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:

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 Solution	Advantages	Disadvantages
Damping Pad	Does not Increase wind loads Impervious to most oils and Solvents Not frequency dependent, works for all Modes of vibration	Can only withstand up to 200° F [93 °C] Difficult to retrofit an existing stack Slight increase in lateral deflection Does not provide enough damping for every stack
Tuned Mass Damper (TMD)	Can Provide high Damping Relatively Low weight and wind drag Can visually see TMD working Proven Design with Decades of Service	Must be near the top of the stack Less economical on short stacks Less desirable with lots of attachments; however, mass can be notched to accommodate a ladder or attachment.
Tuned Liquid Damper (TLD)	Can design for multiple modes No moving parts Relatively low weight and wind drag Shape can be changed as needed	Needs to be near the top of stack Difficult with multiple attachments Can’t see liquid from grade
Helical Strakes	Used successfully for decades Simple design Economical for short stacks	Increase wind loads Expensive for Tall Stacks Difficult to fabricate Little guidance on complex situations
Segmented Strakes	They are easy and cheap to fabricate	They do not work

I have a vibration problem, what’s next?

If you have a stack with wind induced vibration issues, answer the questions in the “Vortex Shedding Solution Wizard” to receive a recommendation on how to best solve the problem.