PREDICTION OF ONSET OF BOILING CONDITIONS IN STEAM-GENERATING ELEMENTS OF NATURAL CIRCULATION POWER UNITS

The stability of the power system operation, taking into account daily and seasonal load schedules, is determined by the technical condition of the power-generating capacities of enterprises within Ukraine’s energy sector. Having had, until recently, a surplus of thermal generation capacities, the issue of ensuring the stable operation of the power system has now become critical due to the extended service life, as well as damage and destruction of thermal power plant units. In the leading countries of the world, power units with similar parameters and equipment characteristics are in operation. However, unlike them, Ukrainian power units mainly operate in regulating capacity modes with significant load fluctuations. Such modes affect the temperature regime and thermal stress state of the metal in furnace wall tubes and superheating surfaces, which determines the technical condition and residual service life of the steam boiler. At present, the fleet of steam boilers, according to the circulation method in evaporative heating surfaces, is divided into once-through and natural circulation types. In once-through boilers, the movement of the working fluid throughout all elements of the steam-water path is provided by feed pumps at all stages of operation-from start-up and load regulation to shutdown. In contrast, in natural circulation boilers, the movement of the working fluid in the evaporative surfaces is ensured by the difference in density between the liquid in the downcomer tubes and the steam-water mixture in the furnace screens. The methodology of thermal and hydraulic calculations for modern steam boilers is based on the use of regulatory documents, which incorporate the results of theoretical and experimental research. It should be noted that the temperature of the tube walls in the heating surfaces being the decisive factor in their reliability does not always correspond to the results of industrial testing. The main reason for this discrepancy is the influence of coupled radiative-convective heat transfer and significant variations of thermal and hydraulic parameters both inside and outside the heating surfaces. Given the large share of equipment failures caused by damage to furnace screens, the analysis of their design features and heat transfer processes remains an urgent and relevant task. Bibl. 17, Fig. 2.