Calculating Thermal Gradients in a B31.1 Welding End Transition Joint by Formulae

Abstract

Fatigue evaluation in B31.1 is currently done based on equations 1 and 2 of ASME B31.1-2007 Power Piping, which only considers the displacement load ranges. However, fatigue damage, in addition to displacement load ranges, is occurring in B31.1 piping 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 article takes the first step in addressing the pressing need to address this additional fatigue damage by quantifying thermal gradients in the prevalent B31.1 welding end transitions in Fig. 127.4.2, or tapered transition joints (TTJs) in Appendix D, of ASME B31.1-2007 Power Piping by formulae to be able to evaluate their contribution to fatigue (see PVP2009-77148 [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]). The disadvantage of this approach is that the conservatisms inherent in the calculations of thermal gradients, as per ASME Section III Subsection NB3600-2007, are also inherent in these calculations and may produce unacceptable results when evaluated as per PVP2009-77148 [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]. If the results are unacceptable, it is a warning that something else needs to be done. The advantage of this approach is that it eliminates the need for a computer program to quantify these thermal gradients, a computer program that is not normally accessible to the B31.1 designer anyway. Instead, the formulae use the data that are available to the B31.1 designer, namely, physical geometry, thermal conductivity, and rate of temperature change in the fluid in the pipe. This 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.