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...

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The construction and materials of heat-exchangers should ensure their trouble-free exploitation over the entire service life and the retention of their efficiency factor. In the case of aggregates that are exploited in the atmosphere of a humid-tropical climate, the main factor that leads to the deterioration of exploitation characteristics is corrosion of the external surface of heat exchangers.The rate of atmospheric corrosion is determined by the corrosivity of the media [1], which, on the one hand, can lead to a thinning of the walls of the heat-transferring surface up to its perforation and, on the other hand, to a reduction in the heat-transfer efficiency due to the formation of dense corrosion products on the surface.At the Kudankulam nuclear power plant (India), aircooled heat exchangers intended for the removal of residual heat emission of a reactor in emergencies (e.g., the loss of all sources of electric power supply) are used as a system that should guarantee the passive safety of the reactor in such an emergency (passive heat removal system, PHRS).It is known that the main characteristic of a tropical climate is high temperature (20-35 ° C) combined with high air humidity for long periods of time [2,3]. The atmosphere in the region of the Kudankulam nuclear power station is characterized, apart from high temperature and air humidity, by a high content of corrosive agents such as chlorides (100 mg/(m 2 day)), sulfates (50 mg/(m 2 day)), sulfur dioxide (0.025 mg/m 3 ), ozone (190 µ g/m 3 ), etc.The most complex structural and technological features, from the point of view of corrosion protection, that wholly determine the service-life capacity are the heat-exchange coils of PHRSs (pipes 25 mm in diameter with walls 2.5 mm thick (steel 20) and ribs made of a ribbon 1 mm thick (steel 10)).The necessity of applying protective coatings (PCs) has been confirmed by the results of determining annual corrosion losses in a humid-tropical climate (corresponding to conditions of maintenance of PHRSs).The appearance of the templates after the tests shows intense corrosion of the surface of the coil elements; the average rate of corrosion of structural materials (steel 20) is, according to the results of metallographic investigations, about 95 µ m/year when there are such local defects as pits with a maximum depth of up to 90 µ m (without allowance for the loss of metal due to general corrosion) and crevice corrosion at sites where the ribs are welded to the pipes.The experimentally obtained values of the rate of general corrosion agree well with the calculation by the equation that is included into the modified standard ISO 9223 [4]; namely, K = 90 µ m/year at T = 25 ° C and Rh = 80%; K = 95.9 µ m/year at T = 28 ° Cand Rh = 80%; K = 99.9 µ m/year at T = 30 ° Cand Rh = 80%; K = 104.9 µ m/year at T = 25 ° Cand Rh = 85%; K = 111.3 µ m/year at T = 28 ° Cand Rh = 85%; and K = 116.6 µ m/year at T = 30 ° Cand Rh = 85%.These results, as might be expected, have unambiguously shown how necessary it is to protect the heatexc...

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