Aconcrete dome is a great choice for storing large. Selecting a cost-effective concrete dome for high-volume powder storage

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1 As appeared innovember 2011 PBE Copyright CSC Publishing Selecting a cost-effective concrete dome for high-volume powder storage Thomas W. Hedrick Hedrick Consulting LLC After describing the advantages of concrete domes over silos for high-volume powder storage, this article outlines some dome basics and what goes into selecting a cost-effective dome. The final sections focus on choosing a pneumatic floor system for powder withdrawal a major factor in the dome s cost and how to select one with lower installation and operating costs. Aconcrete dome is a great choice for storing large volumes of dry bulk powders. Compared with a conventional silo, the dome has inherent design advantages: The dome is fat, making it lower to the ground than a silo of similar volume and more resistant to seismic and wind loads. Unlike a silo, which requires a large foundation, often with pilings, to resist these loads, the dome bears directly on the soil, reducing the dome s cost. Dome advantages Depending on its size, a concrete dome can store from 5,500 to 132,000 tons (5,000 to 120,000 metric tons) of powder. The dome is suited to applications ranging from seasonal plant storage to ship loading and unloading, and it can handle powders with widely varying characteristics. Some common applications are alumina, fly ash, cement, pulverized stone, and ores. While powders traditionally have been stored in silos, design constraints of keeping the silo diameter small relative to the height add to this vessel s cost. A silo is very tall, increasing the cost of its filling equipment and increasing loads on its foundation, which often requires expensive deep pilings to withstand seismic and wind loads. Storing more than about 5,500 tons of powder often requires a cluster of silos and, hence, multiple systems for filling and withdrawing the powder. A concrete dome, on the other hand, is more structurally efficient. The dome is lower and wider than a silo of similar volume. As a result, the cost for the dome s filling equipment is lower. Wind loads on the structure are inconsequential even 200- to 300-mph wind loads typically affect only the equipment attached to the dome. Seismic loads rarely affect the dome s structural design, reducing its foundation costs and eliminating a need for deep pilings. Because the dome s storage volume grows by the cube of the dome radius, increasing the dome s diameter only slightly will greatly increase its storage volume. Dome designers have also come to understand the economies of adding cylinder height under the hemispherical dome shell to balance the dome s construction costs. In addition, only one dome is required to store up to 132,000 tons of powder, eliminating a need for multiple structures that each require filling and withdrawal equipment. Another benefit is that the soil under a dome typically accommodates the dome s storage load by settling. This means that softer and lower-cost soil enhancements (like soil compaction and installing stone columns) are much more effective under domes than the stiff, deep foundations (with driven piles or caissons) used for silos. In fact, as many as 80 percent of domes for storing powder are constructed on natural soil, without deep foundations. Finally, the layers of material comprising the dome shell form a matrix that insulates the structure. This prevents sweating problems common with roofing steel in silos.

2 Some dome basics Major components and construction. The dome consists of a concrete hemispherical shell; a straight-sided cylinder section can also be incorporated into the dome to increase the structure s height and add storage capacity without enlarging the footprint, as shown in Figure 1. The dome s exterior covering, called the air form, is a roofing membrane, inflated and stabilized during construction with low-pressure air and then made rigid with sprayed-on polyurethane foam. After steel reinforcing mats are attached to the insulation, the dome interior is sprayed with layers of shotcrete to produce the desired wall thickness. (Detailed information on how the concrete dome is constructed can be found in the American Concrete Institute [ACI] Code ) The pneumatic floor system is constructed as a series of inclined plane surfaces on the structural fill below the dome. (This fill is screened earthen material that provides a strong, stable base for the dome.) The plane surfaces slope downward toward an outlet (or outlets); the slope angle is matched to the powder s internal friction angle to promote the powder s flow by gravity toward the outlet. Once all the powder above a zone has been removed, the next zone is activated, and so on, until the powder above all zones has been removed. A filling inlet is located at the dome s top center, and filling equipment, which can be pneumatic or mechanical, is mounted on the dome exterior. A floor system for withdrawing powder is located at the dome s base. [Editor s note: A detailed discussion of the concrete dome s many components (such as dust collection equipment, access doors and ladders, safety equipment, and level controls) and related loading and unloading equipment is beyond this article s scope; for more information, contact the author.] More about the floor system. In most high-volume domes for powder storage, the floor system is pneumatic and uses a combination of fluidizing air and gravity to withdraw the powder. (Mechanical floor systems are also available but less commonly used today because they have more moving parts and require more maintenance.) The pneumatic floor s fluidizing air system consists primarily of blowers, solenoid valves, and air piping and is PLC-controlled. The piping, which is buried in the fill below the floor, injects low-pressure (typically 3- to 5-psi) fluidizing air from positive-displacement lobe-type blowers upward through the floor. This fluidizes the bottom few inches of powder in the dome, causing it to run like water along the sloping floor toward the outlet. From the outlet, the powder typically enters a solids pump or other conveying equipment that moves the powder to a transfer terminal (for truck or railcar loadout) or to a ship or barge. The pneumatic floor system operates reliably and is generally maintenance-free; in fact, maintenance for most pneumatic floors is required only after 20 to 30 years of highthroughput operation. Figure 1 Concrete dome for high-volume powder storage Two pneumatic floor types Pneumatic floor systems are available in two basic types: partially fluidized and fully fluidized. In a partially fluidized floor system, as shown in Figure 2a, several airgravity conveyor sections (trough-shaped chambers with permeable fabric covers) are arranged across the floor surface. The floor is made up of multiple zones, each containing several conveyor sections; each zone has a similar total fabric surface area and receives fluidizing air from a dedicated blower. The PLC automatically controls the flow of fluidizing air from the blower through the piping and into a zone s conveyor sections, where the air fluidizes the bottom powder layer above the conveyor sections. Once all the powder above a zone has been removed, the next zone is activated, and so on, until the powder above all zones has been removed. However, the powder on the floor surface between the conveyor sections accumulates rather than fluidizes, forming wedge-shaped dead storage areas, called facets, that reduce the dome s total live storage capacity by 3 to 5 percent. The powder in these facets, whose slope angle conforms to the powder s internal friction angle, can be periodically removed by a front-end loader or similar

