Can I deviate from the Standard?
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
- There 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.”
Based upon this guidance, minor breaks for stiffening rings and other similar attachments should be ok; however, start and stop the strake immediately on each side of the ring. Based upon our experience, strakes are usually not provided on a flare tip, and we are not aware of any issues; however, common sense needs to be applied. If the the upper 1/3 of the stack is 20 ft, and the flare tip is 10 ft, then it doesn’t seem wise to remove 1/2 of the recommended strake length. Conical transitions are a more difficult situation. Vortex shedding is usually limited to one diameter, since the critical wind speed is based upon the diameter. It would seem that it might not be important to strake the conical transition. Our practice is that we try to avoid this situation of having a cone in the upper 1/3 of the stack whenever possible.