Evaluating and Mitigating Fatigue Risk of Choked Flow

Uncontrolled manual operation of high energy vent lines could cause immediate fatigue failure at branch connections. Past failures have been attributed to high kinetic energy created due to choked flow at unprotected small-bore branch connections. In this paper, a risk-assessment method has been developed for choked flow in high energy systems. Computational Fluid Dynamics (CFD) tools were used to simulate formation and propagation of shock wave. A state-of-art method Fluid Structure Interaction (FSI) was used to investigate the dynamic pressure fluctuation in the piping system. Finite Element Analysis (FEA) was used to evaluate stresses and vibration responses at the toe of welds. This study found the fatigue risk of choked flow is caused by high-frequency shell-mode vibration. However, unlike the other high-frequency shell-mode vibration phenomenon (the acoustic-induced-vibration) which propagates in the piping system, the risks of choked flow are found to be only in the immediate vicinity of the choking point and decays exponentially with distance. A vibration velocity index defined in recent publications is used and modified to predict the vibration stress in various pipe sizes and branch fittings. Mitigation options are also discussed in this paper.