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

Abstract

Abstract Low liquid loading flow in wet gas pipelines is a common occurrence in the transport of unprocessed 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 using 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/MMm3. 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, at low liquid loading, there is a broad transition range from stratified wavy flow to intermittent flow. Entrainment can occur in the gas core at relatively low gas velocities, and droplet deposition occurs simultaneously with entrainment. An increase in gas velocity did not increase liquid entrainment fraction over a relatively broad range of gas velocities. However, an increase in liquid flow rate did increase the liquid entrainment fraction. Surprisingly, in the annular flow region, a new phenomenon was observed at certain superficial gas velocities: an increase in liquid flow rate decreased the liquid film flow rate and liquid holdup, and increased the entrainment flow rate. This phenomenon may have significant impact on operations to sweep liquids from wet gas pipelines. Seven correlations of liquid entrainment onset point and entrainment fraction in the gas core are evaluated and modified to better fit the data. A new interfacial friction factor closure relationship is proposed. Based on this new information, a procedure is proposed for calculating low liquid loading in wet gas pipelines. The data obtained, the interfacial correlation developed and the proposed procedure to estimate liquid loading can be used in designing wet gas pipelines, determining pigging frequency, and predicting hydrate formation in deep water wet gas pipelines.