Overview on Recent Failure Investigations of Water Tube Boiler in Fossil-Fired Power Plants

Transcription

1 Overview on Recent Failure Investigations of Water Tube Boiler in Fossil-Fired Power Plants J. Purbolaksono a,b, J. Ahmad c a Department of Engineering Design & Manufacture, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia b Centre of Advanced Manufacturing and Materials Processing, University of Malaya, Kuala Lumpur 50603, Malaysia judha@um.edu.my c Kapar Energy Ventures Sdn Bhd, Jalan Tok Muda, Kapar 42200, Malaysia juliaha@kaparenergy.com.my Abstract: This paper in particular presents overview on some recent failure investigations of water tube boiler of a power generation in Malaysia. Here, the problems are mainly related to water tube boilers which include the tube failures in superheater, reheater and water wall regions. Different materials of tubes and different nature of problems in different locations of the plants are presented. It is particularly important to deduce the failure mechanism and main root cause of each problem. Failure mechanisms presented in this paper cover creep rupture in associated with the water/steam-side scale formation, excessive flue gas temperature, and lacking of operational control. Failure investigations include visual inspection, metallurgical examination, hardness testing, chemical analysis, tube wall and deposit/scale thickness measurements, and finite element modeling. Case studies for different failure mechanism are presented. Keywords: Failure analysis; Water tube boiler; Fossil-fire power plant 1. Introduction Nowadays, a large percentage of power plants worldwide have been in operation for long durations. There are strong economic reasons and technical justifications for continued operation of the power plants. In order to enrich some knowledge for continuing operations in practice, learning from past failure problems would be beneficial to the plant engineers or operators. An extremely hostile environment in the boiler can lead to tube failures to occur in both short-term and long-term periods. The costly recurrent failure problems can be minimized or even eliminated by the establishment of appropriate plant operating procedures including identifications of the problem, corrective actions or mitigations, proper scheduled maintenances, and recommendations on operation. When a power plant is forced to shut down because of a single component failure, the cost of the lost electric-power generation can run to several hundred thousand dollars a day. Failures in boiler tubes have been discussed and summarized in [1-3]. As reported by French [3], 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 thick scales.

2 Jones [2] indicated that under which the failures occurred the conditions of temperature and time are deduced from the morphology of fracture and the change in microstructure. This paper presents the failure case studies that are mainly related to water tube boilers which include the tube failures in superheater, reheater and water wall regions. Different material of tubes and different nature of problems in different locations of the plants are presented. 1. Case Study 1: Reheater In this case study, the investigation includes scale thickness measurements, finite element analysis, and microscopic examination [4]. One of the major contributors to the tube failure is the scale formation developed on internal surface of the boiler tubes. Heat transfer rate across the tube will also decrease due to the accumulated scales inside the tube. Consequently higher temperature in the tube metal increases, and in the prolonged exposure this phenomenon will worsen situation that leads to potential rupture problems. 2.1 Operational Background The failed reheater tube was situated in horizontal position and facing directly to the incoming hot flue gas from furnace region. It was reported that the tube failed after 117,522 hours in service and is made of SA213-T22 with dimension of 50.8 mm OD x 3.5- mm-wall thickness. 2.2 Scale Thickness Measurements Dimensional measurements for the scale thickness of the as-received tube samples (the failed tube and its neighbor tubes) are taken to estimate the average scale growth rates. For estimation of oxide scale growth rate, the average scale thickness is measured from the neighbor tubes to be used as the base line data. It is assumed that the tubes had operated normally. It was reported that the operational pressure of the reheater tubes is initially 40 bar or 4 MPa until 92,526 hours. Then, operational pressure is set to 30 bar or 3 MPa for safety reason. Detailed descriptions for scale thickness measurements and scale growth rate estimation are shown in Table 1. Table 2. Scale thickness measurements and scale growth rate estimation [4]. Descriptions Scale thickness of the failed tube = 2 mm; Average scale thickness of the neighbor tubes = 0.58 mm; Average scale growth rate = 0.58 mm/ 117,522 hours = e-06 mm/ hour; Estimated scale thickness of all tube samples at 92,526 hours = mm; Running hours from 92,526 to rupture = 24,996 hours; Oxide scale built up from 92,526 hours until rupture = mm; Average scale growth rate = mm/ 24,996 hours = e-05 mm/ hour. 2.3 Finite Element Analysis The simulations are conducted in order to determine the temperature distribution in presence of different scales in the tube. Generally, the area is divided into three regions, i.e. steam, scale and tube. Steam flowing through the internal of tube with an inlet temperature of 540 C and a mass flow rate of 18,000 kg/h are considered. The flue gas temperature is 800 C, and the heat transfer along the external surface between the flue gas and the tube wall is considered as a cross flow heat transfer. Material of the formed scale is magnetite (Fe 3 O 4 iron oxide). Steady state analyses of heat transfer are carried out in order to show the influence of different scale thickness to the temperature distribution developed in the tube wall. Phenomenon of heat transfer inside the boiler tube is considered as forced convection with turbulent flow. Temperature distributions of the tube with scale thickness of 0.6 mm and 1.6 mm obtained from finite element analysis are shown in Figure 1. It can be seen that the

