High operational flexibility (e.g. the ability of a power plant for fast start-up and to adjust load output as a function of changing of market requirement) is an essential pre-requisite to ensure economic success in a liberalized market.
Plants under cycling duty are subjected to excursions of temperature, pressure, water flow rate, making very difficult to follow appropriate cycle chemistry guideline limits and greatly influencing the performance of material, especially in power plants where exported steam is produced together electrical energy.
Among failure mechanisms, Flow-Accelerated Corrosion (FAC) is recognized as one of the major area of concern, investing all parts of Low Pressure (LP) sections of combined cycle plants (economizer, evaporators, risers, drum boiler). FAC causes wall thinning (metal loss) of carbon steel piping, tubing and vessels exposed to flowing water (single-phase) or wet steam (two-phase), caused by the solubilization of protective oxides.
FAC factors are well known but the application of these understandings to power plants has not been entirely satisfactory.
The Oxidizing-Reducing Potential (ORP), recognized as the most influencing parameter, has been found to provide FAC warnings, particularly if measured in water at operating temperature and pressure. ORP is sensitive to the materials of construction and to the temperature because of the effects of temperature on the redox reactions. ORP also changes with pH, partial pressure of oxygen or oxygen-scavengers in the flowing water, mass transport properties and flow rates.
In this frame, CESI studies this issue, making use of the AT ORPã measurement system of NALCO, with the objective of reducing the potential for FAC in combined cycle plants, during normal and cycling operations, and to determine the impact of cycling operation on unit life expectancy and reliability.