Thermal Stress Analysis Of Steam-side Oxide Scale Of High Superheater In Ultra Supercritical 600MW Boiler

The failure behavior of oxide scale on steam side of superheater of thermal power plant boilers,such as cracking and falling off during shutdown,poses a serious threat to the safety of start-up and shutdown process of units.In this thesis,the final superheater of a 600 MW supercritical boiler is taken as the research object.The combination of numerical calculation and experimental research is used to calculate and analyze the thermal stress state and exfoliation phenomenon of the steam-side oxide scale during the shutdown process.There are three main aspects:(1)Acoustic emission(AE)testing system and universal mechanical testing machine were used to simulate the stress state of oxide scale during shutdown by unidirectional loading method,and the critical exfoliation strain of oxide scale on steam side was measured.Owing to the slow application of tensile or compressive loads,the oxide scale may release strain energy by cracking or wrinkling,which makes the strain of the oxide scale and the matrix unequal in the measurement process.Therefore,a model is established.The critical exfoliation strain of the oxide scale is calculated by the strain of the matrix during the loading process,and the critical exfoliation strain curves of the oxide scale under tensile and compressive stresses are plotted.The critical exfoliation strain of the oxide scale at ambient temperatures of 200°C and 280°C was measured in a high temperature chamber.The experimental results show that the higher the temperature,the greater the critical exfoliation strain of the oxide scale.(2)The boiler thermal calculation program was compiled to provide data support for thermal stress analysis and exfoliation analysis of the steam-side oxide scale of the final superheater.A numerical calculation model of transient thermal stress of steam-side oxide scale in the last superheater T23 and TP347 alloy tubes during shutdown was established.The distribution and characteristics of thermal stress in the process of steam cooling and sootblowing were calculated respectively,considering the effects of different steam cooling rates and soot-blowing speeds on the thermal stress.The model was also compared with the thermal stress of oxide scale under steady state cooling.It is found that the oxide scale is more likely to fall off in the steam cooling process of the T23 alloy tube section when the boiler is shut down,and the possibility of the oxide scale exfoliation on the leeward side is higher than that on the windward side.The thermal stress of oxide scale in TP347 alloy tube section will be weakened by the transient additional thermal stress produced by the steam cooling process,which will inhibit the possibility of falling off to a certain extent.However,the transient thermal stress generated during the soot-blowing process will increase the thermal stress in the oxide scale,and the possibility of falling off of oxide scale on the leeward side is higher than that of the oxide scale on the windward side.(3)The dynamic response model of the outlet steam temperature and the prediction model of the steam-side oxide scale growth under the inlet steam temperature step disturbance of the final superheater were established and calculated.Combined with the thermal stress state of the steam-side oxide scale and the critical exfoliation strain curve of the steam-side oxide scale during the shutdown process,the relationship between the allowable rate of change of the outlet steam temperature and the running time during the shutdown process is calculated.The calculation shows that when the running time does not exceed 51005 h,the allowable variation rate of the outlet steam temperature during the shutdown process is controlled within 3 °C/min,so that the oxide scale in the final superheater does not fall off in a large area,ensuring the safe operation of the boiler.When the running time exceeds 51005 h,the steam cooling rate should be appropriately reduced during the shutdown process,and the fluctuation rate of the outlet steam temperature should be strictly controlled to prevent large-scale exfoliation of the oxide scale inside the tube.Moreover,in the maintainance process,it is necessary to carry out strict inspection on the oxide scale inside the tube to avoid the exfoliation of the oxide scale in the start-up of the boiler.