Page 267 - Pressure Vessel Design Manual

Special Designs 245

for process towers (trayed columns). They estimate the “per- shedding velocity,” V,,,,. This value is then compared to the
cent critical damping” at 3% for empty vessels and 5% for critical wind speed, V,, and a decision made.
operating conditions. The value actually used by most codes 0 If V, i Vzcrit, vortex shedding loads shall be calculated.
is only a fraction of this value.
0 If V,,, < V, < 1.2VzCfit, vortex shedding loads shall be cal-
culated; however, the loads may be reduced by a factor of
Design Criteria (Vz,nflJ2.
0 If V, > 1.2Vzcrit, vortex shedding may be ignored.
Once a vessel has been designed statically, it is necessary Equations are given for calculating all of the associated
to determine if the vessel is susceptible to wind-induced loads and forces for the analysis. This procedure utilizes
vibration. Historically, the rule of thumb was to do a dynamic the combination of two components off?, one /3 for aerody-
wind check only if the vessel UD ratio exceeded 15 and the namic damping, pa, and one for steel damping, Bs. The two
POV was greater than 0.4 seconds. This criterion has proven values are combined to determine the overall /3.
to be unconservative for a number of applications. In ad&- This standard does not require a fatigue evaluation to be
tion, if the critical wind velocity, V,, is greater than 50 mph, done if the stack is subject to wind-induced oscillations.
then no further investigation is required. Wind speeds in
excess of 50mph always contain gusts that will disrupt uni-
form vortex shedding. Criterion 5
This criterion was amplified by Zorrilla [18], who gave
additional sets of criteria. Criterion 1 determines if an ana- An alternative criterion is also given in the Canadian
lysis should be performed. Criterion 2 determines if the Building Code, NBC. The procedure for evaluating effects
vessel is to be considered stable or unstable. Criterion 3 of vortex shedding can be approximated by a static force
involves parameters for the first two criteria. acting over the top third of the vessel or stack. An equation
is given for this value, FL, and shown is this procedure.

Criterion 1
Dynamic Analysis
0 If W/L@ < 20, a vibration analysis must be performed.
0 If 20 < W/L@ < 25, a vibration analysis should be per- If the vessel is determined by this criterion to be unstable,
formed. then there are two options:
0 If W/LD: > 25, a vibration analysis need not be per-
formed. a. The vessel must be redesigned to withstand the
effects of wind-induced vibration such that dynamic
Criterion 2 deflection is less than 6 in.400 ft of height.
b. Design modifications must be implemented such
0 If WS/LD: < 0.75, the vessel is unstable. that wind-induced oscillations do not occur.
0 If 0.75 < WS/LD~ < 0.95, the vessel is probably unstable.
0 If WG/LD; > 0.95, the structure is stable.
Design Modifications
Criterion 3
The following design modifications may be made to the
This criterion must be met for Criteria 1 and 2 to be valid. vessel to eliminate vortex shedding:
Lc/L<O.5 a. Add thickness to bottom shell courses and skirt to
0 10,000 D,< 8 increase damping and raise the POV.
0 W/ws<6 b. Reduce the top diameter where possible.
0 V, > 50 mph; vessel is stable and further analysis need not c. For stacks, add helical strakes to the top third of the
be performed. stack only as a last resort. Spoilers or strakes should
protrude beyond the stack diameter by a distance of
Criterion 4 d/12 but not less than 2 in.
d. Cross-brace vessels together.
An alternative criterion is given in ASME STS-1-2000, “Steel e. Add guy cables or wires to grade.
Stacks.” This standard is written specifically for stacks. The f. Add internal linings.
criterion listed in this standard calculates a “critical vortex g. Reduce vessel below dynamic criteria.