Page 268 - Pressure Vessel Design Manual

246 Pressure Vessel Design Manual

Dynamic deflection: Deflection due to vortex shedding
Precautions perpendicular to the direction of the wind.
~~ ~
The following precautions should be taken.
Notes
a. Include ladders, platforms, and piping in your cal-
culations to more accurately determine the natural 1. See Procedure 3-3 to determine a vessel's fundamental
frequency. period of vibration (POV).
b. Grout the vessel base as soon as possible after erec- 2. See Procedure 4-4 to determine static deflection.
tion while it is most susceptible to wind vibration. 3. Vessel should be checked in the empty and operating
c. Add external attachments as soon as possible after conditions with the vessel fully corroded.
erection to break up vortices. 4. Concentrated eccentric loads can be converted to an
d. Ensure that tower anchor bolts are tightened as additional equivalent uniform wind load.
soon as possible after erection. 5. JJD ratios for multidiameter columns can be deter-
mined as shown in Note 8.
6. A fatigue evaluation should be performed for any vessel
susceptible to vortex shedding. A vessel with a POV of 1
Definitions second and subjected to 3 hours per day for 30 years
would experience 120 million cycles.
Critical wind velocity: The velocity at which the frequency 7. This procedure is for cylindrical stacks or vessels only,
of vortex shedding matches one of the normal modes of mounted at grade. It is not appropriate for tapered
vibration. stacks or vessels. There is a detailed procedure in
ASME STS-1 for tapered stacks. Multidiameter col-
Logarithmic decrement: A measure of the ability of the umns and stacks can be evaluated by the methods
overall structure (vessel, foundation, insulation, contents, shown. This procedure also does not account for multi-
soil, lining, and internal and external attachments) to dis- ple vessels or stacks in a row.
sipate energy during vibration. The logarithmic ratio of 8. LlD ratios can be approximated as follows:
two successive amplitudes of a damped, freely vibrating
structure or the percentage decay per cycle. LID1 4- LDz + . . . + LxDx -I- LskDsk
D;
Static dejlection: Deflection due to wind or earthquake in
the direction of load. where quantities LDx are calculated from the top down.