Numerical Modeling of Natural Gas Two-Phase Flow Split at Branching T-Junctions with Closed-Loop Network Applications

Open Access
Author:
Alp, Doruk
Graduate Program:
Petroleum and Natural Gas Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 12, 2009
Committee Members:
  • Dr Luis Ayala, Dissertation Advisor
  • Turgay Ertekin, Committee Member
  • Luis F Ayala H, Committee Chair
  • Robert W Watson, Committee Member
  • Dr Mirna Urquidi Macdonald, Committee Member
  • John Harlan Mahaffy, Committee Member
Keywords:
  • uneven phase separation
  • Branching
  • tee
  • T-Junction
  • Double Stream Model
  • Flow split
  • Staggered Grid
  • Finite Volume Method
  • Two-Phase Flow
  • Steady-State
Abstract:
Branching T-junctions are essential components of small and large scale piping systems found in natural gas and oil pipeline networks, as well as various industrial applications. Two-phase flow through branching tees (T-junctions) result in pronounced hydraulic losses and uneven phase separation following the split of flow stream; ultimately causing profound effects on system performance and quality of delivered fluids. Current state-of-the-art modeling software available to oil and gas industry do not address uneven phase separation (route preference) issue and typically do not provide a distinct T-junction component for modeling purposes. A finite-volume (FVM) based two-fluid model has been developed for one-dimensional, steady-state analysis of two-phase flow split and uneven phase separation at branching T-junctions of natural gas networks using steady-state Euler equations and outlet pressure specifications. Based on a comprehensive review of available branching T-junction and phase separation models in the literature, and classification of modeling efforts included in this text, Double Stream Model (DSM) of Hart et al. (1991), essentially a mechanistic phase separation sub-model for low liquid loading conditions (holdup < 0:06), is applied at the junction control volume to capture uneven phase separation. In order to have a consistent model, phasic momentum equations are replaced with gas phase Bernoulli equations at the junction cell and required gas phase loss coefficients (K-factors) are calculated using Gardel correlations. Generalized Newton-Raphson method is applied for simultaneous solution of governing equations, for all the control volumes in the computational grid. Along with the staggered solution of governing momentum equations at CV edges, this allows accounting for the impact of outlet (downstream) pressures on flow split as well as closed-loop network applicability. Analysis is focused on regular, horizontal T-junctions with 90o branching angle. Results from air-water and hydrocarbon mixture studies are in agreement with literature; model captures pressure rise in the main line following the flow split (the Bernoulli effect) and uneven phase separation with changing liquid flow rates and outlet pressures. An important observation is that while pressure in the main line right after the split (entrance pressure of the run) is always higher than the entrance pressure of the branch due to Bernoulli effect, this is not necessarily the case for run and branch outlet pressures. When branch outlet pressure is specified higher than the run outlet pressure, less fluid is diverted into the branch. However, the turning of flow causes branch entrance pressure to be always smaller than run entrance pressure. Slight difference in overall hydrocarbon mixture composition in run and branch arms, after flow split at the tee, suggests that for small holdup values compositional change can be ignored for practical purposes.