (S44)

4.26 7.23 1.56 5.54 5.96

K12 to K248 K24 to K218 K21 to K188 K19.2 to K248 K24 to K218

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673

860

Temperature (K)

840 820 800

Tube Temperature,13:15 25/3/2003 Tube Temperature, 14:00 26/3/2003 Tube Temperature, 16:30 26/3/2003 Tube Temperature,8:30 27/3/2003 Tube Temperature,18:30 27/3/2003

780 760 2

6

10 14 18 22 26 30 36 40 44 48 52 56 60 64 Serpentine (number)

Fig. 7. Temperature profile in tubes E for periods with a change in burner orientation.

close to the mean temperature considering the maximum and minimum differences in each period. Fig. 7 represents the temperature in tubes E in a centered instant of each period, as was done for Fig. 6. The curves present a very similar temperature distribution and it can be concluded that this parameter barely affects the thermal disequilibrium in the reheater. 3.1.2.2. Changes in active rows of burners. A similar study to the previous one was carried out to ascertain the effect of changes in the rows of burners that were operative. Four moments were chosen maintaining full load in all of them; different burner rows being operative at each moment. Table 6 shows a description of the periods, the change introduced in the operating rows of the burners, the maximum and minimum temperature difference in tubes E (with the number of the serpentine where this occurred), and the mean difference in temperature. The summarized results were obtained from the measured temperatures for the thermocouples of the 16 serpentines, which were registered every 15 min. Activation or deactivation of the upper rows of burners (4 and 5) induces greater variation in the temperature of tubes than that produced by changes in the orientation of burners. However, as in the previous study, the temperature distribution does not seem to be substantially affected. For instance, Fig. 8 shows the changes produced in the temperatures of E tubes for the period in which row four is deactivate and the row five is activated. It can be observed that, as assumed the temperature distribution did not change substantially and that the changes are only at the level of the tube temperatures. Table 6 Maximum, minimum and mean temperature difference in tubes E for changes in active rows of burners Period

Orientation change

Maximum difference

Minimum difference

Mean difference

15:30–16:30 25/03/2003 5:00–6:00 26/03/2003 10:30–11:30 26/03/2003 16:00–17:00 27/03/2003

1, 2, 3, 4 and 5–1, 2, 3 and 4 1, 2, 3 and 4–1, 2, 3 and 5 1, 2, 3 and 5–1, 2, 3, 4 and 5 1, 2, 3, 4 and 5–2, 3, 4 and 5

9.45 (S26) 19.57 (S36) 18.13 (S36) 4.73 (S60)

2.50 6.22 8.28 0.98

4.90 13.60 12.53 2.40

(S2) (S56) (S2) (S10)

M.M.P. Gonza´lez et al. / Energy 31 (2006) 665–676

674 860

Temperature (K)

850 840 830 820 810

Tube Temperature, 5:00 26/3/2003 Tube Temperature, 5:15 26/3/2003 Tube Temperature, 5:30 26/3/2003 Tube Temperature, 5:45 26/3/2003 Tube Temperature, 6:00 26/3/2003

800 790 780 2

6

10 14 18 22 26 30 36 40 44 48 52 56 60 64 Serpentine (number)

Fig. 8. Temperature profile in tubes E for changes in active rows of burners.

It may, therefore, be concluded that the available actions on the burners do not substantially affect the distribution of the temperature of the tubes. 3.1.3. Off-line measurements in the gas An off-line measurement plan was carried out in the reheater zone to directly check the possibility of disequilibrium in the fluid-dynamic and thermal properties of the combustion gases, which could cause disequilibrium in the temperature of the tubes. The variables measured were temperature, velocity, pressure and composition of the combustion gases. The measurement points were conditioned by the existing inspection doors, the position of the blowers and the auxiliary measurement point. The points were already indicated in Fig. 2 (a). A probe equipped with a thermocouple, a Pitot tube, and a suction duct to collect gases was inserted in the boiler maintaining stable full load as far as possible in all the measurements. The measurements were carried out in the three sections along the route of the gas through the reheater that corresponded with three planes in which there were available measurement points, named ‘inlet’, ‘intermediate’ and ‘outlet’. These sections are represented in Fig. 9, together with the measurement points, situated at depth 1, 3 and 5 m from the boiler walls. Tables 7, 8 and 9 present the measurement results for the gas: temperature, horizontal component of the velocity and oxygen content, respectively. The gas temperature decreases and horizontal component of its Inlet Section 1

