Research on Remaining Life Evaluation Method of T92 Steel for Superheater Tube Based on Oxide Layer Growth
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Abstract
T92 steel is widely used in high-temperature superheater of supercritical power station boiler, and the researches on its remaining life have always been the hot spot of scholars around the world. In this paper, based on the analysis of high-temperature creep rupture strength data, the optimum C value of Larson–Miller parameter formula for T92 steel is obtained by using isothermal extrapolation method. On this basis, combining the relationship between the thickness of the oxide layer and the operating time, the relationship between the thickness of the oxide layer and the temperature of the tube wall and the relationship between the thickness of the oxide layer and the stress of the tube wall, a new remaining life evaluation formula is deduced. Finally, based on the cumulative creep damage life evaluation method, the service life of superheater tube is divided into three stages, which provides a new reference for the service life evaluation of superheater tube of ultra-supercritical boiler.
Keywords
Larson–Miller parameter Superheater Oxide layer growth Remaining life evaluation Cumulative creep damageIntroduction
Due to the large heat transfer resistance of the oxide layer, the heat exchange between the steam medium and the wall metal is blocked, which leads to the increase in the temperature of the wall metal. Relevant calculations show that for every 0.025 mm oxide increase, the superheater wall temperature increases by about 1.67 °C. At the same time, exfoliated oxide skin will block the steam flow and cause superheater tube over-temperature explosion. This has become the second major cause of boiler tube failure in the world [1]. T92 steel is widely used in high-temperature superheater of supercritical power station boiler. Therefore, it is of great significance to evaluate the remaining life of T92 steel for superheater tubes. Rolf Sandström et al. [2] combined the precipitation hardening model to derive the life assessment model for ductile fracture of austenitic heat-resistant steel. Purbolaksono et al. [3] found that the major contributor which results in the failure of the tube was interaction between the excessive scale formation on the inner surface and outer wall thinning due to coal-ash corrosion. Kapayeva et al. [4] presented the method that considers a combined effect of overheating and wall thinning. They used LMP method for creep evaluation, while adding the effect of wall thinning. The actual data for the thickness of tube wall and thickness of internal oxide layer were taken from measurements using nondestructive testing methods. Ilman et al. [5] found the cause of failure was overheating due to deposit buildup inside the superheater tube. In this paper, based on the LMP formula, considering the relationship between the thickness of the oxide layer and the operating time and the relationship between the thickness of the oxide layer and the temperature and stress of the tube wall, a new remaining life evaluation formula is derived to evaluate the use of the ultra-supercritical boiler superheater tube life.
Establishment of Remaining Life Assessment Method Under Ideal Conditions
- 1.
Considering the formula for the growth rule of the oxide layer with the boiler operating time.
- 2.
Considering the effect of oxide layer thickness on tube temperature and the relationship between them.
- 3.
Considering the effect of oxide layer thickness on tube stress and the relationship between them.
- 4.
It is assumed that the oxide layer is structurally continuous and isotropic and will not fall off during the growth process.
Establishment of the Basic Model of the Life Assessment Method
The life evaluation formula model of T92 steel has been widely concerned at home and abroad. The Larson–Miller parameter method based on creep damage life evaluation is a time–temperature parameter (TTP) method. The method comprehensively considers the stress and temperature during the operation of the boiler superheater tube. And after a lot of verification, the C value in the formula is constantly revised [3], which has played a good guiding role in the life evaluation of the superheater tube of the power station boiler for many years [6].
