Pressurized pipeline systems used in urban heating and water supply are susceptible to water hammer under emergency operating conditions, which may trigger highly destructive liquid column separation and and subsequent cavity-collapse-induced water hammer, thereby posing a serious threat to system safety. For viscoelastic pipelines such as high-density polyethylene (HDPE) pipes, traditional elastic-pipe models often produce considerable deviations in numerical prediction. In addition, variations in operating parameters, such as water temperature and flow velocity, further complicate the fluid-structure interaction mechanisms. Therefore, this study combines experiments, numerical simulations, and theoretical analysis to establish a pump-valve-tank experimental system for investigating liquid column separation and cavity-collapse water hammer in viscoelastic pipes induced by rapid closure of an end valve. The effects of different water temperatures (20-40 °C) and flow velocities (1.68-2.83 m/s) on pressure fluctuations are systematically examined.Firstly, a viscoelastic-pipe model for liquid column separation water hammer is developed based on the discrete vapor cavity model (DVCM) and viscoelastic constitutive equations. Secondly, using an overall energy analysis method, the effects of water temperature and flow velocity on the energy conversion mechanisms during liquid column separation in viscoelastic pipes are investigated. The results show that, with increasing initial flow velocity, the first pressure peak after cavity collapse increases nonlinearly, whereas it decreases linearly with increasing water temperature. In addition, the duration of cavity existence decreases approximately linearly as water temperature rises. During the cavity-presence stage, frictional dissipation dominates the energy loss throughout the pipeline and increases with initial flow velocity. In contrast, during the pure liquid phase after cavity collapse, viscoelastic dissipation becomes dominant, while the trend of mechanical energy attenuation remains similar under different flow conditions. As water temperature increases, energy loss during the cavity-presence stage decreases, whereas energy loss after cavity collapse intensifies, resulting in a lower overall energy conversion ratio. These findings provide an important theoretical basis for the safe design, risk prevention, and operational optimization of viscoelastic pipeline systems.