3 metal temperature with scale thickness of 1.6 mm has a significant increase. scale 0.6 mm scale 1.6 mm Figure 1. Temperature ( o C) distributions of the tube with two different scale thickness of 0.6 mm and 1.6 mm [4]. 2.4 Microscopic Examination Findings of the microscopic examination showed that the tube metal microstructure had complete stage of spheroidization where the carbide particles have coalesced and dispersed uniformly and showed intergranular micro cracks at the external surface of tube as shown in Figure 2. The change of microstructure confirmed that the failed tube had operated at higher temperature. Figure 2. Complete stage of spheroidization with the coalesced carbide particles to form chain of pores and intergranular micro cracks at the external surface [4]. 2.5 Discussion The estimated hoop stress developed in the tube may be determined as t (r + ) σ h = p 2 (1) t where p is operational internal pressure; r and t are inner radius and wall thickness of the tube, respectively. The results of average temperature for different scale thickness are plotted in Figure 3. The temperatures are taken from of the average of the temperatures at oxide scale/ metal interface and outer surface of the tube. It can be seen from Figure 3, the failed reheater tube had been operated at high metal temperature (greater than C) for the last 17,000 hours. Stress, MPa Service hours Allowable stress Service stress Average temperature Figure 3. Stresses in service and allowable stresses of the tube at the corresponding average tube temperatures and service hours [4]. Thermal oxidation for SA-213-T22 will become excessive if the metal temperature is greater than C [2]. It is also shown that from 92,526 service hours the operational pressure is set to 30 bar from 40 bar, however in the last 5000 hours stresses in the tube remain exceeding the maximum allowable stress values for SA- 213-T22 tube (Table 2). The combinations of the phenomena above will worsen the condition and eventually lead to tube rupture. However, in this case overheating in the metal tube is the main factor governing the rupture. Early rupture might occur if the operational pressure remains at 40 bar (4 MPa) Average Temperature, 0 C

