This paper reports on the complex phenomena of pressure fluctuations and vibrations in a large-capacity drainage pump station. Significant pressure fluctuations were observed in suction water tunnels when an axial flow pump was operated without an actual head at some blade angles. To identify the causes, investigations were done by measuring acoustic natural frequencies and pressure fluctuations in the tunnels, lateral and torsional vibrations of the pump shaft, and pressure fluctuations of the oil in the hydraulic system to control the blade angle. The measurements were taken for different blade angles, rotating speeds, as well as acoustic natural frequencies. The natural frequency was changed by inserting air into the suction tunnels with a compressor and by setting air bags. The results showed that acoustic resonance occurred in the tunnels, but it was not a simple resonance. The dominant frequency, which was neither the blade passing frequency nor its higher harmonics, depended on rotating speed in the reverse way: it decreased when rotating speed increased, and vice versa. Pressure fluctuations in the water tunnels and lateral/torsional vibrations of the pump shaft had a strong relationship. However, they had different dominant frequencies and occurred at different blade angles. Several measurements were made in different seasons and it was found that the phenomena were season-dependent (dependent on quality of water). The causes are discussed. It is believed that the excitation source was vortex shedding from the blades, which locked into the acoustic resonance in the water tunnels via vibration of the blades.

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