A comprehensive finite element method (FEM) analysis was conducted on a molten salt/oil heat exchanger used in a solar plant facility. This heat exchanger is one of three identical units connected in series, operating under high pressures and temperatures with significant pressure and temperature cycling throughout day and night. To ensure reliable performance over its operational lifetime, a thorough static design, fatigue assessment, and transient analysis were essential. This assessment adhered to ASME Section VIII Division 2 standards, utilizing ASME Section VIII Division 1 material allowables.
Assessment
The evaluation process involved examining critical areas of the heat exchanger by dividing it into several component models and assessing each individually for primary stress, shakedown, and fatigue.
Primary Assessment: This stage aimed to protect the heat exchanger from plastic collapse or rupture within the shell during operation. High-stress locations in the FEM model were linearized to assess membrane and bending stress components, which were then compared against ASME Section VIII Division 1 material allowables.
Fatigue Assessment: This analysis proved more complex due to several critical factors, including weld type, stress location (toe or root of the weld), quality inspection, and operating stress range. Critical welds were further examined by comparing stress tensor components to identify the operating stress range. This approach enhanced the understanding of cyclic stress behavior, leading to improved assessment results.
Transient Analysis: This analysis focused on daily start-up scenarios at the inlet and outlet nozzles and the tube bundle. The start-up process, characterized by large temperature gradients at the nozzle and tube bundle, was assessed, including the cooling effect of the molten salt, critical temperature heating paths, and variations in the heat transfer coefficient with temperature.
Assessment
The stress analysis revealed several necessary design modifications, including structural adjustments, changes to weld type and inspection quality, and procedural improvements for heat exchanger start-up. The fatigue assessment highlighted welds as the critical aspect of the design. By carefully selecting weld inspection quality and type, fatigue-related issues can be significantly mitigated.
For a comprehensive understanding of cyclic stresses, evaluating tensor components is recommended to accurately assess operating stresses and enhance design reliability.
