Objective Supercritical/dense-phase CO₂ exhibits relatively high density. During pipeline transport, planned or accidental valve closures can induce transient water hammer. Current research on CO₂ pipeline water hammer primarily relies on simulations, with limited experimental investigation into its governing dynamics.

Methods A 238 m-long experimental setup for transient water hammer in a supercritical/dense-phase CO₂ pipeline (56 mm diameter, 20 MPa design pressure) was designed and constructed. Fifteen groups of experiments were conducted at varying initial temperature, pressure, CO₂ flow velocity, and valve closing time. Multiple pressure sensors positioned along the pipeline loop captured pressure fluctuations, enabling detailed analysis of water hammer behavior in supercritical/dense-phase CO₂ pipelines.

Results At a constant initial temperature, increasing the initial pressure raised CO₂ density and fluid kinetic energy per unit volume. When converted to pressure energy, this intensified the water hammer effect, increasing pressure and shortening the cycle. At a constant initial pressure, higher initial temperature reduced CO₂ density, viscosity, sound velocity, and compressibility, thereby weakening water hammer pressure and extending the cycle. Shorter valve closing time increased water hammer pressure, while prolonging closure significantly reduced peak pressure —under 9 MPa and 20 °C, a 1-second delay lowered pressure by about 30％. Increasing CO₂ flow velocity raised system kinetic energy, and under supercritical/dense-phase CO₂ conditions, rapid valve closure released this energy more intensely, amplifying water hammer intensity. However, flow velocity and valve closing time did not affect the water hammer cycle which was mainly governed by initial temperature and pressure.

Conclusion When designing and operating pipeline systems, the risk of water hammer overpressure must be addressed. Reducing operating pressure and increasing temperature can mitigate transient pressure shocks caused by water hammer. Additionally, rapid valve operations under high flow velocity should be avoided. This study experimentally characterizes valve-closing water hammer behavior in supercritical/dense-phase CO₂ pipelines, providing a critical foundation for developing water hammer models for supercritical CO₂ pipelines and guiding CO₂ pipeline design and protection.