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

Abstract

Fatigue evaluation in B31.1-2007 is currently done based on B31.1 Equations 1 & 2 and generally considers only the stress due to displacement load ranges as per Equation 13. Yet, fatigue damage is also occurring due to pressure cycling and thermal gradients. To exacerbate this, 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. This paper addresses the pressing need in today’s power plant environment for additional fatigue evaluation by providing a procedure for assessing an as-welded Butt weld of dissimilar metals and a Weld End Transition (or TTJ) (B31.1-2007 Figure 127.4.2) of similar metals to include the effects of thermal gradients calculated as per ASME Section III-2007 Subarticle NB-3600. The disadvantage of this approach is that the conservatism in the calculation of these thermal gradient stress intensities may produce unacceptable results. In that case, the assessment is a warning that something else needs to be done by way of either monitoring or modifying the thermal operation or more rigorous evaluation. The advantage of this methodology is that it will ensure a fatigue failure does not occur any sooner due to the effects of thermal gradients than would otherwise occur due to other factors. It maintains the traditional B31.1 approach to fatigue with the same limit of SA except that there is now an additional term, STG, to account for the fatigue contribution due to thermal gradients. Considering the effects of these 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.