4 Table 2. The maximum allowable stress value for seamless tube SA-213-T22 [5]. Temperature, 0 C Max. allow. stress, MPa The abnormal scale thickness would contribute significantly to elevate the tube metal temperature over a quite prolonged period of time as indicated by severe metal structure deterioration (Figure 3). Results obtained from finite element analysis are in agreement with the findings from the microscopic examination. 3. Case Study 2: Superheater This section reports the first occurrence involving a high-strength alloy SA213- T91 material in Kapar Power Station Malaysia [6]. The investigation consists of visual inspections, hardness measurements and microscopic examinations. Hardness readings on all the as-received tubes are used to estimate the operating temperature of the superheater tubes. 3.1 Operational Background After operating for 38,012 h, the boiler unit had undergone a planned outage for 45 days. Next, the boiler unit returned into service with average operating steam pressure of 163 bar (16.3 MPa) for serving loading of 500 MW. Unfortunately, the unit was forced to shut down due to the tube leakage involving material of T91 (9Cr-1Mo-1V steel) at the superheater region after just running for 38 h. The failed tube and 11 other neighbor tubes were then removed and replaced. The neighbor tubes experienced a localized wall thinning due to the steam impingement escaping from the failed tube. Figure 4 shows the failed superheater tube after removing of the eleven neighbor tubes. The superheater tubes have outer diameter of 50.8 mm and thickness of 7.2 mm. Figure 4. The ruptured superheater tube in the location after the eleven neighbor tubes were removed [6]. 3.2 Visual Inspections Visual examination on the as-received failed superheater tube revealed the following features as: - The tube was failed by a wide open rupture and leaving thin edges (see Location 4, Figure 5). - A half-width of the tube wall was blown out, and the remaining ruptured part was leaving blunt edges (see Locations 2 and 3, Figure 5). - No evidence of active corrosion on either the external or internal surfaces and no evidence of wall thinning at some distance away of the rupture region. - There was presence of numerous longitudinal marks/ lines along the side of rupture edges. The information most likely indicates that the failed tube had experienced short-term overheating. Figure 5. The failed T91 superheater tube with blunt and thin edges [6]. 4 4

5 3.3 Microscopic Examination The metal structure of the as-received failed tube was examined. Figure 6 shows numerous creep cavities and a crack propagating towards the inner surface of tube (Location 3). The findings of the microscopic examinations confirmed that the failed tube had operated at high temperature operation prior to failure. Figure 6. Photomicrographs showing a crack propagating into the inner surface of tube with numerous cavities (Location 3) [6]. 3.4 Hardness Measurements Hardness measurements on the tube samples which experienced a localized wall thinning due to steam impingement are carried out. The measurement on the failed tube was also conducted. The sample of the failed tube was representatively taken from the location of 60 cm away from the rupture region. The hardness readings are then used for estimating the operating metal temperature at the time of failure [6], considering that the operating service is 38 h. The estimations are tabulated in Table 3. Tabel 3. Estimated operating tube metal temperatures [6]. No. Tube samples HV Tube temperatures, C Failed tube Discussion The thin-edge rupture of the failed tube and the severe deterioration of the metal structure indicated that the failure was failed by the short-term overheating. The estimated metal temperature at the time of failure indicated that the failed tube had operated at much higher metal temperature than the temperature alarm of 570 o C. The overheating is also confirmed by the presence of numerous creep cavities and a macro crack, resulting in tube rupture. Hardness readings of the eleven neighbor tubes also show high metal temperature operation. It may indicate that the high temperature operation had generally occurred in the superheater region. Therefore, the combustion issue is reasonably suspected to be the root cause of the failure. It is more reasonable cause than that by the tube blockage which usually leads to steam starvation and overheating. Inconsistent feeding of the pulverized fuels into burner is identified to cause the fluctuation and uneven distribution of the heats causing a sudden high flue gas temperature. However, after the feeding of the pulverized fuels into the burner is cautiously taken care, the boiler unit may have run normally till the next planned outage. Several recommendations could be considered to avoid a similar failure in the future as follows: - Combustion issue which includes pulverized mill and combustion system inside the boiler furnace should be addressed correctly. - Consistent feeding of the pulverized fuels into burner should be maintained. 4. Case Study 3: Water Wall Tubes In this section, visual inspections, chemical analysis and metallurgical examinations on the failed SA210-A1 rear riser water wall tube of a boiler unit [7] are presented.

