Hydraulic transient analysis in low gas-liquid ratio pipelines under valve closure conditions: Pressure surge characteristics and predictive modeling

The propagation dynamics of water hammer pressure waves in gas-liquid pipelines are predominantly influenced by gas-phase compressibility. To investigate the effects of the gas-liquid ratio and flow regime on the velocity and attenuation of the water hammer pressure wave, transient two-phase flow experiments were conducted in a 50 mm diameter, 145 m long pipeline. The results indicate that, unlike downstream valve closure, upstream valve closure induces liquid column separation, which reduces peak pressure and causes waveform hysteresis in liquid pipelines. As the gas-liquid ratio increases from 0.04% to 12.6%, the water hammer pressure wave velocity decreases from 298 to 146 m/s, accounting for 21.6% to 5.88% of the corresponding values in liquid pipelines. When the gas-liquid ratio exceeds 2.65%, the water hammer effect becomes negligible. Based on these findings, a predictive model incorporating the effects of line packing was developed by integrating interphase compression wave propagation and the unsteady friction mechanism. The prediction errors of this model are below 4.55%, accurately capturing the dynamics of secondary pressurization and wave velocity decay. These advancements provide essential theoretical support for the design and operation of gas-liquid pipelines.