A group of stacks is a bit of a tricky issue.  The ASME STS-1 code doesn’t give a lot of guidance on design of a group of stacks.  STS does have a calculations for the Strouhal number for a group of stacks, and the Strouhal number dictates the critical wind speed at which the stack can vibrate.  MecaStack allows you to model this aspect by entering the “Group” option and specifying the spacing.  

 

 

That’s about as far as ASME STS-1 goes, but there is some additional information that we can add.  We know from personal experience that when you have interference effects from 2 or more stacks, that strakes are not very effective.  We avoid using strakes in this situation, because we don’t trust them to work.  The Strouhal number modification will have no impact on the ability to predict when strakes are effective in a group of stacks, because once you add strakes no additional vortex shedding is performed.  There is some criteria in other codes, such as the Euro code, that looks at “Galloping” of closely spaced stacks.  MecaStack doesn’t do this check, but we have done the calculation by hand a few times.  


The codes vary on what is considered "Closely" spaced stacks, but generally it is in the range of 10 to 15 diameters.  We use 15 diameters to be conservative.  What this means is that if you have a stack that is 1 m OD at the top, then the adjacent stack should be greater than 15 m (center to center) away in order to ignore interference effects.  

 

To keep it simple, when group of stacks are an issue we recommend the following:

 

  1. Run the stack as a “Single” and “Group” and determine which gives the more conservative results, and that’s the one we use.  The group is assuming the wind is acting along the line between the two stacks, so this only occurs for a small range of wind directions.  That’s why we recommend looking at both options. 
  2. Don’t use Helical Strakes in these situations because it’s too big of a risk.
  3. If you don’t have enough damping, then use a Damper (Tuned Mass Damper, Damping Pad or Tuned Liquid Damper)


ASME STS-1 and other codes often refer to groups of stack as being "Identical" stacks.  In our experience, they don't have to be identical.  We have seen instances where a space frame structure was near a stack and the interference from that structure caused vibration problems with the stack.  We consider any closely spaced structure that is within 15 Diameters and is comparable in height (Approx 2/3 or greater of the height of the stack) that interference could be an issue.