274The conditions under which the steam superheat ers of boilers at thermal power stations (TPSs) operate are characterized by nonuniform distribution of cool ant due to the fact that their coils are mechanically nonidentical with one another, as well as due to non uniform fields of flue gas velocity and temperature. Such conditions result in that the metal is subjected to thermal nonuniformity and, consequently, experi ences nonuniform loss of strength [1,2]. The steam superheating path accounts for 60-70% of failures of heating surfaces, which occur mainly due to thermal misalignment. For example, deviation of temperature from its average level toward increasing by 10°С results in that the coil service life decreases by a factor of 1.7-1.9. The aim of examining the state of steam super heaters is to reveal coils operating under the worst temperature conditions and having the minimal resid ual service life.Calculations for estimating the remaining service life of steam superheaters made of Grade 12Cr18Ni12Ti steel with respect to heat resistance conditions are based on the results of analysis on determining the equivalent operating temperature of the coil external surface from the quantity of second ary carbides or the σ phase in the metal structure and from the stress in metal due to internal pressure of coolant [3]. These data are obtained during a labora tory study of a limited number of samples (as a rule, three to six ones) cut from coils based on the results of thickness measurements. Examination of thermal nonuniformity is a necessary operation for cutting representative samples, and the final estimation of remaining service life is made after carrying out a met allographic study.It is unfeasible to carry out total temperature exam ination of steam superheaters by means of thermome tering inserts. At present, ultrasonic thickness mea surements of coils during equipment repairs is the only method for identifying thermal nonuniformity, which requires cleaning of the external surface from scale and takes a long period of time for carrying out measure ments. Thickness is measured on a sampling basis, at accessible locations, and not always on the front side where the maximal thinning occurs. Experience has shown that information on thermal nonuniformity obtained using thickness measurements is unreliable, especially at early stages of operation and for boilers operating predominantly on natural gas. Lack of reli able methods for recognizing thermal nonuniformity does not allow better quality of diagnostics to be achieved.A considerable part of gas and oil and coal fired boilers operates for 1-5% of the time on fuel oil. The temperature of coils made of austenitic steels does not exceed 620°С in the base mode of operation. The rate of high temperature corrosion of Grade 12Cr18Ni12Ti steel in flue gases produced from firing natural gas is a factor of 5.5 lower than that during the firing of fuel oil. The heat resistance of Grade DI59 steel in flue gases produced from firing fuel oil is at least a factor of 5 hi...

The thermal regime in convective steam superheaters with shortened coils is studied. Shortening is found to cause underheating and extends the lifetime of the shortened coils, but creates a temperature wake in the flue gases from the thinned packets that initiates local overheating and nonuniform weakening of the metal of the convective heating surfaces in the downstream flue gas flow. Recommendations are made for reducing the negative consequences of shortening.Damage statistics show that 35% of the damage to heating surfaces occurs on convective steam superheaters. The major reason for their failure is thermal inhomogeneity, prolonged heating at or above the maximum temperature, hightemperature corrosion, and thermal fatigue of the metal. The thermal inhomogeneity and nonuniform weakening of the metal are caused by nonuniform distributions of the steam feed rate and of flue gas velocity and temperature.Damaged coils generally become blocked. After a certain time, they burn up, gas corridors form at the site, and the source region for failure expands. Calculations of the temperature regime and time to failure of such thinned packets [1] have shown that this operating practice leads to a higher temperature and reduced operating lifetimes for the remaining coils, as well as to the formation of a temperature wake in the flue gases at the outlet from damaged packets. In order to enhance the reliability of convective steam superheaters, damaged coils should be replaced in order to prevent the formation of gas corridors out of the thinned packets [1].Sometimes the simplest, but not best, method is chosen for extending the service lifetime of a thinned packetshortening the remaining coils to increase the steam velocity and raise the heat transfer to the steam. The shortening is done without analyzing the change in the temperature regime of the steam superheater, without considering the negative consequences of a nonuniform temperature distribution of the flue gases at the outlet from the thinned packets with shortened coils. The resulting temperature wake can cause overheating and more rapid high temperature corrosion of the metal of the steam superheater downstream in the flow of flue gases.The temperature regime of a convective steam superheater with partially shortened coils is studied in this article.The problem is solved by thermal and physical calculations, magnetic ferritometry, and quantitative metallographic analysis for the case of a type ND-2 second-stage, low-pressure, convective steam superheater (CSS) in a type TGMP-314 flow-through supercritical-pressure boiler. The ND-2 CSS is installed in a counterflow configuration. The steam superheater consists of flows A and B and is made up of 106 twoand three-coil packets with pipe diameters of 42/34 mm in the heated zone. The packets are arranged in a checkerboard pattern. As manufactured, the coils had four passes. The parts of the coils for the first and second passes are made of 12Kh1MF steel and the third and fourth, of 12Kh18N12T. Packets 24,[26][27][28][29...

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