Evaluating Deposit Formation of Iron-Corrosion Products in High- and Low-Pressure Evaporative Circuits When Starting a Combined-Cycle Plant

Abstract

One of the main causes of equipment damage at TPPs is corrosion of structural materials and deposit formation on the heat-transfer surfaces of the steam-water tract. This process occurs during stationary operation of the equipment and during start-ups and shutdowns of the power unit. In this paper, we assess the deposit formation of corrosion products in the evaporative circuits during the start-up of a combined cycle plant with the subsequent prediction of the number of starts after which it is necessary to carry out a chemical washing of the boiler-utilizer. The changes in the product concentration of iron corrosion in water and steam when starting a 450-MW power unit from a cold state are shown. The probability for the deposit formation of iron-corrosion products is estimated based on the experimental data obtained at start-up, with the subsequent calculation of the material balance for iron-corrosion products for low- and high-pressure circuits. A comparative analysis of the probability for deposit formation of iron-corrosion products in the high- and low-pressure circuits during the start-up period and at nominal operating parameters is performed. As a result of studies, it was found that the iron concentration in the feed and boiler water and saturated steam of the high- and low-pressure circuit significantly exceeds the normalized values by the time when the boiler-utilizer reaches its operating parameters. The specific amount of iron-corrosion products that can be deposited on heat-transfer surfaces in one start for the low- and high-pressure circulation circuit is determined. To reduce the probability for deposits of corrosion products during start-up, it is proposed to use feed water with a lower iron concentration or to increase the purge of the boiler-utilizer. Recommendations on the shutdown of the boiler-utilizer for washing when the maximal specific deposit amount of 200 g/m2 is reached taking into account the number of permissible starts for the low-pressure circuit are given.