Slug Flow Analysis using Caesar II Static Method: An Article

Static Analysis of Slug flow: A Presentation for Beginners

The purpose of this article is to explain the static analysis of the piping system having slug flow using Caesar II. One of the major causes of vibration in operating plants is slug-flow. So, its always preferable to design systems to overcome the effects of slug Forces.

What is Slug Flow?

Slug Flow is a typical two-phase flow where a wave is picked up periodically by the rapidly moving gas to form a frothy slug, which passes along the pipe at a greater velocity than the average liquid velocity.

In this type of flow, slugs can cause severe and, in some cases, dangerous vibrations in piping systems because of the impact of the high-velocity slugs against fittings such as bend, Tee, etc.

Is Slug Flow Dangerous?

Slug flows generate dynamic fluid forces, which may induce structural vibration.

Slug Flow
Slug Flow

Excessive vibration may lead to component failures due to fatigue or resonance.

Such vibration problems may be avoided by thorough analysis, preferably at the design stage. Two types of Analysis Methods are prevalent in piping design-

Examples of Slug flow

Process Engineers will Analyze the two-phase flow regimes and inform accurately whether the given fluid can cause slug flow while flowing through the piping system. On a broad scale normally following lines are believed to gave slug tendency.

  • Vacuum Transfer Lines
  • Condenser Outlet Lines
  • Re-boiler Return Lines
  • Fired Heater outlets 
  • Boiler Blow down lines.
  • Various Pipeline Flowlines (Process Discipline to Confirm case by case)

Calculation of Slug Force

Slug force is equal to the change in momentum with respect to time. Refer to the below-attached figure:

Diagram Showing Slug Force Application
Diagram Showing Slug Force Application
  • Use the following equations to calculate the Slug Force.
  • Multiply the calculated value with a suitable DLF. Normally a DLF of 2.0 is common to use.
Diagram Showing Slug Force Equation
Diagram Showing Slug Force Equation

Static Analysis of Piping Systems Carrying Slug Flow

Inputs Required for Static Slug Flow Analysis

  • Stress isometrics of the complete system.
  • Line parameters such as line temperatures, pressures, fluid density, pipe material, corrosion allowance, insulation thickness, density, etc.
  • Parameters required for Slug force calculation like slug density or liquid density, two-phase velocity, etc.
  • Nozzle allowable if connected to equipment.

Assumptions for Slug Flow Analysis

While performing slug flow analysis the following two assumptions are made

  • It is assumed that the slug is formed across the full cross-section of the pipe for the maximum impact. This configuration is least probable for vertically down word flow as no hold – up is possible for the accumulation of liquid and eventual formation of the slug. Hence slug force at elbows for vertically downward flow lines is not considered.
  • It is assumed that the reader knows the normal static analysis of the piping system using Caesar II.

Sample Case Study for Slug Flow Analysis

Let’s assume the shown system is subjected to slug flow. The parameters for the pipe are as mentioned below:

  • Pipe: A106B, 6”, Sch 40
  • CA=3 mm
  • T1=100 degree C
  • T2=75 degree C
  • P1=15 bar
  • Liquid Density=950 Kg/m^3
  • Two-phase Velocity=10.53 m/s
Stress System under consideration
Stress System under consideration

After modeling the piping system following the conventional method we have to calculate the slug force and apply the same into the system. Normally all organizations have their excel spreadsheet to calculate Slug Force. A typical excel spreadsheet for slug force calculation is shown in the below-attached figure for your reference.

Excel Spreadsheet for Slug force calculation
Excel Spreadsheet for Slug force calculation

So if we use a DLF of 2 then each of axial and orthogonal force will be 4240N. We have to incorporate this force in the Caesar II input spreadsheet. Check the below-mentioned figure for the direction of forces.

Slug force in Bends with Application direction
Slug force in Bends with Application direction

Now we will input the axial and orthogonal forces at all changes in direction as shown in the attached figure.

