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Figure from:Failure analysis and retrofitting of superheater tubes in utility boiler

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In order to determine the chemical composition of the tubes, a chemical analysis was performed. The results, as shown in Table 1, indicated that the chemical composition was complied with the 12Cr1MoV original composition of the tube.

Table 1 In order to determine the chemical composition of the tubes, a chemical analysis was performed. The results, as shown in Table 1, indicated that the chemical composition was complied with the 12Cr1MoV original composition of the tube.

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Fig. 2. The image of failed tube. Fig. 1. Schematics of Sahand superheater system with two attemperator.
Fig. 2. The image of failed tube. Fig. 1. Schematics of Sahand superheater system with two attemperator.
Chemical composition of scale. Table 2
Chemical composition of scale. Table 2
Chemical composition of material. 3.3. Scale analysis Table 1
Chemical composition of material. 3.3. Scale analysis Table 1
Fig. 3. (a) The outer surface of damage tube, (b) brittle failure of the damaged tube.
Fig. 3. (a) The outer surface of damage tube, (b) brittle failure of the damaged tube.
Fig. 4. X-ray diffraction pattern of the tube outer surface scale.
Fig. 4. X-ray diffraction pattern of the tube outer surface scale.
Fig. 5. The nature of carbides spheroidization in 12Cr1MoV steel [11]. From Level 1 to 5, the lamellar in pearlite changes to a particle structure. Microstructure examinations on the ruptured and un-ruptured sections (120 cm away from the rupture area) of the tube are conducted, the result is illustrated in Fig. 6. An advanced stage of spheroidization can be seen from this figure, which indicates the probability of creep failure. To confirm the occurrence of overheating (creep phenomenon) in the middle superheater tubes, creep analysis was performed and results are discussed in the following.
Fig. 5. The nature of carbides spheroidization in 12Cr1MoV steel [11]. From Level 1 to 5, the lamellar in pearlite changes to a particle structure. Microstructure examinations on the ruptured and un-ruptured sections (120 cm away from the rupture area) of the tube are conducted, the result is illustrated in Fig. 6. An advanced stage of spheroidization can be seen from this figure, which indicates the probability of creep failure. To confirm the occurrence of overheating (creep phenomenon) in the middle superheater tubes, creep analysis was performed and results are discussed in the following.
Fig. 7. Curve of stress vs. Larsen-Miller parameter of CrMoV steal [12]. As it was mentioned earlier, superheater tubes suffer from long term overheating (creep phenomenon), which is occurred over a period of months or years, therefore the creep rupture could be originated by local overheating. During the normal operation, superheater tubes will experience increasing temperature and strain over the life of the tube until the creep life is expended. In order to evaluate the deviation of operating conditions from design conditions, the steam outlet temperature of middle superheater and the mass flow rate of spray water in several conditions were recorded. These results are reported
Fig. 7. Curve of stress vs. Larsen-Miller parameter of CrMoV steal [12]. As it was mentioned earlier, superheater tubes suffer from long term overheating (creep phenomenon), which is occurred over a period of months or years, therefore the creep rupture could be originated by local overheating. During the normal operation, superheater tubes will experience increasing temperature and strain over the life of the tube until the creep life is expended. In order to evaluate the deviation of operating conditions from design conditions, the steam outlet temperature of middle superheater and the mass flow rate of spray water in several conditions were recorded. These results are reported
Fig. 6. (a) Microstructure of the rupture region, showing an advanced stage of spheroidization, (b) microstructure of the tube metal in the distance o 120 cm from rupture region (magnication of 500x).
Fig. 6. (a) Microstructure of the rupture region, showing an advanced stage of spheroidization, (b) microstructure of the tube metal in the distance o 120 cm from rupture region (magnication of 500x).
Operating results of Sahand Power plant boiler On March 2005. Table 3
Operating results of Sahand Power plant boiler On March 2005. Table 3
Fig. 8. Creep-rupture curve for 12Cr1MoV steel tested at 600 °C [13].
Fig. 8. Creep-rupture curve for 12Cr1MoV steel tested at 600 °C [13].
Fig. 10. Comparison of water spraying mass flow rate in boiler unit 1 and 2 at the same condition.
Fig. 10. Comparison of water spraying mass flow rate in boiler unit 1 and 2 at the same condition.
Fig. 9. Tube arrangement in the middle superheater of Sahand boiler (the removed tube was identified by red line). (For interpretation of the references to color in this Fig. 9 legend, the reader is referred to the web version of this article.)
Fig. 9. Tube arrangement in the middle superheater of Sahand boiler (the removed tube was identified by red line). (For interpretation of the references to color in this Fig. 9 legend, the reader is referred to the web version of this article.)
Modification of middle superheater structure. Table 4
Modification of middle superheater structure. Table 4
Comparison of spray water mass flow rate between boilers unit 1(retrofit boiler) and 2 of Sahand Power Plant. Table 5
Comparison of spray water mass flow rate between boilers unit 1(retrofit boiler) and 2 of Sahand Power Plant. Table 5