Helical Strakes are aerodynamic stabilizers which are sometimes used to reduce the forces and deflections of the stack experienced due to vortex shedding.  Strakes consist of three (3) vanes which can be wrapped in a helical pattern on the upper 1/3 of the stack.  They have the appearance of a "Snake" which spirals around the stack.

When strakes are added the drag coefficient of the stack is increased greatly.  A smooth cylinder will have a shape factor of 0.7, while the same stack with strakes will increase its shape factor to 1.4.  Consequently the load on the top 1/3 of the stack is doubled.  Since a stack is like a large cantilever beam, increasing the forces at the top by a factor of 2 will increase the loads at the base by approximately 1.5.  Similarly, the deflections will also increase significantly.
The design is simple
Double the wind load on the upper 1/3 of the stack, increasing stack weight significantly
In short stacks or stacks that have excess stiffness, they are the most economical solution
For tall stacks, usually more expensive than a vibration damper
Difficult to fabricate
Can interfere with other attachments (i.e. ladders, platforms, piping, etc.)
Foundation & anchor bolts usually increase in size
Do not work in the case of interfering stacks
Some codes do not allow their use for Scruton Number <8 (which covers most stacks with vibration issues)
Segmented Helical Strakes
Sometimes the helical strakes are fabricated as straight segments and placed in a helical pattern around the stack.  Although these are attractive from a fabrication standpoint, they do not always work. 

Most codes do not permit strakes of this type, the only one that we are aware of that allows their use is API-560 (Heater Stack).  They have been used quite often, and so it is possible that they do work in some situations; however, there are many known situations where they were used and they DID NOT work.  Therefore, Meca does not use them in our designs, and we do not recommend their use.
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Meca Enterprises Inc.
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