SLUG FLOW ANALYSIS by CAEPIPE 3D+

Slug flow is a two-phase flow pattern commonly observed in pipelines and vessels, where both gas and liquid phases are present. In slug flow, elongated packets or "slugs" of one phase alternate with packets of the other phase flowing through the system. These slugs can vary in length and velocity, and they are separated by regions where the two phases are more evenly mixed, known as the slug interface or slug front. The word 'slug' usually refers to the heavier, slower moving fluid in the two-phase flow.

In a piping system, when liquid slugs directly impact bends and T-junctions, the system will likely exhibit dynamic behavior. Fig. 2 illustrates the schematic of slugs flowing through a bend in a piping system, highlighting the forces exerted on the bend by these slugs. These forces can influence the structural integrity and performance of the piping system, requiring careful consideration and design to mitigate any potential issues.

SAMPLE PROBLEM IN CAEPIPE (included in CAEPIPE 3D+)

In the piping system illustrated in Fig.3a, the 2-phase Flow directions are as shown. Within this system, slug formation occurs, as shown in Fig.3b, where the force-time response at Tee is depicted. The average velocity and fluid density of the working fluid (i.e., slug) are given as 3.41 m/s and 1000 kg/m^3 respectively. Also, the pipe section with bends and an equal Tee has a nominal diameter of 65 mm with a thickness of 3 mm and a bend angle of 90 degrees.

The fluid coming at a velocity (V) will be diverted into two symmetric streams at the T-junction. For the same pipe cross-section, applying momentum and mass balance at T-junction for the given case results in a final velocity of (V/2) in both the opposite directions as shown below. The forces acting in Z and X directions at the T-junction and the 2 bends are as follows.

We have three options to analyze the slug flow problem in CAEPIPE as follows.

1. Static loading with Dynamic Load Factor (DLF): Slug Forces (F_x and F_z) multiplied by an appropriate DLF are applied in the respective directions at the Tee and both the bend nodes. This is implemented by activating the “Static Seismic 1” with a minimal (0.001 g) g force. The steps to apply this option can be seen in the “Force” section of the CAEPIPE Technical Reference Manual [1]. For illustrative purpose, a DLF value of 2.0 is chosen for the present analysis.
2. Time History Method: Actual slug force as a function of time (Force-Time History) is applied at the respective Tee (see Fig. 3b) and bend node points. This time history analysis is turned ON by activating the “Time history” in load cases. The steps to input the relevant data can be seen in the sections titled “Time functions” and “Time History Load” in CAEPIPE User’s Manual [2].
3. Force Spectrum Method: The Force-Time History is converted to a force spectrum (Force as a function of Frequency) using in-built CAEPIPE option “Convert time function” and applied at the respective Tee and bend node points. This force spectrum analysis is turned ON by activating the “Force Spectrum” in load cases. The steps to input the relevant data can be seen in “Force Spectrum” section of the CAEPIPE Technical Reference Manual [1]. As a sample, the time-history data shown in Fig. 3b is converted to Force Spectrum using CAEPIPE as shown below (Fig. 5).

All the above 3 options can be implemented in CAEPIPE. See the below snapshots of CAEPIPE implementation for your reference. For further details on the analyses and the associated model files, please visit us at "sstusa.com" (Click Here)

References

[1]. SST Systems, Inc., CAEPIPE Technical Reference Manual. Retrieved from https://www.sstusa.com/pdf-download.php?id=4

[2]. SST Systems, Inc., CAEPIPE User’s Manual. Retrieved from https://www.sstusa.com/pdf-download.php?id=3

The Snapshots below show the CAEPIPE model implementing the above 3 methods with a few results.

Snapshots of Layout, Graphics and Force Inputs:

Snapshots of Time-Varying Load Input:

Snapshots of Force Spectrum Input:

Snapshots of a Few CAEPIPE Results: