Overview Steam turbines: Part 1 – Operating conditions and impurities in condensates, liquid films and deposits

Abstract

A review of the literature and discussions with plant operators have provided the basis for this overview of present knowledge and understanding of the chemistry of the condensates formed on steam turbines and the link to system operation. This has been supplemented by Part 2 of this study: a review of the effect of test variables on environment assisted cracking, which is to be published in Corrosion Engineering, Science and Technology. Steam turbine operating conditions are described including the steam cycle, chemical control of the steam/water circuit and the distribution of temperature, pressure and steam wetness in the turbine. Service experience indicates that cracking occurs mainly in regions where condensates form. The saturation temperature, at which condensation occurs, varies from station to station, depending on the turbine design and operating conditions. Calculations based on thermodynamic data under steady flow conditions show that the concentration of non-volatile species (e.g. chloride, sulphate, sodium, etc.) in the liquid phase can be 100 times higher than that in the steam. Much higher concentrations of these species may be present in deposits in the regions where the steam is dry or dry/wet cycles occur but these regions do not correlate with observed cracking in service. The concentration of volatile species (e.g. oxygen, carbon dioxide) in the liquid phase is usually less than 1 ppb under normal operating conditions. However, concentrations of oxygen and carbon dioxide in the condensates can be as high as several ppm under transient conditions (forced condensation, air leakage, off load and startup).