If your stack has a vortex shedding problem, there are a few options that you can consider:
Helical Strakes – These can be added (Usually to the top 1/3 of the stack) and will disrupt vorticies from forming. They are not to be used if the stack is nearby (Within about 15 diameters) of another tall stack or structure, because the interference effects of adjacent structures can negate the effectiveness of strakes. The shape factor (Drag coefficient) of a smooth cylinder is normally about 0.7, and when you add helical strakes that coefficient doubles to 1.4. This adds a tremendous amount of additional wind load on the structure, which can increase the overall loading on the structure and can cause your stack to get heavier in order to withstand these loads. If you add strakes to your MecaStack file, then the software will do no further a vortex shedding checks. The reason is that the codes (CICIND, ASME STS-1, etc.. ) offer no way to Quantify how effective the helical strakes are in disrupting vorticies. If you add helical strakes, MecaStack will assume that you know that they will solve the problem, and NO further vortex shedding checks will be performed. Go here for more information: http://www.mecaenterprises.com/Helical-Strakes.html
Damping Pad - A damping pad is a pad that goes under the stack, and is used under each anchor bolt washer. In some instances it is a good economical solution to suppress vibration of a stack due to vortex shedding; however, it doesn't work in every situation. We have to analyze the stack to determine how much damping can be expected with the addition of the damping pad, to see if it will be a good solution on a particular stack. Once we analyze your specific stack, we give you a damping value that can be expected, and then you will enter that value into the Damping Pad screen in MecaStack. In addition, the material can only withstand 200 °F, and so if temperatures at the baseplate are greater than this value then the pad is not an acceptable solution. Go here for more information: http://www.mecaenterprises.com/Damping-Pads.html
Tuned Mass Damper – A tuned mass damper can be added to the top of the stack, and it will significantly increase the damping of the stack. Increasing the damping will suppress vortex shedding to a much lower value. The design of a tuned mass damper (TMD) is a very specialized and proprietary nature and it’s not something handled by MecaStack. If you send us your MecaStack file, then we can determine if a TMD would be a suitable solution and then size the TMD and quote it to you. Go here for more information: http://www.mecaenterprises.com/Tuned-Mass-Dampers.html
Change Geometry – Sometimes you can change diameters and or mass distribution in order to solve a vortex shedding problem. Generally this is most feasible if your critical wind speed is very low or very high. If it’s very low, you may be able further lower it by changing geometry (Make stack more flexible, or add mass to the top) and decreasing the critical wind speed to a level where it’s no longer significant. Similarly, if the critical wind speed is very high, then you may be able to further raise (Make stack stiffer or reduce mass) to the point where it can be ignored. Once the critical wind speed is very high then the code allows you to ignore it.
There are several factors in determining the right solution on a particular stack. If you would like our help in performing this evaluation then please send us your MecaStack input file and we can evaluate your options and make a recommendation. If one of our damping solutions is a good option, then we will send you a quote. If you have not used MecaStack to design your stack, then please send us a sketch or drawing. Please make sure that we also have the following information on the following information:
• Refractory or Insulation (Density, thickness, etc..)
• Ladders and Platforms
• Baseplate Dimensions
Please send us your MecaStack input file or stack details, and we will help to determine the best solution for your particular stack.
Here is an article that we published that gives more details on this topic: