Acoustic, fluid-structure and decoupled seismic analysis of piping systems

A multi-disciplinary review of the current procedures used for the dynamic analysis of the piping system identified shortcomings in three areas: (i) finite element formulation for concentric acoustic damping, (ii) fluid-structure interaction at a control valve, and (iii) decoupled seismic analysis. Numerical studies were performed in each of these areas to illustrate the shortcomings and to develop techniques to overcome the shortcomings.; The current formulation in the industry standard finite element packages (ABAQUS, 2002, and ANSYS, 2002) for concentrated acoustic damping is inadequate for transient acoustic analysis. It gives high acoustic damping at anti-node locations of the pressure modeshapes, while the actual measurements indicate that the damping is most effective at the pressure node locations. Two new formulations based on pressure and fluid velocity were developed to properly model the concentric acoustic damping for transient conditions. These formulations were benchmarked against experimental results.; A study of a self-excited vibration problem in a nuclear station was performed to identify the self-excited vibration mechanism. A coupled fluid-structural dynamic model developed to perform a numerical simulation of the valve closing transient. The simulation indicated that there is a positive feedback between valve motion and the hydraulic forces acting on the valve. This feedback introduces a negative hydraulic stiffness on the structural system and negative damping in the fluid system. An analytical study of the negative damping identified the range of operating parameters with unstable valve behaviour. A procedure to calculate the steady-state limit cycle response was also presented and benchmarked against test results.; The existing seismic decoupling criteria used to assess suitability of performing decoupled seismic analysis was found to be unacceptable for the case of finely tuned systems when the coupling location was close to the node location of the primary system mode. At this location, due to localization of the energy in the piping upstream of the coupling location, the system response increases upstream of the coupling location and decreases downstream of the coupling location. The mechanism identified was illustrated using an experiment and a numerical example. A new criterion was developed that evaluates the energy localization effect.