Types of Phase Flows and Gas Liquid System

Introduction

Pipelines carrying two-phase fluids (Liquid and gas) are called two-phase flow. The analysis of two-phase flow piping is more complex and less understood than that of incompressible or compressible fluid flow. 

Two-phase Flow Capabilities

• Complete thermodynamics: phases appear and disappear as conditions warrant. 

Two-phase heat transfer correlations are built-in or user-defined

Two-phase pressure drop correlations are built-in or user-defined

Automatic flow regime mapping

From quasi-steady homogeneous equilibrium to fully transient two-fluid modelling.

Optional slip flow modelling (separate phasic)

Optional, no equilibrium transients (separate phasic energy and mass equations)

Optional tracking of liquid/vapour interfaces

Capillary modelling tools for static or vaporising wicks

 

Two-phase Mixture Capabilities

• Mixtures of up to 26 liquids and/or gases

• Optional condensable /volatile component in mixture, including effects such as diffusion-limited condensation.

• Optional dissolution of any number of gaseous solutes into any number of liquid solvents, including homogeneous nucleation models

Types of Two-phase Flow

• Gas – liquid
• Gas – solid
• Liquid – solid

Focus

• Gas liquid system
• Orientation of flow
• Whether horizontal or vertical flow

Gas Liquid System

Both gases and vapours correlate similarly in two-phase systems except for certain conditions of continuous vapour condensation or liquid flashing in the flowing system. 

Two -phase Flow Regimes and Characteristic Linear Velocity

Dispersed Flow

Also referred to as spray or mist flow, dispersed flow occurs at very high gas velocities with the liquid phase dispersed as droplets throughout the gas phase. The liquid droplet velocity approaches the gas phase velocity in this flow regime because the droplet terminal velocity is negligible and the slip velocity approaches zero.

Annular Flow

Annular flow occurs at relatively lower gas velocities than dispersed flow. The liquid phase forms an annulus about the circumference of the pipe with the gas flowing through the central core. There is significantly more slip with annular flow than with dispersed flow.

Stratified Flow

Stratified flow occurs only in horizontal pipes when the gas phase velocity is not great enough to maintain an annulus of liquid about the circumference on the pipe. One form of stratified flow, called “wavy flow” is characterised by the formation of waves on the surfaces of the liquid phase. Wavy flow is formed close to the transition point where stratified flow can be transformed into slug flow with a further increase in gas velocity.

Slug Flow

Slug flow is characterised by an intermittent pattern of alternating liquid phases and gas phases along the length of the line. The entire pipe cross-section area can be occupied by a slug of either liquid or gas at different points along the flow path.

Plug Flow

Plug flow occurs when the liquid phase forms a nearly continuous phase with large elongated bubble plugs of gas located within the liquid phase.

Bubble or Froth Flow

Bubble or froth flow like the plug flow, has a dominant liquid phase, but the liquid phase in bubble flow is at a higher velocity than the liquid phase in plug flow. This higher velocity causes the vapour phase to disperse into many smaller bubbles within the liquid phase.

Two-phase Flow Types

Flow Regimes

• Each flow regime behaves differently.
• Each flow regime has its own set of empirical correlations for predicting flow behaviour.
• The most often used method to determine the flow regime is the baker
• Baker plot horizontal axis:

Note: 1. Steam condensate in return lines flashing into steam

2.Two-phase feed lines entering distillation columns

3. Process plant refrigeration return lines.

Mitigating Erosion

Depending upon the flow regime, the liquid in a two-phase flow system can be accelerated to velocities approaching or exceeding vapour velocities. In some cases these velocities are higher than desirable for a process piping system. Such high velocities can cause a phenomenon known as “erosion corrosion” in equipment and piping systems. There are no general correlations that predict the rate of erosion corrosion in piping systems, but Coulson has proposed an index based on velocity head to determine the range of mixture densities and velocities below which erosion corrosion should not occur. The index takes the form

Numerical

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