An Analysis Of Two-Phase Flow In Inclined Pipes With Zero Net Liquid Production

Abstract

Abstract A two-phase gas-liquid flow in upward inclined pipes with zero net liquid production is considered. The models of slug and stratified flow regimes are proposed for predicting the pressure drop. Estimated values of pressure loss compare well with the experimental results obtained with 26 mm and 51 mm pipes at angles of inclination of + 1 degree, + 5 degrees and + 9.2 degrees. Introduction Transportation of hydrocarbon mixtures by pipelines has been recently gaining in importance as Canada begins an era of increasing dependence on production from offshore fields. Although the commercial pipelines follow the terrain variations, most of the research endeavor has been focused on horizontal and vertical flow. The subject of this study is to investigate a special case of the gas-liquid flow in upward inclined pipes: a case of zero net liquid production. Such a situation may occur under any of the following conditions:pipeline is shutdown and a hydrocarbon mixture is cooled below its bubble point;liquid to gas ratio is very low; andliquid slug is introduced into the pipeline as a "kick " from a gas well. In all these cases fluid accumulates at the low sections of a pipeline. After the start-up some of the liquid is sheared off by the flowing gas and the remaining liquid circulates in the uphill pipeline section resulting in a zero net liquid flow rate. The liquid presence significantly affects the pressure drop in a pipeline initially designed for gas flow only. The main goal of this investigation is to describe the physics of the problem to the extent [hat the pressure drop could be correctly determined in design calculation. Model Development Gregory et al. (1981) conducted an experimental study on systems featured with zero net liquid production. The intermittent flow pattern was observed at superficial gas velocities up to 2 m/s. At higher rates fluid stratification occurred. Due to the zero liquid flow rate no comparison with existing flow maps is possible. Any step increase in gas velocity was accompanied by a corresponding decrease in liquid holdup. Intermittent Flow Over the past two decades a considerable amount of research has been done on intermittent flow in horizontal pipes. A major contribution was made by Dukler and Hubbard (1975) who proposed a semi-empirical model for slug flow. A major drawback of the model is the fact that quantities such as slug frequency and gas holdup in liquid slugs cannot be predicted from the first principles. Later on, Nicholson et al. (1978) modified this model. Recently attempts have been made to close the model: Barnea et al. (1983) suggested a method of predicting the slug holdup, and Maron et al. (1982) described the slug characteristics in terms of the boundary layer theory. Difficulties of the gas liquid slug flow modelling may be appreciated by inspecting Figure 1 which show the schematics of this flow pattern together with the symbols used in the mathematical description.