Liquid drop-out in natural gas pipelines is becoming increasingly common because of high changes in the composition and low quality of the natural gas supply. Predictions of possible locations where liquid drop-out occurs are, on occasion, very difficult to obtain. Moreover, estimating the amount of liquid condensation in the gas pipeline is even more challenging. From an operating gas company prospective, it is fundamental to identify those issues and take the appropriate actions to solve them before they significantly affect the operation of the entire pipeline system Modeling of pipeline networks has increased in the past decade due to the use of powerful computational tools that provide good quality representation of the real pipeline conditions. Therefore, a commercially available single-phase pipeline network flow model was used to model a very complicated transmission network that covers an entire country. The system includes approximately 4400 miles of interconnecting pipelines, 11 main compressor stations, 21 different injection points and 110 extraction locations. The developed model takes into account heat transfer with the surroundings, changes in elevation, flow and pressure regulations points as well as diverse operating conditions. Simulation scenarios cover a wide variety of flow and pressure conditions. A baseline model is developed and tuned with real operating conditions. Parameters such as roughness, heat transfer coefficients and ground properties are obtained by using real operating data. It is essential to note that conventional assumed friction factors and heat transfer coefficients are affected significantly with the presence of liquids that originate a two-phase transport mechanism. In order to obtain predictions of possible locations of liquids drop-out, simulation results are compared with the phase envelope of the different gas streams along the pipeline network. Pressure, temperature and velocity parameters define the conditions for hydrocarbons condensation. Model results are compared with snapshots of the operational conditions to complement the validation of the model and improve simulation results. Recommendations for managing the liquids in the pipeline are presented. This paper provides a methodology on how to determine the possible hydrocarbon liquid drop-out in a pipeline network by combining a modeling tool with mathematical calculations. In addition, results for a case study are presented and discussed. It will also provide a solution for predicting liquids accumulation to improve the general operation of the pipeline by minimizing maintenance and operational costs.
Simulations of the system operating under steady-state conditions can provide velocity, temperature and pressure profiles of every single pipe branch. There are several specific conditions that can be identified with velocity, temperature and pressure profile data, in which condensate formation is most likely. Utilizing an equation of state and an initial gas composition, the phase envelop of the gas transported is generated. Comparison of the pressure and temperature results obtained from the pipeline simulation with the phase envelop data is used to determined if the gas flow is under possible mixture conditions that could result in liquid drop condensation.
