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

Fatigue evaluation in B31.1 is currently done based on Equation 1 & 2 [B31.1-2007] which considers only 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 paper takes the first step in addressing the pressing need to address this additional fatigue damage by considering thermal gradients. Granted there are several configurations where thermal gradients could be calculated. As the first step, this paper provides formulae to quantify the thermal gradients in the prevalent B31.1 Welding End Transitions Fig. 127.4.2, or Tapered Transition Joints (TTJ) Appendix D [B31.1-2007] which produce bending loads in the pipe around the full circumference and add to the fatigue damage of these welded joints. 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]. If results are unacceptable, it is 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 data that is available to the B31.1 designer, namely physical geometry, Thermal Conductivity and the Rate of temperature change of the fluid in the pipe. Calculating the magnitude of thermal gradients in a B31.1 TTJ is an essential step in evaluating their fatigue effects for design and in plant life extension considerations (see PVP2009-77148). 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.