Modeling of Flashing Fluids Discharging through a Puncture in a Pipeline

Abstract for 2017 AIChE Spring Meeting and 13th Global Congress on Process Safety March 26-30, 2017 San Antonio, TX Session Selection: 51st Annual Loss Prevention Symposium (LPS) (T1C)

 

Modeling of flashing fluids discharging through a puncture in a pipeline

Tatiana Flechas, Delphine Laboureur, and M. Sam Mannan

Mary Kay O’Connor Process Safety Center Artie McFerrin Department of Chemical Engineering Texas A&M University College Station, Texas 77843-3122, USA +1(979)985-0265, tatianaflechas@tamu.edu

ABSTRACT

 Large amounts of substances are transported in pipelines worldwide. This activity represents a hazard that needs to be quantitatively assessed through discharge models that are capable of accurately predict the outflow when a pipeline ruptures. The calculation of the discharge rate of pipelines transporting gases or liquids is well understood and accurate models exist for this purpose; less well understood is the discharge of flashing liquids. The expansion following the rupture causes some liquids such as dense phase CO₂, LNG or LPG to produce a two-phase release. Current models do not describe the behavior of a multiphase outflow following a pipeline puncture accurately. Usually, when there is a puncture in a pipeline, the decompression rate is lower in comparison to a full bore rupture and a stratified regime predominates inside the pipeline during the release. Most of the models in the literature assume a homogeneous equilibrium approach, which cannot describe the flow stratification. When a pressurized pipeline transporting a liquefied gas punctures, a process of formation and motion of vapor bubbles happens due to the pressure drop. Very limited investigations and analyses have been found in the open literature to understand the phase transition and the two-phase configuration inside the pipeline, during the rupture of lines transporting liquefied gases. Little has been done to understand how different parameters (e.g., hole size, initial pressure, initial temperature) affect the inner flow regime during the puncture of this type of pipelines. The main objective of this research is to investigate the effect of geometric and operational parameters on the inner flow regime during the rupture of pipelines transporting liquefied gases, using computational fluid dynamics. A parametric analysis will be performed in ANSYS Fluent in order to study how the puncture size and different initial conditions affect the vapor volume fraction and the velocity profile inside the pipeline. This will enable one to have a better picture of the phase transition and the inner two-phase configuration when a pressurized pipeline ruptures. In order to validate the simulation results, release experiments with dense-phase CO₂ found in the open literature will be used. The validation step will evaluate the performance of the default phase transition and multiphase models available in ANSYS Fluent; furthermore, it will help to identify and fill the gaps of the existing models (e.g., volume of fluid, mixture model, Eulerian model) in order to represent the physics of the release process. It should be highlighted that the behavior of the fluid inside the pipeline has an impact on the characteristics of the discharge. In this sense, the study of the inner flow regime during the rupture is a topic of great importance and it may create the basics to improve the formulation of pipeline discharge models currently available, especially for the puncture case, where the physics are less well understood than for the full bore rupture scenario.

Keywords Computational Fluid Dynamics, Consequence Modeling, Pipelines, Liquefied Gases, Flow Regime