Linear fitting results of lgt_{r} and temperature 1/T under different stress values
σ/Mpa | T/°C | Fitting equation | C value |
---|---|---|---|
80 | 625–650–675 | y = − 20.92 + 23159.23x | C_{1} = 20.92 |
90 | 625–650–675 | y = − 24.89 + 26614.63x | C_{2} = 24.89 |
100 | 625–650–675 | y = − 27.29 + 28551.98x | C_{3} = 27.29 |
110 | 625–650–675 | y = − 32.30 + 32905.4x | C_{4} = 32.30 |
130 | 600–625–650 | y = − 29.41 + 29714.09x | C_{5} = 29.41 |
140 | 600–625–650 | y = − 26.76 + 27209.87x | C_{6} = 26.76 |
160 | 600–625–650 | y = − 24.98 + 25100.97x | C_{7} = 24.98 |
180 | 550–575–600 | y = − 35.29 + 33641.73x | C_{8} = 35.29 |
190 | 550–575–600 | y = − 33.96 + 32126.39x | C_{9} = 33.96 |
210 | 550–575–600 | y = − 38.91 + 35803.57x | C_{10} = 38.91 |
Fitting results of LMP principal curve equation of T92 steel
C value | lgσ = a+b × P+c × P^{2} + d×P^{3} | R ^{2} | Adj. R^{2} | |||
---|---|---|---|---|---|---|
a | b | c | d | |||
C_{1} = 20.92 | −14.97876 | 0.00216 | −8.23746E^{−8} | 8.9505E^{−13} | 0.97927 | 0.97739 |
C_{2} = 24.89 | −6.81824 | 8.6542E^{−4} | −2.09939E^{−8} | 2.80638E^{−14} | 0.99065 | 0.9898 |
C_{3} = 27.29 | 0.85624 | −4.96756E^{−5} | 1.36089E^{−8} | −3.6646E^{−13} | 0.99317 | 0.99255 |
C_{4} = 32.30 | 20.10741 | −0.00187 | 6.81892E^{−8} | −8.4807E^{−13} | 0.99235 | 0.99165 |
C_{5} = 29.41 | 8.64311 | −8.5289E^{−4} | 3.97613E^{−8} | −6.2093E^{−13} | 0.99362 | 0.99304 |
C_{6} = 26.76 | −0.95225 | 1.53245E^{−4} | 6.42646E^{−9} | −2.8983E^{−13} | 0.99283 | 0.99217 |
C_{7} = 24.98 | −6.58054 | 8.37503E^{−4} | −1.9729E^{−8} | 1.25266E^{−14} | 0.99078 | 0.98994 |
C_{8} = 35.29 | 32.40352 | −0.00281 | 8.96105E^{−8} | −9.7377E^{−13} | 0.98949 | 0.98853 |
C_{9} = 33.96 | 26.91026 | −0.00241 | 8.09986E^{−8} | −9.9158E^{−13} | 0.99092 | 0.99009 |
C_{10} = 38.91 | 47.11671 | −0.00375 | 1.06526E^{−7} | −1.0262E^{−12} | 0.98474 | 0.98335 |
\( \bar{C} \) = 29.47 | 8.8808 | −8.75846E^{−4} | 4.0458E^{−8} | −6.2715E^{−13} | 0.99361 | 0.99303 |
Relationship Between Growth Thickness of Oxide Layer and Operation Time of Boiler
In general, the index n is related to the actual working condition, and the value ranges from 1 to 2. When the reaction kinetics curve shows linear oxidation, the index n = 1; when the reaction kinetic curve shows parabolic oxidation, the index n = 2. According to the experimental study of high-temperature steam of T92 steel, the reaction kinetic curve of steam oxidation of T92 steel is parabolic oxidation [9], so the index n in the formula is 2.
The Arrhenius constant A is related to the composition of the metal alloy material. When the Cr content in the alloy is 0–2%, A = 3.70; when the Cr content in the alloy is 9–12%, A = 230.5, so A of T92 material is 230.5 [10].
The process rate control activation energy Q is related to the metal material and operating conditions. In the 9–12% Cr alloy, when the temperature is between 290 and 700 °C, Q is about 146 kJ/mol.
The gas constant R is the ratio of the product of the absolute pressure p of the ideal gas and the specific volume v to the thermodynamic temperature T, and R is 8.314 J/(mol*K).
The maximum temperature T of tube operation is 953 K according to the T92 steel manual.
The Formula of Remaining Life is Obtained by Combining L–M Formula
Remaining Life Assessment Method Based on Cumulative Creep Damage
According to Eq 17, the value of the integral upper limit t of f(t) can be calculated when the pressure tube is identified as cumulative creep damage failure, that is, when D_{1} = 1. After the calculation of Mathematica1 1.2 software, when the operating time of a power plant unit is t = 49202.1, D_{1} = 1. It means that the boiler superheater tube has been identified as cumulative creep damage failure after the unit has been running for 49,202.1 h. From the perspective of safe operation of the boiler, this situation requires the replacement of a new superheater tube to ensure safe operation of the units.
Conclusion
- 1.
It is deduced that the optimum C value of LMP formula based on T92 steel is 29.40979.
- 2.
According to the remaining life assessment method derived in this paper, in the practical situation, the power plant operators detect the oxide thickness of the tubes and then substitute the measured data into Eq 8 in the text to calculate the running time. Finally, the calculated running time is substituted into Eq 12 in the text to get the remaining time.
Notes
References
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