2

3

4

5m 3m 1m 5

EAST 6

Intermediate Section

5

4

3

2

EAST 3

WEST 7

1

4

2

Outlet Section

1

2

EAST

WEST 4

1

3

4

3

3

2

1

WEST

5

6

Fig. 9. Measurement points in the gas for inlet, intermediate and outlet sections.

5

4

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675

Table 7 Gas temperature in the measurement sections Inlet section

Intermediate section

Outlet section

Point

East

West

Point

East

West

Point

East

West

1 2 3 4 5 6 7 Mean

1096 1122 1134 1147 1153 1293 – 1182a

– – 1148 1061 1078 1263 1513 1138a

1 2 3 4

1057 1063 1049 1125

1059 1008 1118 1101

1 2 3 4 5

968 986 996 921 1040

934 918 919 1093 1099

1074

1072

982

993

a

Points without a symmetrical are not included in mean (1, 2 and 7, inlet section).

Table 8 Horizontal component of velocity in the measurement sections Inlet section

Intermediate section

Outlet section

Point

East

West

Point

East

West

Point

East

West

1 2 3 4 5 6 7 Mean

7.81 8.51 3.50 3.37 4.07 3.67 – 3.65a

– – 2.10 2.03 1.66 3.20 2.29 2.25a

1 2 3 4

5.51 6.77 1.56 1.51

4.89 3.10 5.18 3.15

1 2 3 4 5

7.32 8.88 8.40 3.81 6.69

5.99 6.60 7.28 8.43 9.73

3.83

4.08

7.02

7.61

a

Points without a symmetrical are not included in mean (1, 2 and 7, inlet section).

Table 9 Oxygen content in the measurement sections Inlet section

Intermediate section

Outlet section

Point

East

West

Point

East

West

Point

East

West

1 2 3 4 5 6 7 Mean

3.34 3.15 3.49 3.16 3.12 2.87 3.16a

– – 3.86 3.56 3.60 3.52 2.30 3.64a

1 2 3 4

3.24 3.18 3.23 3.52

3.53 3.34 4.16 4.01

1 2 3 4 5

3.14 3.58 3.23 3.85 3.69

4.10 3.96 4.03 3.42 3.52

3.29

3.76

3.50

3.81

a

Points without a symmetrical are not included in mean (1, 2 and 7, inlet section).

676

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velocity increases from the inlet to the outlet section. There are no significant differences in the oxygen content. Significant differences were not observed in these measurements with respect the position east– west. The possibility of gas disequilibrium is rejected as a cause of the disequilibrium in the temperatures of the reheater tubes. Other possible causes to consider in future research would be: incorrect distribution of the steam inside the reheater or irregular fouling of the external surface of the tubes. Taking into account the visual inspections of the boiler, the first possible cause is suggested as the next to be investigated.

4. Conclusions In this paper, an on-line and off-line study was carried out of thermodynamic variables that affect the performance and the tube temperature of a power plant reheater that presented problems of thickness losses and repetitive tube fissures. The fissures were expected to be associated with the effect of a thermal mechanism. A non-uniform temperature distribution was found in the steam entering the reheater, as well in the oxygen content in the gases. Important differences were also appreciated in the tube temperatures at the reheater outlet, decreasing at low power regimens. Burner orientation and the activation of some burner rows were found to influence the temperature of the tubes, but the burners did not produce a notable variation in tube temperature distribution. The off-line gases assays to investigate non-uniform flow and temperature discards these as the cause of the problem. The deduction process presented in the research development indicates that the thermal disequilibrium in the reheater will probably be promoted by an incorrect distribution of steam inside the tubes.

Acknowledgements We wish to express our gratitude to the Scientific and Technological Research Organization (F.I.C.Y.T.), and to the company Hidrocanta´brico Generacio´n SAU for their financial support under grant CN-02-199-IE-CIS01-11, which made this research work possible.

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