Wind loads tall vessels

H p = the height of the concentrated load above the column base. 

The load imposed on any structure by the action of the wind will depend on the shape of the structure and the wind velocity. 

Cd = drag coefficient (shape factor), pa = density of air, uw = wind velocity. 

For a smooth cylindrical column or stack the following semi-empirical equation can be used to estimate the wind pressure  

If the column outline is broken up by attachments, such as ladders or pipe work, the factor of 0.05 in equation 13.79a should be increased to 0.07, to allow for the increased drag. 

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Wind loading will only be important on tall columns installed in the open. Columns and chimney-stacks are usually free standing, mounted on skirt supports, and not attached to structural steel work. Under these conditions the vessel under wind loading acts as a cantilever beam, Figure 13.19. For a uniformly loaded cantilever the bending moment at any plane is given by ... 

Skirt supports are recommended for vertical vessels as they do not impose concentrated loads on the vessel shell they are particularly suitable for use with tall columns subject to wind loading. 

When an estimator costs pressure vessels such as reactors and distillation columns, care must be taken to ensure that the wall thickness is adequate. The default method in IPE calculates the wall thickness required based on the ASME Boiler and Pressure Vessel Code Section VIII Division 1 method for the case where the wall thickness is governed by containment of internal pressure (see Chapter 13 for details of this method). If other loads govern the design, then the IPE software can significantly underestimate the vessel cost. This is particularly important for vessels that operate at pressures below 5 bara, where the required wall thickness is likely to be influenced by dead weight loads and bending moments from the vessel supports, and for tall vessels such as distillation columns and large packed-bed reactors, where wind loads may... 

The walls of pressure vessels are usually relatively thin compared with the other dimensions and can fail by buckling under compressive loads. This is particularly important for tall, wide vessels such as distillation columns that can experience compressive loads from wind loads. 

Bending stresses resulting from the bending moments to which the vessel is subjected. Bending moments will be caused by the following loading conditions The wind loads on tall, self-supported vessels (Section 13.8.2) ... 

We will now consider the special problems in tall tower design which are not described in the ASME Code for Unfired Pressure Vessels. As discussed previously, circumferential stresses control the design of cylindrical vessels if external loads are of small magnitude. In tall vertical vessels, four major factors (wind load, seismic loads, dead weight and vibration) may contribute to axial stresses — in addition to axial stress produced by the operating pressure or vacuum of the vessel. 

Wind loads become a special problem if the vessel is located in hurricane areas such as the U.S. Gulf Coast. Winds apply force which causes the tall vertical vessel to be loaded as a vertical cantilever beam that is fixed at its base. 

Standard calculation forms can save considerable time in pressure vessel design. These forms also systematize the mechanical design procedure to insure that nothing is omitted. Most engineering contractors have developed their own vessel calculation forms. Basically, all are alike in that they correlate, in easy-to-use fashion, the design rules set forth in Section VIII of the ASME Boiler and Pressure Vessel Code for Unfired Pressure Vessels. They also include design considerations not covered by the code, such as wind loading for tall vessels. (Text continues on p. 139.)... 

Vessels will vibrate based on an exciting force such as wind or earthquake. There are two distinct types of loadings as a result of wind. The first is the static force from wind loading pressure against the vessel shell. The second is a dynamic effect from vortex shedding due to wind flow around the vessel. Tall, slender, vertical vessels are more susceptible to the effects of vortex shedding. 

Iluilding Code are different from those given in the ANSI standard. A typical distribution of wind loads at various elevations for a tall vessel is shown in Fig. 16.4. 

When a taU vessel supported in the vertical position is subjected to internal l rc sure and external loading om such sources as earthquake or wind, both the tension and compression sides of the cylinder must be examined. These items lire similar to those for a tall vessel under internal pressure only, except for the hitter only one of the sides needs examination. The earthquake loading or the wind loading is resolved into an overturning moment that is further resolved into tensile and compressive loads. 

Wind, seismic and vibrational stresses and accumulated dead weight compression loadings primarily affect the axial stress and produce only a small effect as a result of Poisson s relationship on the circumferential stress. Therefore, the shell thickness of the upper portion of a tall vertical vessel designed to operate under either internal pressure or vacuum is determined by the circumferential stress. 

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