ASME STP-PT-079-2016 pdf download

ASME 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. Geometry The configuration modeled consists of the piping with a band of ceramic electrical resistance heating elements. This in turn is covered by two layers of insulation over the heating band and one layer of insulation extending a distance beyond the heating band. The entire assembly is contained in a domain representing the surrounding air. Figure 2-1 and Figure 2-2 show the configuration of the half-symmetric model, with the ambient domain shown in blue, the piping shown in yellow, the heating layer in green, and the insulation layers in gray and purple. Heat flows from the heating band into the piping and to the insulation via conduction. Heat is lost to the surroundings via natural convection and radiation. Figure 2-2 shows the configuration of the soak band (SB), the heat band (HB), and the gradient control band (GCB). For all cases, the domain was assumed to be ten times the pipe length in the axial direction, and five times the pipe length in the transverse directions.
The effect of contact resistance must be included to obtain the proper temperature distribution. In the case of the piping heating system, the contact resistance must be included between the heating layer and piping to obtain the physical temperature distribution. Contact resistance (or conductance) is a function of the contact area between two bodies on a microscopic scale. For the piping system, this contact resistance is a function of the heating element size, element geometry, element layout (pattern), contact pressure (“tightness” of the wrap), pipe size, and pipe surface condition (including roughness and cleanliness). Unlike the pipe, the insulation blanket can conform easier to the heating elements, resulting in a different contact resistance. When solving the CHT problem using CFD, the thermal contact resistance can be directly specified at a contact interface. Values of thermal contact resistance are difficult (or impossible) to determine analytically, and therefore are typically determined through experimental measurement. For this analysis, the thermal contact resistance value was the “tuning” parameter used to match the computational solution to experimental measurements. Using thermal contact resistance as a tuning parameter allows the heating layer to be treated as uniform, rather than having to include detailed element layouts in the models. Note that since the actual temperature distribution is a function of the thermal contact resistance, which is a function of the particular heating elements used, the results are strictly valid only for the exact equipment used for the heat treating experiments. Other heat treating providers, alternative equipment, or alternative designs could impact the thermal resistance, and thus the resulting thermal distribution. It is suggested that the heat treating experiments be repeated using alternate equipment or an alternate provider. Heat flows from the heating element into the piping and to the insulation via conduction. Heat is then lost to the surroundings via natural convection and radiation. Heat is applied to the system through a prescribed power input governed by a series of temperature probes. These temperature probes correspond to thermocouples used for control zones during PWHT.