Measurements of junction coupling during water hammer in piping systems

For the analysis of the effects of fluid–structure interaction (FSI) during water hammer in piping systems, a test facility has been designed and constructed. The research objective is to show on the basis of two specific examples that the necessity of considering FSI is strongly dependent on the boundary conditions of the system. Resonance experiments on movable bends in two piping system configurations focused on junction coupling were carried out. These configurations differ in the length of the hydraulic system and in the geometry of the oscillating bend. The displacement of the bend and the pressure inside the pipe were measured for various free oscillating lengths of the bend while the rest of the piping system was restrained. The results are displayed in resonance curves and frequency spectra for the different configurations. In both cases a correlation between the pressure and the displacement spectrum shows a transfer of momentum from the fluid to the structure, but only in the configuration with the long oscillating pipe section can a reaction of the fluid on the motion of the structure be identified. Frequency shifts of the pressure and a splitting of the pressure peak were observed. The time signals confirm that the effects of FSI are most significant in one system configuration which is strongly influenced by the bend geometry. Furthermore a parameter is presented which quantifies the effects of junction coupling based on the geometrical and hydraulic properties of the bend and the system.