What is Slug Flow? Steps for Slug Flow Analysis (With PDF)

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 piping vibration in operating plants is slug flow. So, it’s 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-

• Static Analysis and  
• Dynamic Analysis

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 give slug tendency.

• Vacuum Transfer Lines
• Condenser Outlet Lines
• Re-boiler Return Lines
• Fired Heater outlets 
• Boiler Blowdown 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 when 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 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 in Caesar II

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 to 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 axial and orthogonal force will be 4240 N. 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 the Caesar II spreadsheet.
• Provide the node number and magnitude of forces with the proper direction.
• Similarly input forces in all bends (other than vertically downward bends).

The next step is to prepare the required load cases. Some 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 occasional
• Use combination methods such 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 pipe 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 (FAQ)

References

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