How to Model Slug Flow Loads

Read about START-PROF pipe stress analysis software

The loads from slug flow calculated using PASS/Hydrosystem software, then exported to PASS/START-Prof to perform the pipe stress analysis. The algorithm is following:

1. The piping model is created in PASS/START-PROF and saved into the file (.ctp)
2. The file .ctp imported in the PASS/Hydrosystem software
3. Then performed the hydraulic analysis of two-phase gas-liquid flow in the PASS/Hydrosystem software and loads are exported into the .ctpf file
4. The loads are imported from .ctpf file into the PASS/START-PROF software using the operation mode editor (Add > Force Loads from File)

Additional dynamic loads caused by the slugs hitting the bends and tees must be considered. It can lead to the dropping of piping from the supports, exceeding the allowable nozzle and pump loads, etc.

One of the methods for calculating the effect of slug flow loads is the static method. Vertical, horizontal and resultant forces F can be determined by the formulas:

where

F - dynamic load, N

- bend angle (90 degree, 45 degree etc.)

- fluid density, kg/m3

v - slug velocity at the moment then it hits the bend, m/s

A - internal pipe cross-section area, m2

DLF - dynamic load factor. For static method recommended value is DLF=2

For straight tees load calculated the same as for 90 degree bend.

Calculated forces should be applied to the bends that slug heats step by step.

Example

Example project file: SlugFlow.ctp

Input data:

• Diameter of discharge pipe: 0.219 m

• Wall thickness of discharge pipe: 0.016 m

• Product: steam + water

• Velocity 12.65 m/s

• Water density ρ=1000 kg/m³

Calculation:

A=p∙(D-2t)²/4 = 3.14159∙(0.219-2∙0.016)²/4 = 0.027465 m²

Loads on the bend 2 will be:

F=DLF*ρ*v*v*A*(1-cos 90)=2*1000*12.65*12.65*0.027465*(1-cos 90)=8790 N=879 kgf

F=2*1000*40*40*0.027465*sin 90=8790 N=879 kgf

Loads on the bend 3 will be:

F=DLF*ρ*v*v*A*(1-cos 90)=2*1000*12.65*12.65*0.027465*(1-cos 90)=8790 N=879 kgf

F=2*1000*40*40*0.027465*sin 90=8790 N=879 kgf

Loads on the bend 4:

F=DLF*ρ*v*v*A*(1-cos 60)=2*1000*12.65*12.65*0.027465*(1-0.5)=4395 N = 439 kgf

F=2*1000*12.65*12.65*0.027465*sin 60=7610 N=761 kgf

The smaller the angle of bend, the less the load on the bend, because the direction change angle of slug is smaller.

Using load case editor we should create one main operating mode (1) and three additional modes that will model slug loads on bends 2,3,4 (1.1, 1.2, 1.3). The mode type should be occasional.

On the picture below three load modes are shown:

To apply loads we should add additional node near the bend 2 and apply calculated loads:

The same for bend 3:

And for bend 4:

Support loads caused by slug impact at the bend 3:

Piping stress caused by slug impact at the bend 3:

In order to prevent the piping falling from the supports, reduce the support loads, reduce the stresses, you need:

- add limit supports (limit stop supports with a gap, sway brace, snubber), that will take the huge dynamic loads

- try to change the piping layout to make bend angles as small as possible

- eliminate slug flow effect