major factors in the dome s overall cost. Of course, if you re storing a valuable powder in the dome, a fully fluidized floor system may be your only option.

3 Figure 2 Pneumatic floor systems a. Partially fluidized conveyor sections Strategies for selecting a cost-effective pneumatic floor system The pneumatic floor system is integral to the dome, and which system you choose is one of the major factors in the dome s overall cost. Of course, if you re storing a valuable powder in the dome, a fully fluidized floor system may be your only option. But if you re storing an inexpensive powder, you can use the less costly partially fluidized system, and there are ways to configure this system s floor geometry, air-gravity conveyor sections, piping, and related components to further reduce the floor s construction and operating costs. b. Fully fluidized Fabric panels equipment. Because the partially fluidized floor system doesn t provide complete dome cleanout, it s generally used for less valuable powders that don t cake easily, like pulverized stone, alumina, and fly ash. In a fully fluidized floor system, as shown in Figure 2b, large, thick permeable fabric panels are bolted to the floor, covering its entire surface. As with the partially fluidized floor, the floor slopes toward the outlet, and the piping for each floor zone is individually activated by the PLC to fluidize the powder above one zone at a time. The zone number and size depend on the application requirements, such as the dome size and blower size. Unlike the partially fluidized floor, this entire floor is fluidized so powder can t accumulate in dead areas and the dome can be fully cleaned out. The drawback is that this system s greater fluidizing air volume requires larger blowers and larger-diameter piping and, hence, more energy to operate than the partially fluidized floor. As a result, the fully fluidized floor system is typically used for more valuable powders like cement. Some background. Understanding how to select a cost-effective partially fluidized floor system will be easier if we start by reviewing a common limitation with older, less cost-effective partially fluidized floors: the location of the fluidizing air system s components. The fluidizing air system typically consists of one blower and blower header per floor zone and multiple solenoid valves at each header; the valves control airflow into the piping that runs through the fill under the floor to each airgravity conveyor section in the zone. Each solenoid valve is connected to instrument-air and electrical supplies and to the system s PLC, which is typically remotely located. In some older domes, the blower headers, instrument-air lines, and electrical connections run around the dome s perimeter. Each blower header penetrates the dome s shell. Not only does this require many holes in the shell that compromise the dome s structural integrity, but the piping must be very long and can be damaged by truck impacts and similar mishaps. The small-diameter instrument-air line is also hard to support and easily broken. Installation outside the dome exposes the valves to sunlight, which eventually destroys them. Nearby workers are also exposed to high levels of noise from the operating air equipment. Using a cross tunnel and double-dihedral floor design. Installing the valves, blower headers, blowers (in some cases), and instrument-air and electrical lines in a cross tunnel under the floor is a cost-effective way to overcome these problems. An example of a partially fluidized floor system with a cross tunnel is shown in Figure 3; this system is installed in the fly ash dome in Figure 1. In this case, the floor has a double-dihedral design that is, the floor is formed as two dihedrals, which each have two plane surfaces. Each of the four plane surfaces is low at the outlets (at 3 and 9 o clock in the figure) and higher at the center (between 6 and 12 o clock), with the floor s highest elevation at 6 and 12 o clock. The angle from 6 to 3 o clock is also the same as that from 3 o clock to the center. The cross tunnel, which contains the blower headers, valves, and instrument-air and electrical lines, runs under the floor from 6 to 12 o clock (Figure 3). The tunnel not only saves on piping and electrical costs but avoids the structural dis-

