Experimental Study of Low-Liquid-Loading Gas-Liquid Flow in Near-Horizontal Pipes

Abstract

Summary Low-liquid-loading flow in wet-gas pipelines is a common occurrence in the transport of raw gas. The most important parameters governing the flow behavior are pipe geometry (inclination angle and diameter), operating conditions (flow rate, pressure, and temperature), and physical properties of the gas and liquid (density, viscosity, and surface tension). In this study, extensive experiments were conducted with a test loop made of 50.1 mm diameter acrylic pipe with inclination angles from the horizontal of −2°, −1°, 0°, 1°, and 2°. Gas and liquid superficial velocities ranged from 5 to 25 m/s and from 0.001 to 0.053 m/s, respectively. In-situ liquid loading ranged from 150 to 1800 m3. The flow patterns studied were stratified (smooth and wavy) and annular. Measured parameters included gas and liquid volumetric flow rates, liquid-film flow rate, pressure drop, temperature, liquid holdup, and droplet deposition rate. The experimental results show that there is a broad gas velocity range for the transition from stratified to intermittent flow at low liquid loading. Entrainment can occur in the gas core at relatively low velocities, and droplet deposition occurs simultaneously with entrainment. An increase in gas velocity did not increase the liquid entrainment fraction over a relatively broad range of velocities. However, an increase in the liquid flow rate did increase the liquid entrainment fraction. In the annular flow region, a new phenomenon was observed at certain superficial gas velocities; an increase in the liquid flow rate decreased the liquid-film flow rate and liquid holdup and increased the entrainment flow rate. This phenomenon may have a significant impact on operations to sweep liquids from wet-gas pipelines. Seven correlations of the liquid entrainment onset point and the entrainment fraction in the gas core are evaluated. The interfacial friction-factor closure relationship should be different for upward flow when compared to horizontal and downward stratified flow, owing to the existence of counter-current flow in upward-inclined pipes.