Flange Assembly Protocols: Analyzing Preheating Strategies for High-Temperature Applications

Abstract Flange assembly is a critical process in various industrial applications, where maintaining a secure and leak-free joint is essential for operational integrity. Specifically, in high-temperature assemblies, the differential thermal expansion between bolts and flanges poses an additional challenge that can lead to joint loosening and potential leaks during startup. A widely used practice to address this issue is the start-up retorque (“hot torque”), which consists of tightening the bolts in a circular pattern until the nuts no longer turn. This is typically performed while the unit is online, at temperatures between 150°C (300°F) and 230°C (450°F), or within 24 hours if the joint temperature remains below 150°C (300°F). This procedure restores gasket stress and reduces the risk of leakage by compensating for long-term gasket relaxation. The objective of this study is to evaluate an alternative approach presented in ASME PCC-1-2022 Pressure Boundary Bolted Flange Joint Assembly, Appendix I, where electric heaters are used to preheat the flange before system startup, allowing for retorque at elevated temperatures before the unit is online. A test rig was designed for this purpose, consisting of a 16” ASME B16.5 Class 300 flange, equipped with thermocouples and strain gauge bolts to monitor temperature and bolt tension.