Turbine Inlet Valve Fitness for Service
In a recent project, Powertech conducted a fitness-for-service (FFS) analysis to establish the integrity of a turbine inlet valve(TIV) for continued use in a generating station. The TIV was a butterfly valve installed at a peaking plant in 1951. Many surface cracks were found along the central location on both sides of the disc. Replacement of the valve would be costly and require significant downtime. The Powertech FFS project sought to determine if the TIV could be used as-is or would require repair at specific locations to restore its integrity.
The project was conducted in six phases:
1. NDT assessment. Dry powder magnetic particle inspection was conducted on the disc’s upstream side to map cracks.
2. Residual stress analysis of the weldment. A hole-drilling strain gauge method was used to measure residual stress.
3. Metallurgical analysis. Samples were subjected to chemical analysis, scanning electron microscopy, energy dispersive x-ray spectroscopy, Fourier transform infrared spectroscopy, metallographic examination, and microhardness testing.
4. Mechanical testing. Material was removed to test for Charpy V-notch impact energy, fracture toughness, yield strength, ultimate tensile strength, and elongation to failure.
5. Stress analysis. A finite element analysis was carried out to estimate maximum stresses and their locations when the disc is closed.
6. Fitness-for-service analysis. Analysis was conducted for static and cyclic stress conditions.
Analysis indicated that, although the detected flaws were not vulnerable to fatigue cracking under normal operational cyclic stresses of start/stop operations, they may be vulnerable to crack instability under potential high static loading conditions. A plate-cladding weld repair procedure was developed to restore the integrity of the TIV disc.