Effects of foam on slug generation mechanism in a hilly terrain pipeline

Abstract

Liquid accumulation is one of the most common challenges associated with natural gas gathering processes, particularly in pipelines where mechanical pigging is infeasible and there is a risk of terrain slugging. Aqueous foam drainage technology is a potential method to remove liquid accumulation owing to its low risk and cost, and lack of constraints. An experimental study of gas–liquid two-phase flow with foam was conducted to clarify the effects of foam on the slug generation mechanisms at the elbow of upward-inclined pipelines. The experiments were conducted in horizontal-upward pipelines at different inclination angles (5°, 10°, and 20°). The flow pattern was captured by a high-speed camera and the real-time differential pressures along the pipeline were recorded. The results indicate that the slug generation mechanisms at the elbow mainly consist of an interfacial wave growth mechanism at low gas velocities and an interfacial wave aggregation mechanism at high gas velocities. In the presence of aqueous foam, the interfacial wave growth mechanism still exists. In contrast, the interfacial wave aggregation mechanism disappears, and the flow pattern transforms into wavy foam flow under the same conditions. In addition, the increase in the inclination angles intensified the liquid backflow in the upward-inclined section and contributed to the liquid accumulation at the elbow. Furthermore, owing to the Bernoulli effect, the increase also enhanced the liquid-carrying capacity of the gas in the divergent section of the elbow, resulting in increased interfacial fluctuations and promoting the generation of liquid slugs. Meanwhile, the eigenvalues extracted from the fast Fourier transform (FFT) of the differential pressures correlate well with the flow characteristics at the elbow, and the average values at separated flow conditions are all below 0.05, which proves that the method enables an accurate prediction of whether the liquid slug is generated at the elbow of the upward-inclined pipe.