Investigation into Factors Causing Damage to Low-Pressure Loop Evaporating Tubes of Large-Capacity Heat-Recovery Steam Generators

The article addresses factors causing damage—in particular, flow accelerated corrosion wear (FAC) of tube walls—to the evaporator operating in the low-pressure loop of the Ep-258/310/35-15.0/3.14/0.44-540/535/263 (P-132) heat-recovery steam generator used as part of a 795-MW combined-cycle plant. Factors influencing the FAC, namely, improper water chemistry, availability of gas shunts in the steam generator, and high mixture motion velocity in the tubes in combination with the flow path configuration, are described. The damages inflicted to the tubes and their locations in the heat-recovery steam generator low-pressure loop are described. It is shown that gas shunts in the heat-recovery steam generator result in an increased heat absorption and higher velocity of medium in the boundary tubes of low-pressure evaporator tube banks, which intensifies the tube’s wear process. It has been found that the water chemistry used for the Kirishi District Power Plant (DPP) heat-recovery steam generator was not the factor that caused damage to the low-pressure loop tubes. The Kirishi DPP steam generator was compared with other similar large-capacity heat-recovery steam generators. For analyzing the low-pressure evaporator performance indicators, the loop and the processes occurring in it were modeled using the Boiler Designer software. An analysis of the results from comparison of the steam generator design and performance characteristics has shown that it is necessary to change the two-phase mixture motion velocity in the Kirishi DPP steam generator low-pressure loop by modifying the design or operating parameters. The wear of the low-pressure evaporator tube walls was mathematically modeled, the results of which confirm that erosion wear is one of the main factors causing damage to the tubes. The erosion is caused by the intense dynamic effect of two-phase, high-velocity flow jets on the tube walls. General recommendations for decreasing the wear of heat-recovery steam generator tubes are given. It has been determined that the increased wear of tubes in the low-pressure loop of the P-132 steam generator at the Kirishi DPP is caused by a combination of a few factors, such as high velocity of steam–water mixture, availability of bends, and unsatisfactory quality of aligning the tubes at their welding places.