6 4.1 Operational Background The failure at a rear riser water wall tube located 2 meter away from the boiler bottom drain header was discovered during the outage session. It was reported that the operating temperature and pressure of the steam are around 350 o C and 165 bar (16.5 MPa), respectively. The failure occurred at a tube as counted from the right hand side of the rear riser water wall tube region. The failed tube was then cut at around 1 m in length and replaced by a new similar tube 4.2 Visual Inspections Visual examination of the failed rear riser water wall tube revealed the following features as The tube was failed by a rupture with thick edges, and there was hard adherent scale on the internal surface accumulated at the hottest part of the tube (see Figure 7) The removal of the scale by hammer revealed split rupture, and showed a macro crack from the rupture and gouging contour on the internal surface of the tube (see Figure 8). V notch crack was formed and clearly seen when the sample was subjected to hard pressing, indicating positive sign of hydrogen damage (see Figure 9). Figure 8. Removal of the scale after hammering revealed split rupture (top) and gouging contour on the internal surface (bottom) [7]. Figure 7. Tube failed by a rupture (top) and hard adherent scale on the internal surface (bottom) [7]. Figure 9. A form of V notch crack under a hard pressing [7].

7 4.3 Chemical Analysis The internal scale taken from the rupture region was analyzed under the quantitative element detection by using atomic spectrophotometer and flame photometer. The chemical constituents of the scale are tabulated in Table 4. It can be seen that the major compound identified in the internal scale was iron from corrosion product with traces of manganese, sodium, potassium, etc. Table 4. Chemical constituents of the internal scale [7]. No. Elements Water soluble, % 1 Fe/Fe3O / Mn/MnO / Cr/CrO / Mo/MoO / Ni/NiO / Mg/MgO / Na/Na2O / K/K2O / Cl / Microscope Examination The metal structures at the rupture region and away from the rupture region of the as-received were examined by the metallurgical microscope for microstructural assessment. It can be seen from Figure 10 that the metal structures underneath the scale were found to have significant evidences of decarburization and showing discontinuous intergranular micro cracks. 4.5 Discussion The presence of trace amount of sodium in scale as confirmed by chemical analysis (Table 4) was believed to have concentrated beneath the heavy deposits during operation. The effect of the sodium in the form of caustic attack would remove the protective magnetite layer and would cause corrosion where the gouging contour would be the common appearances as shown in Figure 8. The evidences obtained from the investigation, revealing the hard adherent internal scale, gouging contour beneath the scale and presence of trace amount of sodium in scale, a form of V notch crack of the sample under hard pressing and decarburization with presence of numerous discontinuous intergranular micro cracks underneath the thick hard scale, confirmed that the failure mechanism of the riser water wall tubes was caused by hydrogen damage. 5. Concluding Remarks Three case studies including different material of tubes and different nature of problems in different locations of the plants were presented. The failure mechanisms were deduced and the main root cause of each problem was identified. The operating procedures are necessarily put in place to prevent the recurring problem and to monitor the damage of the water tube boiler. Acknowledgement The authors wish to thank Kapar Energy Ventures Sdn Bhd Malaysia for permission of utilizing all the facilities and resources while conducting this study. Figure 10. Photomicrographs of metal structure underneath heavy scale consisting of numerous discontinuous intergranular micro cracks accompanied by decarburization [7].

8 References [1] Robert D. Port, Harvey M. Herro, The NALCO Guide to Boiler Failure Analysis, Nalco Chemical Company, McGraw-Hill Inc, 1991 [2] D.R.H. Jones, Creep failures of overheated boiler, superheater and reformer tubes, Engineering Failure Analysis., Vol. 11, , [3] David N. French, Metallurgical Failures in Fossil Fired Boilers, A Wiley- Interscience Publication, John Wiley and sons inc, New York, [4] J. Purbolaksono, Y.W. Hong, S.S.M. Nor, H. Othman, B. Ahmad, Evaluation on Reheater Tube Failure, Engineering Failure Analysis, Vol. 16, Issue 1, , 2009 [5] ASME International Electronic Stress Table, Table 1A: The Maximum Allowable Stress Values for Ferrous Materials, Section II, Part D of The ASME Boiler and Pressure Vessel Code, Copy Right 1998 ASME International. [6] J. Ahmad, J. Purbolaksono, L.C. Beng, A. Ahmad, Failure evaluation on a high-strength alloy SA213-T91 superheater tube of a power generation, Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering 224 (4), , [7] J. Ahmad, J. Purbolaksono, Hydrogen damage in rear riser water wall tube of a power plant, Engineering Failure Analysis, Vol. 17, Issue 5, , 2010.