Steam Turbine Operating Conditions, Chemistry of Condensates, and Environment Assisted Cracking – A Critical Review

A review of the literature and discussions with plant operators has been undertaken to establish 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 an overview of the effect of test variables on environment assisted cracking. In Section 1, steam turbine operating conditions are described including the steam cycle, the 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. On the other hand, 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 start-up). In Section 2, stress corrosion cracking (SCC) data for disc steels are reviewed critically. The effect of material properties (steel cleanliness, yield strength), environment (oxygen, carbon dioxide, chloride level), temperature, and stress conditions, on the initiation and growth of stress corrosion cracks is assessed. For long cracks, growth rates are claimed to be independent of oxygen and CO2 concentration, and steel cleanliness, but there are some exceptions that suggest that the apparent independence of these variables may be linked partly to the spread in growth rate data. Unquestionably, the variables having the most dramatic effect are strength level and temperature. Dissolved gases and steel cleanliness have a more profound influence on “initiation” of stress corrosion cracks for medium strength disc steels, certainly in relation to pit development and growth but the impact on short crack growth is less well established.