Objective Ammonia occupies an important position in the development of the new energy system. Pipeline transportation of ammonia supports large-scale ammonia delivery and effectively mitigates bottlenecks in high-pressure hydrogen storage and transportation. Nevertheless, liquid ammonia pipelines are prone to leakage induced by internal and external factors during operation. The complex variation of in-pipe thermodynamic characteristics during the leakage process acts as a major trigger for catastrophic brittle fracture of pipelines.

Methods In this study, a self-designed experimental system is built for liquid ammonia pipeline leakage. Synchronous measurement and observation of in-pipe pressure, temperature and flow regime near and far from the leak orifice are carried out. The effects of different initial temperatures (20 °C, 25 °C, 30 °C) on the evolution of in-pipe thermodynamic parameters during leakage are investigated.

Results During leakage, in-pipe pressure drops rapidly before transitioning to a gradual and then steady decline, with pressure variations tending to synchronize at the near and far measuring points. In-pipe temperatures initially fall — progressing from a mild decline to a rapid drop and subsequent deceleration — before rising at a progressively decreasing rate. The far measuring point exhibits a higher temperature drop rate than the near point. Fluid phases sequentially transition from a single liquid phase through wavy stratified and stable stratified gas-liquid two-phase flows to a single gas phase. Phase evolution at the far measuring point is closer to the equilibrium state. Elevating the initial temperature from 20 °C to 30 °C significantly affects the in-pipe thermodynamic characteristics. As the initial temperature rises, the total leakage duration increases from 4,200 s to 6,100 s at a moderating growth rate, and the average pressure drop rate in the early leakage stage increases from 269 Pa/s to 486 Pa/s. In terms of thermal response, the near measuring point exhibits a decreasing minimum temperature and a continuously increasing maximum temperature drop, whereas the far measuring point exhibits non-monotonic variations in both minimum temperature and maximum temperature drop. Furthermore, a higher initial temperature delays the phase transition from the gas-liquid two-phase state to the single gas phase inside the pipeline.

Conclusion The findings offer guidance for risk assessment and safety control of liquid ammonia pipelines under leakage conditions, and provide a data basis for the development and validation of relevant prediction models. In engineering practice, the complex coupling relationship and spatiotemporal differences of in-pipe thermophysical parameters during leakage should be fully considered to achieve reasonable layout of measuring points and proper design of protective measures.