Exam Prep - Multiphase

Question 1

For designing multiphase flow pipeline, what are the main concerns different from the single phase pipeline?

Answer 1

• Flow regime
• Flow pattern
• Pressure loss estimation
• Heat loss estimation
• Physical properties
• Thermodynamic properties
• Hydrate formation management
• Wax formation management

Question 2

What are the main parameters affecting the multi-phase flow regime map?

Answer 2

The flow regime map depends on a number of parameters:

• Pipe diameter
• Pressure
• Velocity
• Water Cut
• Pipeline Inclination

Question 3

What is multiphase homogeneous model for calculating frictional pressure gradient? What is its limitation?

Answer 3

(dp/dx)f = 𝑓 * (ρu^2/2D)

It assumes no slippage between the liquid and the gas.
applicable for ρl/ρg < 10 or G > 2000 kg/m^2s

Question 4

For multi-phase pipe flow, what is the Flow Regime Map? With an increasing of gas flow rate, for a horizontal pipe, write down the main flow regimes and describe their features. What are the main parameters affecting the multi-phase flow regime map?

Answer 4

A flow regime map gives the regime of two phase flow for an increasing gas flow rate.

For increasing gas flow rate in a horizontal pipe, the main flow regimes are:

• Dispersed Bubbles - high gas flow rate
• Slug - frothy waves
• Annular - ringed flow
• Wavy - ripple flow
• Stratified - layered flow

The flow regime map depends on a number of parameters:

• Pipe diameter
• Pressure
• Velocity
• Water Cut
• Pipeline Inclination

Question 5

List down two main flow analysis methods for multi-phase flow, and describe their pros and cons.

Answer 5

• Large scale modelling which includes the use of transient and steady-state two fluid models using empirical and semi-empirical correlations.
• Small scale modelling which relies on the direct solution of fluid dynamic equations. A limitation is it reduces the scope, however greatly reduces the need for empiricism.

Question 6

For multi-phase pipe flow, what is the Flow Regime Map? With an increasing of gas flow rate, for a vertical pipe, write down the main flow regimes and describe their features. What are the main parameters affecting the multi-phase flow regime map?

Answer 6

A flow regime map gives the regime of two phase flow for an increasing gas flow rate.

For increasing gas flow rate in a vertical pipe, the main flow regimes are:

• Dispersed Bubbles - high gas flow rate
• Bubble - high liquid flow rate
• Slug or Churn Flow - frothy waves
• Annular - ringed flow

The flow regime map depends on a number of parameters:

• Pipe diameter
• Pressure
• Velocity
• Water Cut
• Pipeline Inclination

Question 7

Define the technical terms:

i) Superficial velocity
ii) Liquid holdup
iii) Gas void fraction
iv) Slip velocity

Answer 7

i) Superficial velocity is a hypothetical flow velocity calculated as if the given phase or fluid were only one flowing or present in the pipe. Calculated using: (V=Q/A)

uSL = QL/A, uSG = QG/A
QL,QG: Liquid and gas flow rates
A: Area

ii) Liquid hold up is the cross sectional area occupied by the liquid in the pipe carrying the wet gas flow. It is a function of both space and time.

HL = VL/V
VL: Liquid Occupied by the pipe
V: Pipeline Volume

iii) The gas void fraction is defined as the fraction of the cross-sectional area of the channel that is occupied by the gas phase.

αg = Vg/V
Vg: Liquid Occupied by the pipe
V: Pipeline Volume
αg + HL = 1 !!

iv) Slip velocity is defined as the ratio of the velocity of the gas phase to the velocity of the liquid phase. In the homogeneous model of two-phase flow, the slip ratio is by definition assumed to be unity (no slip).

US = UG - UL
UG: Average Gas Velocity
UL: Average Liquid Velocity

Question 8

What are the three empirical methods?

Answer 8

• Single Phase Flow
Moody Diagram

• Multiphase Homogeneous Flow Model
(dp/dx)f = 𝑓 * (ρu^2/2D)
ρm = (1-HL)ρG + HLρL

HL: Liquid Hold-up

It assumes no slippage between the liquid and the gas.
applicable for ρl/ρg < 10 or G > 2000 kg/m^2s

• Multiphase Separated Flow Model
Lockhart-Martinelli Correlation
(dp/dx)f SL = 𝑓SL * (ρLuSL^2/2D)
(dp/dx)f SG = 𝑓SG * (ρGuSG^2/2D)

uSL: Supercritical Liquid Velocity
uSG: Supercritical Gas Velocity

Large flow rates give errors in the change in pressure.

Question 9

Multiphase pressure drop formula

Answer 9

(dp/dx) = (dp/dx)f + (dp/dx)g + (dp/dx)a

pressure gradient due to: friction, gravity, acceleration

Question 10

Describe the flow regimes

Answer 10

Stratified: The bottom layer has liquid flow and the top layer has gas flow (gravity separation)

Stratified Wavy: Similar to stratified however the higher gas velocity creates ripples in the liquid.

Annular: Liquid forms around the annulus area and high gas velocity passes through the centre of the pipe.

Slug: Frothy waves of liquid can almost take up the full cross section of the pipe and forms slugs of flow.

Bubble: Small bubbles form in the liquid and are more elongated as they reach the top of the pipe.

Dispersed bubble: More liquid and less gas is present, small bubbles form near the top of the pipe.