Literature and experimental investigations indicate that a stratified flow pattern (with or without liquid droplet entrainment) is obtained when gas and small quantities of liquids flow concurrently in a pipe. The liquid load is typically less than 10 barrels per million cubic feet of gas (56 m3/million m3 of gas) for transmission pipelines and less than 100 barrels per million cubic feet of gas (560 m3/million m3 of gas) for gas gathering systems. Very few correlations exist for the prediction of hold-up and pressure drop for these systems and fewer still give satisfactory results. Experimental studies for an air-oil system flowing through small diameter plastic and steel pipes ranging in size from 1-inch to 3-inches were studied. The experiments were carried out at the multiphase flow laboratories of Imperial College in London and the University of Calgary in Canada. Data from the 2-inch and 3-inch pipes are discussed in this paper. Results to date indicate that pressure drop increases could be as high as 1.5 times the dry gas value for a liquid hold-up in the order of 0.005. These results were complemented by those of actual operating gas pipeline systems transporting small amounts of hydrocarbon liquids. Based on the experimental results and the operating data two approaches for modeling these systems were evaluated:
A single phase approach for liquid loads of 10 bbls/MMCF of gas or less, typical in gas transmission systems. Preliminary results indicate that a liquid load of 1 bbl/MMCF of gas could reduce the pipeline transmission factor by about 1%.
A stratified two-phase approach for liquid loads of 10-100 bbls/MMCF of gas, usually found in gas condensate systems with consideration given to :
The reduction in the available flow area due to the presence of the liquid film.
The interfacial friction factor between the liquid film and the gas.
This interface was observed to be either flat or curved. c) Possibility of droplet entrainment in the gas stream and the dynamic mass and momentum transfer between liquid film and droplets. Only the single phase solution is addressed in detail in this paper; a solution procedure for the two-phase approach is provided.
In the petroleum industry the transportation of gas and low loads of liquids occurs frequently in natural gas gathering and transmission pipelines for both onshore and offshore operations. The accompanying liquids, are usually heavy hydrocarbon fractions and water and may be introduced from several sources. The most significant contribution is from the reservoir formation which produces substantial amounts of connate liquids. Liquids from compression facilities (e.g. lube oil) and treatment plants (e.g. glycol), as well as, products of condensation may also accompany the gas during transportation. An adequate design and modeling of gathering and transmission facilities for these gas-liquid systems will require a good prediction of the flow regime as well as the liquid hold-up and corresponding pressure drop which are consistent with the thermodynamic and hydrodynamic properties of the mixture.