Cycle chemistry influenced failures and performance losses have increased recently on fossil plants worldwide. These directly affect the overall performance and cost the industry enormous amounts of money in availability loss and replacement power. For instance the number of hydrogen damage incidences is at an all-time high, and the percentage of organizations experiencing each of the major chemically influenced boiler tube failures has increased drastically since 1997. The paper analyzes this situation in a number of ways. Firstly, the case is made that cycle chemistry is the major business in a fossil plant, and to be successful an organization needs to address the performance gaps associated with day-to-day operation, as well as the introduction of new technology and new ways of doing business. Secondly, a benchmarking process is introduced that has been applied to around 100 fossil plant organizations/plants/units. Deeper analysis of the areas "behind" the benchmarks illustrates how organizations need to link together all aspects of cycle chemistry around the plant, and that dealing with only one or two items is not sufficient and will lead to an increasing number of chemically influenced activities. Finally, analysis of all the results from this large number of organizations/ units/plants leads to an indication of the most reliable chemistry treatment/plant system/materials combination. Thus, the way is clear for any organization that desires to improve or build in reliability and performance either in a new unit or as retrofit.
A brief review is presented of the electrochemical mitigation of intergranular stress corrosion cracking (IGSCC) in water-cooled reactor heat transport circuit structural materials. Electrochemical control and mitigation is possible because of the existence of a critical potential for intergranular stress corrosion cracking and due to the feasibility of modifying the environment to displace the corrosion potential to a value that is more negative than the critical value. However, even in cases where the corrosion potential cannot be displaced sufficiently in the negative direction to become more negative than the critical potential, considerable advantage is accrued, because of the roughly exponential dependence of the crack growth rate on potential. The most important parameters in affecting electrochemical control over the corrosion potential and crack growth rate are the kinetic parameters (exchange current densities and Tafel constants) for the redox reactions involving the principal radiolysis products of water (O2, H2, H2O2), external solution composition (concentrations of O2, H2O2, and H2), flow velocity, and the conductivity of the bulk environment. The kinetic parameters for the redox reactions essentially determine the charge transfer impedance of the steel surface, which is shown to be one of the key parameters in affecting the magnitude of the coupling current and hence the crack growth rate. The exchange current densities, in particular, are amenable to control by catalysis or inhibition, with the result that surface modification techniques are highly effective in controlling and mitigating intergranular stress corrosion cracking in reactor coolant circuit materials.
PowerPlant Chemistry 2002, 4 (6)
Andy Rudge, Phil Turner, Ajit Ghosh, Wolfgang Clary, and David R. Tice
For the first time in their operational lives, UK advanced gas-cooled reactor once-through boilers have been chemically cleaned. Chemical cleaning was necessary to avoid lost output resulting from boiler pressure drops, which had been increasing for a number of years. Chemical cleaning of these boilers presents a number of unique difficulties. These include lack of access to the boilers, highly sensitised 316H superheater sections that cannot be excluded from the cleaning flow path, relatively thin boiler tube walls and an intolerance to boiler tube failure because of the role of the boilers in nuclear decay heat removal. The difficulties were overcome by implementing the clean in a staged manner, starting with an extensive materials testwork programme to select and then to substantiate the cleaning process. The selected process was based on ammoniated citric acid plus formic acid for the principal acid cleaning stage. Materials testwork was followed by an in-plant trial clean of six boiler tubes, further materials testwork and the clean of a boiler tube in a full-scale test rig. An overview is presented of the work that was carried out to demonstrate that the clean could be carried out safely, effectively and without leading to unacceptable corrosion losses. Full-scale chemical cleaning was implemented by using as much of the existing plant as possible. Careful control and monitoring was employed to ensure that the cleaning was implemented according to the specified design, thus ensuring that a safe and effective clean was carried out. Full-scale cleaning has resulted in significant boiler pressure drop recovery, even though the iron burden was relatively low and cleaning was completed in a short time.
PowerPlant Chemistry 2002, 4 (6)
Keiko Kudo, Masako Yasutomi, Hirohisa Kubota
Thermally Stable Anion Exchange Resin and Postulated Effect by Application in Condensate Polishing System
A strongly basic anion exchange resin which has a butylene spacer between the ion exchange group (trimethylammonium group) and the benzene ring has been found to have higher thermal stability than conventional anion exchange resin, which places the trimethylammonium group in the benzyl position. Thus it is now possible to operate anion exchange resin at higher temperatures than was previously acceptable. Elevated temperatures decompose the chemical structure of anion exchange resin at the anion exchange site. The rate of thermal decomposition is much higher with conventional anion exchange resin as compared to thermally stable anion exchange resin. In addition, it is known that in condensate polishing systems polystyrenesulfonic acid leaches out of cation exchange resin. These polystyrenesulfonic acid species can be adsorbed at anion exchange sites of anion exchange resin. Thus, thermally stable anion exchange resin exhibits a higher adsorption capability for polystyrenesulfonic acid than does conventional anion exchange resin, especially when they are utilized at elevated temperatures.
In condensate polishing systems, the requirement for anion exchange resin with higher sulfate capability at elevated temperatures has been increasing in order to generate higher power levels for longer periods and to maintain the reliability of the power plant.
PowerPlant Chemistry 2002, 4 (6)
Tony C.-T. Lam and Robert P. Dewey
Probabilistic Analysis of Turbine Disc Stress Corrosion Cracking Incubation and Propagation
Stress corrosion cracking (SCC) is a common problem found on aging low pressure turbine rotors that operate in a wet/dry stream environment. While much has been published on the growth rate of SCC in turbine rotor disk materials, incubation time is rarely addressed. Since no effective way has been demonstrated to prevent disk rim SCC from occurring other than to replace the damaged rim with a weld repair of higher chromium content, a better understanding of incubation time could provide operators with a means to treat SCC before cracks are large enough to start to grow. This paper discusses the critical mechanisms involved in the SCC incubation process and describes a probabilistic approach to make meaningful assessments of incubation time. Data published for General Electric turbine rotors is used to test the model.
The information presented in the previous seven Conferences on Unburned Carbon (UBC) on Utility Fly Ash, dating from 1995 to 2001, is reviewed. The objectives of the conferences have been (1) to provide a forum for ash associations, boiler manufacturers, entrepreneurial companies, federal and state government agencies, research personnel and utilities to discuss technical, regulatory and economic issues associated with UBC, and (2) to facilitate progress toward mitigating the problem.
Past conference subjects have consisted of regulatory updates, experiences and observations, predictive performance tools, processing and utilization of high-UBC fly ash, high-UBC fly ash characterization, effects on specifications and resultant sales, and economics. The impact of the conferences towards understanding and mitigating the problem of UBC on fly ash is analyzed.
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