A Procedure to Include the Additional Fatigue Effects of Thermal Gradients in a B31.1 Butt Weld of Dissimilar Metals and Welding End Transition of Similar Metals for Design and Plant Life Extension

Fatigue evaluation in B31.1-2007 is currently done in accordance with para. 102.3.2 and generally considers only the stresses due to displacement load ranges and other cyclic loads. Yet fatigue damage is also occurring due to thermal gradients which are not considered. To exacerbate and complicate this additional type of fatigue damage, power plant design pressures and temperatures are rising, new materials are being introduced, pipes and attached components are becoming increasingly thick, and owners are requiring power plants to heat-up and cool-down at faster rates. Also, power plant owners are more and more interested in extending the life of power plants beyond their original design life. Although the fatigue design of ASME nuclear Class 1 piping components has long required thermal gradients to be considered, no attempt has been made to incorporate this knowledge into the B31.1 Power Piping Code (other than Creates, D. H., 2009, “A Procedure to Evaluate a B31.1 Welding End Transition Joint to Include the Fatigue Effects of Thermal Gradients for Design and Plant Life Extension,” PVP2009-77148, Proceedings of the ASME 2009 Pressure Vessels and Piping Conference, Volume 3, Design and Analysis, A. Segall, ed., ASME, New York, PVP-Vol. 3, pp. 101–110). To address this pressing need in today’s power plant environment this paper now provides a fully comprehensive methodology for assessing an “as-welded” Butt weld of dissimilar metals and a Welding End Transition (B31.1-2007, Fig. 127.4.2) of similar metals to include the additional fatigue effects of thermal gradients calculated in accordance with ASME Section III-2007 Subarticle NB-3600. The disadvantage of this approach is that the conservatism in these calculations may produce unacceptable results. In that case, this assessment is a warning that something else needs to be done by way of monitoring, modifying the design or the thermal operation, or performing a more rigorous evaluation. The advantage of this methodology is that it maintains the traditional B31.1 approach to fatigue with the same limit of S_A except that there is now an additional term, S_TG, to account for the fatigue contribution due to thermal gradients. Considering the effects of thermal gradients in this way will further help to preserve the integrity of the piping pressure boundary and consequently, the safety of personnel in today’s power plants and into the future.