The Use of Hydrants for Transient Test-Based Diagnosis of the Water Distribution Systems of Milan, Italy

In a pilot study for the diagnosis through transient tests of the water distribution systems of Milan, Italy, hydrants were used for both pressure measurement and pressure wave generation. In the field tests, the maneuver of the hydrant valve used for the pressure wave generation led to an increase in the measured pressures due to the trapping of the pressure waves inside the hydrant connection. In this paper, the trapping phenomenon was analyzed in time and frequency domains to evaluate analytically the effects of the connection geometric characteristics on the pressure variations. The results of field and laboratory tests are shown to confirm the theoretical considerations and extend the investigation to viscoelastic pipes. Because the same phenomenon can also take place in user connections, which present a similar combination of pipe impedance and geometry, the similarity of hydrant and user connections in terms of the response to transients was also explored. Results of the case study and their interpretation can be relevant for the diagnosis of water distribution systems through transient tests and for the design of hydrants and user connections.