Helical strakes

Helical strakes (strips) are fitted to the tops of tall smooth chimneys to change the pattern of vortex shedding and so prevent resonant oscillation. The same effect will be achieved on a tall column by distributing any attachments (ladders, pipes and platforms) around the column. 

FLOW-INDUCED VIBRATION-IMPEDING DEVICES Helical Strakes... 

The metal strips should be the same material as the stack. The effectiveness of the system is not impaired by a gap of 0.005D between the inner edge of the helical strake and the outside surface of the stack. 

FIGURE 3.8 Helical strakes fabricated using the method described in Example 3.2. 

In the area where helical strakes are attached to the stack, ring stiffeners may be omitted if you can prove that the helical strakes provide adequate stiffness. 

Devices can be built into stacks to counter the vortex shedding, which causes dynamic instability. Helical vortex strakes are the most common and practical vibration inhibitors for stacks. They are generally too awkward to use on process columns because of external attachments, such as ladders, platforms, and piping. 

The application of helical vortex strakes to vertical cylindrical towers has shown remarkable results. I independently developed the method presented here over 20 years ago in a fabrication shop in Houston, Texas. Others developed the strake concept long before that, but the challenge for me at the time was to figure out how to fabricate and install than onto stacks using simple shop tools. This information was published first in a technical journal and then later in [1]. 

---