  • To enter forces click on the Forces button in Caesar II spreadsheet.
  • Provide the node number and magnitude of forces with proper direction.
  • Similarly input forces in all bends (other than vertically downward bends).
Caesar Spreadsheet Showing input methodology of Slug Force
Caesar Spreadsheet Showing input methodology of Slug Force

The next step is to prepare the required load cases. Few additional load cases need to be prepared for static analysis of slug force. The same has been shown in the below-mentioned figure.

Caesar II Load cases for Slug Flow Analysis
Caesar II Load cases for Slug Flow Analysis
  • Prepare the load cases as mentioned in the figure.
  • Make stress types as occasional
  • Use combination methods as Scalar

Understanding the Output Report

  • Additionally, We have to check code compliance for load cases L14 to L17  and ensure that the values are well within code allowable values.
  • We have to check forces and displacements for load cases L1 to L9.
  • Refer below-mentioned figures for reference:
Caesar II Code compliance check report
Caesar II Code compliance check report
Caesar II Restraint Summary check report.jpg
Caesar II Restraint Summary check report

Keep all stresses, forces, and displacements within the allowable limit. If exceeds then try iteration with the support location change, support type change or routing change.

Few more Resources for you…

Slug Flow Analysis Using Dynamic Spectrum Method in Caesar II
How to Model Slug Flow Loads
Piping Stress Analysis using Caesar II
Piping Stress Analysis Using Start-Prof

Frequently Asked Questions

What is Slug Flow in Piping?

Slug Flow is a typical two-phase flow where a wave is picked up periodically by the rapidly moving gas to form a frothy slug, which passes along the pipe at a greater velocity than the average liquid velocity. slugs can cause severe and dangerous vibrations in piping systems because of the impact of the high-velocity slugs against fittings such as bend, Tee, etc and it can cause the failure of the piping system.

What is Slug Force?

Slug Force is equal to the change in momentum with respect to time, i.e, Force F=dp/dt. The equation of slug force for a piping elbow is given by:Diagram Showing Slug Force Equation

What are the lines that are prone to Slug Flow?

Process Engineers will Analyze the two-phase flow regimes and inform accurately whether the given fluid can cause slug flow while flowing through the piping system. On a broad scale normally following lines are believed to gave slug tendency.
1. Vacuum Transfer Lines
2. Condenser Outlet Lines
3. Re-boiler Return Lines
4. Fired Heater outlets
5. Boiler Blow down lines.
6. Various Pipeline Flowlines (Process Discipline to Confirm case by case)

References

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/slug-flow

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13 thoughts on “Static Analysis of Slug flow: A Presentation for Beginners”

  1. Dear Anup,
     This is very good article for beginners and use full one.

    In the slug force calculation spread sheet all input units are in KG then how force is calculated in newton.
    I think the calculated force will be 20797.2 N and after considering DLF the force will be 41595 N.

    1. May be I have made a mistake..I will check that in the original spreadsheet..The figure is just for illustration purpose…thanks for your response…

  2. Very good Explanation. I have One doubt. Which velocity and density shall be considered in the slug force calculation? Is it Liquid density and velocity or Fluid ( liquid combined with gas) density and velocity?

  3. If we are calculating slug force so why we have to use liquid density to calculate slug force,
    Instead cant we use homogeneous density given by process department

  4. Can you explain again the meaning:

    “It is assumed that the slug is formed across the full cross section of the pipe for the maximum impact. This configuration is least probable for vertically down ward flow as no hold – up is possible for accumulation of liquid and eventual formation of slug. Hence slug force at elbows for vertically downward flow lines are not considered”

    I understand that we don’t need to input slug force on downward bend. But I don’t understand the philosophy.

    Thank you

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  6. Why slug forces are considered as occasional load. It should be directly added with operating case and should be treated as operating load. As long as pipe is in operation it would always hit by slug hence it obviously is not occasional case.

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