Water hammer modelling in heterogeneous pipelines - analysis of critical loading on existing infrastructure

Abstract

This paper addresses the engineering challenge associated with the safety assessment during the phased rehabilitation of existing water supply systems. The subject of this study is a 9084 m long hybrid main transmission water pipeline, where newly designed ductile iron sections (DN800) are integrated with over 50-year-old steel pipes (ø920/6) and legacy cast iron pipes (ø800) installed in the 1970s. Using the NUASG and IMHUASG software, numerical modelling of water hammer was conducted across more than 10 normal and emergency scenarios. The simulations account for the different wave celerities of the materials (1054 m/s for cast iron, 908 m/s for steel) and the necessity for throttling due to the low surface roughness of the new pipes. The results demonstrate that under normal operation, thanks to the energy dissipator, the system remains safe, with maximum pressures not exceeding 150 mH2O (meters of water column). However, bypassing the energy dissipator (ED) combined with the rapid closure of the DN800 butterfly valve at the Drinking Water Treatment Plant (DWTP) generates surge pressures of up to 219,43 mH2O, which exceeds the theoretically allowable limit of 214,03 mH2O for the ø920/6 steel pipes. The study substantiates the necessity for mandatory ultrasonic non-destructive testing (NDT) of the legacy pipeline sections before their commissioning.