AN INFORMATIVE BULLETIN PROVIDED BY
Warren Environment, Inc.
HELICAL SPOILERS
Many devices have been developed through the years to prevent harmful stack vibrations. These include helical spoilers, tuned mass dampers, absorption pads, impact damping, hanging chains and wind shrouds. As a practical matter helical spoilers and tuned mass dampers are the primary tools for the prevention of excessive vibrations. For additional information on all available prevention designs refer to ASME Steel Stack Standard.
Helical spoilers, or helical strakes, are an aerodynamic approach to the vortex shedding problem in steel stacks. This means that the intent of the spoilers is to deal with the phenomena causing the vibration, i.e. the periodic shed vortices. Spoilers disrupt the air flow thereby weakening the negative pressure generated by the vortices. By accomplishing this, the self-induced cross wind motion is greatly reduced. Spoilers are primarily successful in cases where stacks stand alone without influence from other nearby structures.
The helical spoiler design was invented by the British National Laboratory in the 1960’s and has been used on many stacks throughout the world. Spoilers are supplied as three continuous fins in a helical pattern. They extend radially outward from the stack over the top one-third of the stack height. The pitch of the helix is one and one-half times the stack diameter. The fins protrude outward from each side of the stack one-tenth of the stack diameter.
There is, however, a large price to pay for the helical spoiler design. The total wind pressure of a given stack is increased by the addition of spoilers. The total width of the spoilers is 1.2 x D. The drag, or force coefficient factor is nearly doubled over the length of the spoiler from 0.7 to 1.2. The net result is that the increase in wind load over the top one third of the stack is: 1.2 x 1.2 / 0.7 = 2.1 times. This considerably increases the moment and shear in the stack shell, flanges, inlet reinforcing, base plate, foundation, etc.; thereby increasing the overall cost of the installed stack.
