Analysis of a Resonance Phenomenon in a Water Supply System

The paper deals with resonance phenomena occurred at the downstream end of a long pipe in a water supply system realized in Northern Italy. The involved pipe connects a large artificial reservoir with a group of storage tanks; it is about 30 km long and conveys water by gravity with a difference in level of 104.5 m. At the downstream end, just before the arrival in the storage tanks, the pipe is subdivided in four branches (a “comb” distribution pipe); two water hammer preventing valves and a regulating valve are installed on each branch. During the closure operation, in certain conditions, a severe pressure oscillation with high frequency and increasing amplitude was observed, suggesting the occurrence of a resonance phenomenon; it caused ground vibrations along the conduit path, also at a large distance from the valve chamber, and damages to the water hammer preventing valves. The Authors carried on an in situ monitoring activity, recording water pressure in some locations of the distribution pipe during the unsteady phenomena. In order to identify the causes of the resonance, the data collected were analyzed by means of an impedance analysis of the hydraulic system and the dynamic analysis of the spring element of the water hammer preventing valve. The main results, reported in the paper, suggest that the triggering of the resonance phenomena can be ascribed to the water hammer preventing valves, combined with the particular configuration of the “comb” distribution pipe.