Experiments and numerical simulations on the characteristics and protection of water hammer in long-distance liquid ammonia pipelines

With the expansion of ammonia energy applications, long-distance liquid ammonia pipelines are expected to support large-scale cross-regional ammonia transport. However, rapid valve closure during pipeline operation can generate severe water hammer pressures, posing significant threats to system safety. In this study, a horizontal liquid ammonia loop system was established to capture the temperature and pressure responses under water hammer conditions, and a one-dimensional transient two-fluid model was validated and developed using OLGA software to simulate the transient water hammer characteristics in industrial-scale long-distance liquid ammonia pipelines. The influence of operating pressure, flow velocity, pipe diameter, valve closure time, and valve closure mode on water hammer behaviors was systematically investigated. The results show that under low operating pressures, the coupled pressure–temperature trajectory crosses the vapor–liquid equilibrium curve, triggering flash evaporation and causing a simultaneous sharp drop in temperature. Under high operating pressures, the trajectory remains in the liquid phase region, and both pressure and temperature exhibit damped oscillations. Under liquid phase conditions, the peak water hammer pressure increases significantly and approximately linearly with flow velocity, but decreases slightly with increasing pipe diameter. Compared with linear valve closure, both prolonging the valve closure time and adopting the rapid-then-gradual nonlinear closure strategy can effectively suppress the water hammer pressure. These findings provide practical guidance for water hammer risk assessment and mitigation in long-distance liquid ammonia pipelines.