Gas-liquid two-phase flow exists extensively in the transportation of hydrocarbon fluids. A more precise prediction of liquid holdup in near-horizontal, wet-gas pipelines is needed in order to better predict pressure drop and size downstream processing facilities. The most important parameters are pipe geometry (pipe diameter and orientation), physical properties of the gas and liquid (density, viscosity and surface tension) and flow conditions (velocity, temperature and pressure). Stratified flow and annular flow are the two flow patterns observed most often in near-horizontal pipelines under low liquid loading conditions. Low liquid loading is commonly referred to as cases in which liquid loading is less than 1,100 m3/MMm3 (200 bbl/MMscf).
A previous study by Meng  was carried out on a new low liquid loading flow loop. A transparent test section (50.8-mm inner diameter and 19-m long) could be inclined within ± 2° from the horizontal. Mineral oil was used as the liquid and air was used as the gas phase. A surprising phenomenon was observed with air-oil flow; at high gas velocities (annular flow), liquid film flow rate, liquid holdup and pressure gradient decreased as liquid velocity increased.
Low liquid loading gas-liquid two-phase flow in near-horizontal pipes was studied for air-water flow in the present study, in order to investigate the effects of the liquid properties on flow characteristics. This study was carried out on the same 2-in. ID flow loop used by Meng. The measured parameters included gas flow rate, liquid flow rate, pressure, differential pressure, temperature, liquid holdup, pipe wetted perimeter, liquid film flow rate, droplet entrainment fraction and droplet deposition rate. A new phenomenon was observed with air-water flow at low superficial velocities and with a liquid loading larger than 600 m3/MMm3. The liquid holdup increased as gas superficial velocity increased.