Helical Strakes are aerodynamic stabilizers which are sometimes used… — Refinery training

Refinery training@Refinerytraining · 4.5K subscribers Open in TelegramOpen

Refinery training🟢 Helical Strakes are aerodynamic stabilizers which are sometimes used to reduce the forces and deflections of the stack experienced due to vortex shedding.

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.

If there is another stack (or tall structure) that is nearby, then the interference effects will cause the strakes to become inadequate for stopping vortex shedding. The rule of thumb is that no other tall stacks or structures within about 10 to 15 diameters of the stack. For example, if the stack was 3 m OD, then we need to make sure that we don’t have any other tall structures within 3 m x 15 = 45 m.
The one exception to this is the Euro Standard EN 1993-3-2. In Appendix B Section B.2 if the strake criteria is met, then they provide a way to calcualte the reduced the vortex shedding loads on the stack. The strakes must meet the following:
Check 1: 4.5b <= Strake Pitch <= 5b
Check 2: 0.1b <= Strake Depth <= 0.12b
Check 3: 0.3h <= Strake Length <= 0.5h
Check 4: Unstraked portion at top <= b
Check 5: Scruton Number > 8
where b = Stack Diameter, h = Stack Height
If all this criteria is met then the vortex shedding loads are multiplied by this factor:
Alpha = (1 – Ls / h)^3 , where Ls is the length of stack with strakes

The standard for fabricating strakes is as follows:
Strake Width = 0.1 * Stack Diameter
Strake Pitch = 5 * Stack Diameter
Three (3) strakes, 120 deg apart
Strake Length = 1/3 of the Stack Height
There are many instances where it is not desireable to meet all of these standards, and here are some common examples:
Don’t want strakes on a flare tip here are stiffening rings and so the strake can’t be continuous There is a conical transition in the upper 1/3 of stack Strakes interfere with ladders and platforms
The ASME STS-1 standard provides the following guidance on these issues:
“Each strake is to be aerodynamically continuous except at specific locations where cuts may be necessary to clear ring stiffeners or other attachments.”

Follow me in Linkedin: https://www.linkedin.com/in/esmaeil-esmaeilzadeh-2216b27a?utm_source=share&utm_campaign=share_via&utm_content=profile&utm_medium=android_app

t.me/Refinerytraining/3574