What is Slug Flow? Steps for Slug Flow Analysis and Prevention

Slug flow is a phenomenon that occurs in multiphase flow systems, particularly in pipes and pipelines carrying gas and liquid phases. It is characterized by the intermittent presence of large, cohesive bubbles or slugs of gas that travel through the liquid. This type of flow can lead to significant operational challenges, making slug flow analysis crucial for engineers and operators in industries such as oil and gas, chemical processing, and water treatment.

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 bends, tees, etc.

Characteristics of Slug Flow

• Intermittency: Slug flow is marked by intermittent movement of gas bubbles within the liquid. These bubbles can merge to form larger slugs, causing a non-uniform flow.
• High-Pressure Fluctuations: The movement of slugs can lead to pressure surges within the pipeline, which may damage equipment and create safety hazards.
• Liquid Hold-up: During slug flow, there is a significant amount of liquid held up in the pipeline, which can impact the overall efficiency of the transport system.
• Flow Regime Transitions: Slug flow can transition into other flow regimes, such as annular or stratified flow, depending on changes in flow rates or other conditions.

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, including leaks or hazards
• High Back Pressure
• Increased Corrosion
• Reduced efficiency
• Increased maintenance cost

Creation of Slug Flow

A Slug can be created by

• the liquid trapped in the pipeline in low spots due to imbalances in the distribution of gas and liquid phases.
• a flow rate change.
• pipe geometry change
• Pigging
• significant density differences, etc

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. The equation used for the calculation of slug forces is provided in the 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.

From the above slug force calculation equation it is quite clear that:

• The value of slug force will increase with an increase in fluid density
• The slug force is directly proportional to pipe size; with an increase in pipe size, the resultant slug force will increase.
• The calculated slug force is directly proportional to fluid velocity, which means with an increase in fluid velocity the impact of slug force will increase.
• Also, the magnitude of slug force is directly proportional to the momentum flux (Rho X A X V²)

Alternate Equation for Slug Force Calculation

Alternatively, you can use the following slug force calculation equation as provided in Shell DEP 31.38.01.25, Feb 2023. As per clause 2.2.7, point no. 7 of the mentioned DEPs,

Slug flow piping system shall be designed to accommodate the reaction forces and vibration fatigue. The maximum reaction force Fmax shall be calculated in accordance with Equations 9 and 10.

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 is shown in the figure below.

• 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 to the 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.

Slug Flow Prevention/ Mitigation

To effectively mitigate the risks associated with slug flow, a variety of preventive measures can be adopted. Here are some essential strategies for prevention:

1. Optimized Pipe Configuration

Designing the piping layout to minimize the potential for liquid accumulation is crucial. This includes avoiding configurations that create pockets where liquids can be collected. A well-thought-out design can significantly reduce the likelihood of slug formation.

2. Low Point Drains and Traps

Incorporating low-point drains, traps, or bypasses in the pipeline is an effective way to prevent liquid accumulation. These features allow any trapped liquid to be removed, maintaining a more consistent flow and reducing the chance of slugs developing.

3. Regular Insulation Checks

Periodic inspections of insulation can help minimize condensate formation, which can contribute to slug flow. Ensuring that insulation is intact and effective reduces the risk of excess liquid buildup in the system.

4. Complete Draining Design

Piping and equipment should be designed to allow for complete drainage. This design consideration eliminates pockets where liquid can become trapped, further preventing slug flow.

5. Maintenance of Traps and Valves

Including traps, pressure control valves, safety valves, and rupture disks in your preventive maintenance program is vital. Regular checks on these components ensure they function correctly and help manage flow conditions effectively.

6. Review of Abandoned Equipment

Conducting a review of any abandoned equipment and piping systems can identify potential sources of liquid buildup. Addressing these areas can help prevent slugs from forming in the active portions of the system.

7. Thoughtful Material Selection

Choosing materials with adequate tensile strength is important for withstanding dynamic transients and impact loads. For example, avoiding brittle materials like cast iron can prevent structural failures due to slug impacts.

8. Proper Support Design

Ensuring that supports are designed to accommodate transient performance is essential. This consideration helps maintain the integrity of the piping system during fluctuations in flow.

9. Use of Slug Catchers

Integrating slug catchers at the pipeline outlet can effectively manage and collect slugs. These devices help minimize the impact of slugs on downstream equipment and maintain a more stable flow.

Conclusion

Few more Resources for you…

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

Frequently Asked Questions (FAQ)

References

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

Online Course on Static Slug Flow Analysis

If you still have doubts, then the following online course is just perfect for you:

• Slug Flow Analysis in Caesar II (Both Static and Dynamic)