STP-PT-079-2016 pdf download.LOCAL HEATING OF PIPING: THERMAL ANALYSIS.
2 ANALYSIS METHODOLOGY The approach was to perform conjugate heat transfer (CHT) analysis using the Star CCM+ computational fluid dynamics (CFD) software [2]. This is a fully functional and validated commercial CFD solver. It has the capability of performing the CHT analysis and solving for temperature distributions in the piping and in the surrounding air. The advantage of using a CFD solver, as opposed to using a finite element analysis (FEA) code, is that the natural convection on the solid surfaces can be directly accounted for, rather than applying approximate boundary conditions. The project was broken up into two phases, a calibration phase and a prediction phase. The calibration phase consisted of two pipe geometries with different heating band configurations. Experimental temperature data was collected and provided to Quest Integrity by ASME. This data was then used to calibrate the CFD models by tuning the contact resistance between the heating band and pipe. The prediction phase expanded on the calibrated CFD models to examine post weld heat treatment (PWHT) in pipes of differing diameters and thicknesses. For the prediction phase, five different pipe diameters with three different schedule thicknesses were modeled, changing the heat band length iteratively until a maximum 15°F difference existed in the soak band. These results were then used to suggest new PWHT heat band sizing guidelines.
The use of the CFD solver allows the buoyancy-driven flow pattern throughout the system to determine the film coefficients. This is advantageous as the natural convection heat flow can be directly computed, rather than estimated. In addition, this allows 3D effects (top vs. bottom vs. sides of piping) to be included. This is important to determine an accurate temperature distribution around the weld. During the heat treatment, the surrounding air (especially inside) the pipe will be expected to heat locally, resulting in spatially varying sink temperatures for a steady state analysis. Using CFD-based analysis allows the air temperature to be directly computed, rather than using an estimated (likely uniform) sink temperature. Note that sufficient mesh refinement is required to accurately capture boundary layer convective effects. The y+ value provides a measure of mesh refinement in the boundary layer. It is defined as the distance from the wall normalized by the viscous length scale [4]. A value of 50 or less is recommended and a value of 5 or less is highly preferred to ensure boundary layer accuracy. In all cases the y+ value was significantly less than 50 and only exceeded one at a limited number of points remote from the area of interest.STP-PT-079 pdf download.