HELICAL STRAKES

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.

For more info on helical strakes check out our article: Getting Twisted about Helical Strakes

If you are unsure that helical strakes are the right option to reduce the effects of vortex shedding (wind induced vibrations) on your stack, then click on our “Vortex Shedding Solution Wizard” to receive an email about our general recommendation based on your parameters entered.

Vortex Shedding Solution Wizard

Help for near another tall structure

To determine if you are near another tall structure you want to make sure that another structure that is 3/4 of the height of your stack is not within 15 diameters of your stack (i.e. stack that is 100 ft [30.5 m] tall with a 5 ft [1.5 m] base diameter would be near another tall structure if the structure was 75 ft [22.9 m] tall and within 75 ft [22.5 m] of the stack)

Help for temperature at base

If you don't have hot gases near the baseplate, then you typically would expect the temperature at the baseplate to be less than 200 °F [93 °C]. 

Help for overall stack height

What is the overall height of the stack from grade to the top of the stack.  If the stack is supported on top of other equipment or another structure, include the height of that supporting structure as well by going all of the way down to grade. 

Example of Helical Strake
Example of Helical Strake
Example of Helical Strake
Example of Helical Strake
Example of Helical Strake

Advantages

The design is simple

In short stacks or stacks that have excess stiffness, they are the most economical solution

Disadvantages

Double the wind load on the upper 1/3 of the stack, increasing stack weight significantly

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.