# Slug Flow and Slug Flow Analysis

The purpose of this article is to explain the slug flow in piping and 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.

Excessive vibration may lead to component failures due to fatigue or resonance. Other reasons for worrying about Slug Flow are

• Damage to facilities
• High Back Pressure
• Increased Corrosion

## Creation of Slug Flow

A Slug can be created by

• liquid trapped in the pipeline at low spots.
• a Flow Rate change.
• Pigging

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:

• 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.

## 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

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.

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.

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).

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.

• Prepare the load cases as mentioned in the figure.
• Make stress types as occasional
• Use combination methods as Scalar

## Understanding the Slug Flow Analysis Output

• 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:

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…

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: 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

Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

## 15 thoughts on “Slug Flow and Slug Flow Analysis”

1. sridhar says:

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. want2learn says:

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. Joel says:

Review Units: (kg/m^3) x (m/s)^2 x m^2 = kg x m/s^2 = Newton

1. want2learn says:

right…

3. Prashant says:

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?

1. want2learn says:

liquid density..

4. Patrick says:

Hi there,

I would like to know, how do you derive the the vectors for the forces during input?

5. SANDEEP says:

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

6. sodsai says:

It is force due to momentum of fluids. So density should be fluids and velocity be steam or gas..

7. socstress says:

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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9. Kevin says:

Why is the length of the slug not in this equation. A very short slug is not going to the same as a long slug is?

10. Surajit Ghosh says:

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.

1. Wayne says:

Do any form of nozzle load assessment as an OPE load, however for code stress assessment, this is an OCC load. It’s a primary load.
There is no code stress for OPE loads.

11. williams says:

Hi Anup.
Your articles about pipe stress and related topics are very interesting. I learnt a lot with them.
Do you have any about how to model Water hammer and PSV in dynamic state?