MIL-HDBK-1003/6
d) Loadings.
Wind, earthquake and dead (including moment load
from deflection).
e) Reaction Forces. After structural adequacy has been
determined, both static and dynamic analyses should be made of the loads.
f) Vibration Effects. A steady wind can produce large deflections
of welded steel stacks due to Karmen Vortices phenomenon. If the frequency of
these pulsations is near the stacks' natural frequency, severe deflections can
result due to resonance.
g) Emissions Dispersion. Plant location, adjacent structures, and
terrain affect stack design. Consult 40 CFR 51, Chapter 1, GEP (Good
Engineering Practice) Methods of Determining Stack Height, Dispersion
Techniques, and Restraints on Stack Design.
h) Stack Accessories. Cleanout doors, OSHA-approved climbing
devices including ladder and painter trolleys, EPA flue gas testing ports and
platforms, lightning protection and aviation warning lights as required shall
be supplied.
i) Stack Height Limitations if near Airfield. The stack may be
necked down to increase the discharge velocity but this will also add
additional flow resistance which must be overcome with mechanical means.
j) Stack Construction. Stack height and diameter, support,
corrosion, and economic factors dictate type of construction. Stack
manufacturers must be consulted. Static and dynamic structural analyses must
be made of the wind, earthquake, dead, and thermal loads. Vortex shedding of
wind loads must be considered to be assured that destructive natural-frequency
harmonics are not built into the stack.
Stacks are generally made of concrete or steel because of the high
cost of radial brick construction. If stack gases are positively pressurized,
or if flue gases will be at or below the dew point of the gases, corrosion
resistant linings must be provided; linings must be able to withstand
temperature excursions which may be experienced in the flue gas if flue gas
scrubbers are bypassed.
Stacks of steel or concrete construction shall be insulated to
avoid condensation and smut formations when the internal surfaces may drop
below 250 degrees F (121 degrees C). This requirement does not apply when
scrubbers are used with low temperature discharge (150 to 180 degrees F (65.6
to 82.2 degrees C)) into the stack because the flue gas is already below dew
point temperature.
A truncated cone at the top of the stack will decrease cold air
downdrafts at the periphery of the stack and will help maintain stack
temperature, but stack draft will also decrease considerably.
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