How to Model Slug Flow Loads

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

θ – bend angle (90 degree, 45 degree etc.)
ρ – fluid density
v – slug velocity at the moment then it hits the bend
A – internal pipe cross-section area
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

. See how to open the piping model file

Input data:

• Diameter of discharge pipe: 21.9 sm
• Wall thickness of discharge pipe: 1.6 sm
• Product: steam + water
• Velocity 40 m/s
• Water density ρ=0.001 kgf/sm³

Calculation:

A=p∙(D-2t)²/4 = 3.14159∙(21.9-2∙1.6)²/4 = 274.65 sm²
Loads on the bend 2 will be:
F=DLF*ρ*v*v*A*(1-cos 90)=2*0.001*40*40*274.65*(1-cos 90)=879 kgf
F=2*0.001*40*40*274.65*sin 90=879 kgf
Loads on the bend 3 will be:
F=DLF*ρ*v*v*A*(1-cos 90)=2*0.001*40*40*274.65*(1-cos 90)=879 kgf
F=2*0.001*40*40*274.65*sin 90=879 kgf
Loads on the bend 4:
F=DLF*ρ*v*v*A*(1-cos 60)=2*0.001*40*40*274.65*(1-0.5)=439 kgf
F=2*0.001*40*40*274.65*sin 60=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 operation mode 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:

Piping stress caused by slug impact at the bend 3:

Support loads 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

Download this piping model file. See how to open the piping model file