Guyed stack

Guyed Stack - This type is usually the least expensive to build but in some cases the guy wires result in restrictions on the use of adjacent land, in addition to normal spacing restrictions. 

Trussed towers (heestanding or guyed), guyed stacks, and chimneys 3 1.0... 

The derrick or mast must also be designed to withstand wind loads. Wind loads are imposed by the wind acting on the outer and inner surfaces of the open structure. When designing for wind loads, the designer must consider that the drill pipe or other tubulars may be out of the hole and stacked in the structure. This means that there will be loads imposed on the structure by the pipe weight (i.e., setback load) in addition to the additional loads imposed by the wind. The horizontal forces due to wind are counteracted by the lattice structure that is firmly secured to the structure s foundation. Additional support to the structure can be accomplished by the guy lines attached to the structure and to a dead man anchor some distance away from it. The dead man anchor is buried in the ground to firmly support the tension loads in the guy line. The guy lines are pretensioned when attached to the dead man anchor. 

The cheapest and least durable chimney is the guyed steel stack. Self-supporting metal stacks and those of reinforced concrete come next. Radial and hollow brick stacks, properly designed, are cheaper than those of common brick of equal excellence. 

When the tower is very high, it is sometimes desirable to maintain stability by means of guy wires rather than a large foundation. Although it is not uncommon to find two or even three sets of guy wires on one stack, towers seldom have more than one set, and even these are rare. This discussion, therefore, will be confined to towers with one set of guy wires. 

For guyed towers, or stacks, the shear load from the overturning moment, S5, does not apply, but is replaced by... 

We asked the local guys running the boiler why they thought the top portion of their stack was so narrow. They simply answered,... 

GUY CABLES, REMEDIATION DEVICES, AND SUPPORT OF FLARE STACKS... 

Guy cables can be remediation devices to stabilize a dynamic unstable stack or process column, or as a built-in design support mechanism for a tail and narrow diameter stack. A flare stack is more involved than other types of stacks because there are thermal gradients to complicate matters. 

Determining the pre-tension required for guy cables on flare stacks is necessary for proper support. The method discussed here has been used in practice and is therefore tried and proven. 

Flare stacks supported by guy cables are ubiquitous in use and seem quite innocuous to the untrained eye. However, their failure during the flaring of a stack could have dire consequences. This section is based on actual pretensioning cases that have proven to be successful. 

This section concerns the static problem of guy wires. Experience has shown that if guy cables are properly tensioned, flow-induced vibration (FIV), induced by vortex shedding, is of minimal concern. The reason is that the natural frequency of the stack is well above the resonant range. One reason for pre-tensioning the cables is to avoid FTV. 

FIGURE 3.10 Flare stack guy wire thermal movements. Points A, A, and A" are at the cable connection on the upper portion of the flare stack Point C is at the base of the stack and Point B is where the cable ties into the ground with a dead man.. . . The term dead man refers to an anchor in the ground that has sufficient rigidity to restrain the forces in the cables. Typically they are concrete masses embedded in the ground. 

The guy wires therefore must accommodate the various thermal conditions. ASTM S A-333 pipe material (3V2 Ni—comparable to BS 3603 HF5503 LTIOO CAT.2) is used for the stack. When cold, this material will not experience brittle fracture under wind loads. However, the use of this material does not mean that the stack is always cold when flaring. In many instances, the flare metal temperature is hot and will experience thermal growth. Therefore, the flare must be designed for both cases. 

Because the flare stack gets hot and grows in height, the cable has to incorporate sag to acconunodate stretching under tension. Figure 3.10 illustrates the general flare scheme with the guy wires. 

A guy cable is 1935.07 inches long and weighs 0.154lb ,/in. At operating flare conditions, the guy cable connection on the upper part of the stack (guy cable support ring) is 700°F and at grade the temperature is 500°F. The elevation of the guy cable connection is 114.042 feet. The ambient air temperature is 104°F (40°C). The material of construction of the flare stack is A333 Gr 3. 

The average temperature of the stack from the guy cable support ring connection to grade is calculated as... 

Escoe, A. Keith, Guyed Flare Stacks, Hydrocarbon Engineering, Farnham, Surrey, England, Sept., 1998. 

There is also a discussion about the application of gny cable supports for stacks in regards to dynamic response and wind loads. Of particular interest is a discussion about flare header stacks and how to design guy cables for these tall and slender structures. 

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