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 countercurrent flow in upward-inclined pipes. Introduction Gas-liquid two-phase flow occurs extensively in several industries, such as the petroleum, nuclear, and chemical industries. The most important parameters are pipe geometry (diameter and orientation), physical properties of the gas and liquid (densities, viscosities, and surface tension), and flow conditions (velocities, temperature, and pressure). In horizontal or near-horizontal gas-liquid two-phase flows, observed flow patterns include stratified, intermittent, annular, and dispersed bubble flow. In gas-liquid two-phase flow, droplet entrainment in the gas phase was observed at high gas velocities. Droplet entrainment refers to the presence of liquid in the gas stream in the form of droplets. The onset point of droplet entrainment is the lowest gas velocity at which droplet entrainment occurs in gas-liquid two-phase flow. Under low-liquid-loading conditions, the liquid flow rate is low, the gas flow rate is high, and the gas superficial velocity is very close to the real gas velocity. Entrainment fraction is defined as the ratio of the liquid flow rate in the form of droplets to the total liquid flow rate. Entrainment rate is defined as the mass of liquid in the form of droplets detached from a liquid film per unit area per unit time. Deposition rate is defined as the mass of droplets deposited on the liquid film per unit area per unit time. Entrainment rate and deposition rate indicate the mass transfer rate between the film and gas streams. Low liquid loading is commonly referred to as situations in which the liquid fraction is less than 200 bbl (1123 m3) at standard conditions. Liquid loading is defined as the liquid volumetric flow rate divided by the gas volumetric flow rate at standard conditions. However, in designing pipeline systems, the knowledge of in-situ flow behavior is important. Therefore, the liquid loading in this paper is defined as the liquid volumetric flow rate divided by the gas volumetric flow rate at in-situ conditions. In a pipeline, the pressure is often 100 to 1,000 times the standard pressure. Thus, the in-situ liquid loading is usually 100 to 1,000 times the liquid loading at standard conditions. Among the many theoretical and experimental investigations conducted on gas-liquid pipe flows, only a few studies in recent literature focused on low-liquid-loading two-phase flow. However, it is exactly in this liquid-loading domain that most wet-gas pipelines are operated. Even when single-phase gas enters a pipeline, condensate traces can be formed by retrograde condensation. The presence of condensate traces can lead to a significant increase in pressure loss along a pipeline. For instance, condensate traces as small as 0.5% by volume can result in a 30% greater pressure loss than in single-phase gas flow.1 Liquid holdup is an important factor in determining pigging frequency and designing downstream facilities. It is also important in analyzing corrosion/erosion, wax deposition, and hydrate formation in wet-gas pipelines. The development of an accurate predictive model for this particular case (gas-liquid two-phase flow) is of significant importance for the design of wet-gas pipelines. Literature Review The most commonly encountered flow patterns in low-liquid-loading pipelines are stratified and annular flow. The literature survey focuses on stratified and annular flow studies in horizontal and near-horizontal pipes. Stratified flow is one of the most dominant flow patterns for low-liquid-loading two-phase flow in near-horizontal pipelines, particularly in downward-inclined sections. Liquid entrainment can occur in the gas stream, and droplets can deposit on the pipe wall. Liquid flows mainly along the bottom of the pipe in the form of a film. Over the years, various theoretical models, with different degrees of complexity, have been proposed for this flow pattern. Significant recent works include the models of Taitel and Dukler,2 Cheremisinoff and Davis,3 Shoham and Taitel,4 Oliemans,5 Issa,6 Hamersma et al.,7 Hart et al.,1,8 Wu et al.,9 Paras et al.,10 Grolman,11 and Chen et al.12 Modeling the interfacial friction and the interfacial perimeter are very important. Droplet entrainment was observed by these authors10–1 but never modeled. In annular flow, the film distribution and droplet entrainment fraction are the most important parameters. Several entrainment-fraction correlations for vertical and horizontal annular flow in pipes have appeared in the literature.13–23 Liquid friction factor in two-phase flow was assumed to be the same as in single-phase flow - a constant film thickness was usually assumed. However, the applicability of these correlations to low-liquid-loading, gas-liquid annular flow in near-horizontal pipes is questionable. Williams et al.22 found that the entrainment did not depend on pipe diameter and increased as gas velocity increased.

Low liquid loading gas/liquid pipe flow

“…Meng (Meng, 2001) used mineral oil (m L ¼ 5.66 cp, r L ¼ 877 kg/m 3 , s ¼ 30 dyn/cm at 23.9 C) and air for his low liquid loading experiments in 2-in ID pipes of inclination angles 0 , AE1 and AE2 from horizontal. Fig.…”

“…In the petroleum industry, low liquid loading generally occurs when this ratio is less than 1100 m 3 /MMsm 3 (200 stb/MMscf) (Meng, 2001). Gas/liquid pipe flow with low liquid loading is often encountered in natural gas production wells and transmission pipelines.…”

On the characteristics of the roll waves in gas–liquid stratified-wavy flow: A two-dimensional perspective

“…The segregated coflow of the liquid phase with the gas phase, referred to as stratified flow, is one of the flow regimes occurring in a horizontal pipe. Stratified flow with low liquid loading conditions is common in gas production wells and transmission pipelines, where low liquid loading refers to a ratio of the liquid volumetric flow rate to the gas volumetric flow rate (at standard conditions) less than 1100 m 3 / MMsm 3 [2]. In addition, small amounts of liquid present in this flow configuration is representative of the operational conditions for the wet-gas pipelines, where liquid condensation occurs during the transport of the single-phase natural gas through the pipe.…”

“…1, which is taken from Chen et al [9]. Related to these flow structures, different mechanisms are proposed for the transport of the liquid phase; secondary gas flow structures [12], wave spreading [13], and entrainment/deposition [14,15]. There are several research studies on low-liquid loading [4,[14][15][16][17].…”

“…There are several research studies on low-liquid loading [4,[14][15][16][17]. Meng et al [14] showed that liquid entrainment into the gas phase plays an important role in the variation of the liquid holdup. Badie et al [15] showed that the intermittent waves on the gas-liquid interface are responsible for liquid entrainment.…”