Instrument Nozzles Fatigue Strength Reduction Factor
Recent industry experience has raised concerns with the NRC regarding the fracture toughness of small partial penetration instrument nozzles. Historically, small-bore instruments have not been evaluated when Pressure-Temperature curves were developed. Typical instrument nozzles have an outside diameter of 3¼ inches or less and a wall thickness of 11/16 inches or less. They were considered to be exempt per ASME XI Appendix G-2223(c), “Fracture toughness analysis to demonstrate protection against non-ductile failure is not required for portions of nozzles and appurtenances having a thickness of 2.5 in. (63 mm) or less, provided the lowest service temperature is not lower than RTNDT plus 60°F (33°C).” Now it appears that the effect of the nozzle discontinuity on the stresses in the beltline region of the shell must be assessed. This paper will show that historical interpretation of the code is correct and a fatigue analysis of the vessel shell adjacent to these instrument nozzles is unnecessary. The industry experience with instrument nozzle leakages is typically related to the quality of manufacturing as suggested by J.H. Gross. Fabrication and inspection range quality relative to FSRF is ke = 10 for poor and ke < 5 for good quality. This approach appears to correlate well with this consideration and industry experience. This paper will also demonstrate the use of fabrication travelers and/or inspection information that can be used to determine an appropriate FSRF to assess the relative health of these and other components.