Two Phase Line Sizing

The Two-Phase Line Sizing Calculator is one of the most complex hydraulic utilities in process engineering. When liquid and gas flow concurrently in a pipe, they interact in highly non-linear ways, resulting in various flow regimes (e.g., bubble, slug, annular, stratified) and significantly higher pressure drops than single-phase flow.

Proper sizing of two-phase lines is critical to avoid destructive slug flow, excessive vibration, and mechanical fatigue. This tool evaluates the Lockhart-Martinelli parameter and predicts the flow regime, pressure drop, and liquid holdup.

Two-phase flow analysis cannot rely on simple Darcy-Weisbach equations. This tool utilizes empirical and mechanistic models, such as the Lockhart-Martinelli correlation or Beggs and Brill methods.

The methodology involves calculating the superficial velocities of both the liquid and gas phases. These velocities are mapped onto a flow regime map (such as the Baker or Mandhane map) to predict the flow pattern. The frictional pressure drop is then calculated using two-phase multipliers ($$\Phi^2$$), and the static pressure drop is calculated based on the liquid holdup fraction.

Two-phase line sizing is critical in applications where phase changes occur or fluids are inherently mixed:

• Reboiler Return Lines: The mixture of liquid and vapor exiting a thermosyphon reboiler back to the distillation column must be sized to prevent slugging and ensure stable column operation.
• Flashing Lines: Lines downstream of a pressure letdown valve (e.g., separator feed lines) where liquid flashes into vapor.
• Oil & Gas Gather Lines: Transporting mixed wellhead fluids (oil, gas, and water) to processing facilities.