Vortex Shedding | Stack Vibration | Meca Enterprises Inc

STACK VIBRATIONS

Every stack has a critical wind speed at which vortex shedding occurs. Vortex Shedding is the instance where alternating low pressure zones are generated on the downwind side of the stack, as shown in the figure. These alternating low pressure zones cause the stack to move towards the low pressure zone, causing movement perpendicular to the direction of the wind. When the critical wind speed of the stack is reached, these forces can cause the stack to resonate where large forces and deflections are experienced. The critical wind speed can be calculated as follows:
Vc = f * D/S f – Natural Frequency of stack D – Diameter at the top S – 0.2 (unless other structures are nearby)

Visit our Youtube channel for a short video explaining the phenomena of vortex shedding, with some amazing actual video of stacks that are vibrating due to vortex shedding here: http://youtu.be/hvg0QQt8Ifk. (If link doesn’t work, go to http://www.youtube.com/mecaenterprises )

For more information on vortex shedding, check out our article: Take Vortex Shedding Seriously

Since Meca specializes in Stack design, we frequently encounter problems with new and existing stacks experiencing vortex induced vibration. It’s the single biggest problem that faces stack designers. As a result, we began offering services to assist Engineers in solving vortex shedding problems. We can help you with evaluation of a specific stack vibration problem, but here are the common solutions as well as their advantages and disadvantages.

 

Vortex Shedding SolutionAdvantagesDisadvantages
Damping Pad
  • Easy to install on NEW stack
  • Low Cost
  • Doesn’t require much change to the stack
  • Can only withstand up to 200° F
  • Adds flexibility to Stack
  • Can’t easily be installed onto EXISTING stacks.
  • Anchor bolts need to be approximately 2.5″ longer projection
Tuned Mass Damper (TMD)
  • Provides the greatest damping
  • Can visually see if the TMD is working
  • All Stainless Steel components
  • Relatively low weight and wind drag
  • Higher cost
  • Needs to be near top of the stack
  • Complicates design if there are attachments (Piping, L&P’s, etc.)
Tuned Liquid Damper (TLD)
  • Shape can be altered to fit available space
  • No moving parts, only the internal liquid sloshes
  • Can be used as a platform
  • Can only be used in certain situations
  • If stack temperature is high, then steps must be taken to prevent liquid from boiling
  • Cannot visually see if the liquid is present from grade
  • Many clients have the perception that they will corrode due to the presences of liquid
Continuous Helical Strakes
  • Design is simple
  • Very common and understood by many
  • Due to increased wind drag, they can increase the wind loads on the stack by 30 to 50%, which can add significant steel to the stack
  • They do not work if there is another tall structure within 15 diameters of the stack
  • They are very difficult to fabricate
Segmented Strakes
  • They are easy and cheap to fabricate
  • They do not always work, don’t use them

MECA ENTERPRISES INC.