Metal fatigue is a potentially life limiting mechanism for pressure retaining components in nuclear and fossil-fueled power plants. Fatigue cracking may occur at high stress locations due to the accumulation of startup-shutdown cycles and other thermal transients or load cycles during plant operation. This is accentuated in fossil power plants because of higher operating temperatures and the interaction between creep and fatigue effects.
Under sponsorship of the Electric Power Research Institute (EPRI), an automated fatigue lifetime monitoring technology has been developed that utilizes plant process data to perform a continuous prediction of fatigue damage accumulation in critical components. The methodology was demonstrated in two field tests at operating nuclear plants and has since been installed on a production basis in a number of plants. A demonstration fossil plant application has also been developed and installed, adding creep-fatigue damage and crack growth algorithms to the basic nuclear plant technology.
The methodology has been incorporated into a PC-based computer program called FatiguePro. This program acquires plant process computer data (temperatures, pressures, flows, valve positions, etc.) and interprets them to predict local loads and temperatures in the monitored components. Green's functions are then used to determine transient thermal stresses at high stress locations in these components and to account for thermal history effects. A fatigue evaluation is then made based on the computed transient stress history. The fatigue analysis, based on Miner's Rule, is accomplished in accordance with ASME Boiler and Pressure Vessel Code stress and fatigue analysis procedures. Methodology is also available to perform fatigue and creep crack growth analyses.
The major benefit of on-line monitoring is that the actual challenge to structural integrity is measured, instead of simply counting design-basis cycles, as was the practice before the development of this system. Since actual plant operating cycles are typically less severe than those included in design, extension of predicted design lifetime can usually be obtained.