Figure 3 Partially fluidized floor with double-dihedral geometry and cross tunnel conveyor sections Figure 4 Partially fluidized floor with conical geometry, cross tunnel with central vault, and

Elevation view Direction of gravity flow Outlet Outlets Solids pump Central vault advantage of having to penetrate the dome shell with piping and electrical lines; it also protects the valves from

Plan views Designing the floor with a cross tunnel also better accommodates the movement of the piping in the structural fill as the fill settles during the first few months of dome operation.

4 Figure 3 Partially fluidized floor with double-dihedral geometry and cross tunnel conveyor sections Figure 4 Partially fluidized floor with conical geometry, cross tunnel with central vault, and hub-and-spoke air-gravity conveyor system a. Elevation view Direction of gravity flow Outlet Outlets Solids pump Central vault advantage of having to penetrate the dome shell with piping and electrical lines; it also protects the valves from sunlight and protects workers from equipment noise. b. Plan views Designing the floor with a cross tunnel also better accommodates the movement of the piping in the structural fill as the fill settles during the first few months of dome operation. During this period, the weight of the powder loaded into the dome causes the fill to settle. Because the floor s plane surfaces form peaks and valleys, with more powder weight at low points than high points, the fill settles unevenly. The fill also settles more at the center than at the dome shell edges. Locating the blower headers and valves in the tunnel rather than outside the dome allows shorter piping runs to the individual air-gravity conveyor sections in the floor, reducing piping movement caused by settling. Using a cross tunnel, central vault, and conical floor design with hub-and-spoke fluidizing air system. Now let s take a look at an even more cost-effective partially fluidized floor design with a conical floor, both a cross tunnel and a central vault below the floor, and a hub-and-spoke air-gravity conveyor system. An example of this design is shown in Figure 4; this dome is similar in size to that in Figure 1 and also stores fly ash. conveyor sections in hub-and-spoke design The conical floor (Figure 4a) is a series of plane surfaces that together form a cone with a center outlet. The central vault, an enlarged section of the cross tunnel, is directly below this outlet and holds a solids pump that conveys the discharged fly ash to loadout. The conical floor s lowered center somewhat reduces the amount of fill needed to construct it compared with the double-dihedral floor shown in Figure 3. Central vault Structural fill

5 This dome s major cost savings is in the floor s fluidizing air system design. The floor has a modified hub-and-spoke air-gravity conveyor system (Figure 4b) with a limited number of blower headers compared with the system in Figure 3. In the hub-and-spoke system, the central hub (forming one zone) is fluidized first to remove most of the stored powder; then each spoke zone is individually activated to withdraw the rest of the powder. This floor design s advantage is that it uses fewer zones, much less piping, and many fewer connections than the floor in Figure 3, reducing its installed cost. [Editor s note: Contact the author for much more detailed information on evaluating structural, mechanical, and other costs for concrete domes.] Choosing a cost-effective and virtually maintenancefree partially fluidized floor system will have the greatest effect on your dome s overall cost. and barge, railcar, or truck loading and unloading; conveying, weighing, and electrical equipment; civil systems (such as roads, docks, office buildings, and stormwater collection); and many others. The end result will be a concrete dome that provides the high-volume powder storage you need while operating reliably and cost-effectively for decades to come. PBE Reference 1. Available from American Concrete Institute, Country Club Drive, Farmington Hills, MI 48331; , fax ( For further reading Find more information on storage vessels in articles listed under Storage in Powder and Bulk Engineering s comprehensive article index (in the December 2010 issue and at PBE s website, and in books available on the website at the PBE Bookstore. You can also purchase copies of past PBEarticles at Some final cost-reduction advice for your dome project For high-volume powder storage, a concrete dome provides many cost-saving design advantages over a silo. The dome rarely requires the deep pilings that a silo does, greatly reducing the dome s foundation cost. Limiting the dome s diameter reduces its installation cost by reducing the amount of land the dome consumes, further cutting its foundation costs, and reducing its floor area and related floor system costs. When the application requires adding cylinder height to increase the dome s storage capacity, this increases only the dome s shell cost, which is more than offset by the cost savings afforded by maintaining a smaller dome diameter. Thomas W. Hedrick, PE, owns Hedrick Consulting LLC, 102 Larkspur, Sikeston, MO 63801; , mobile (1twhedrick@gmail.com). He holds BS and MS degrees in civil engineering from the University of Missouri at Columbia and has over 40 years experience in materials handling and engineering management. Choosing a cost-effective and virtually maintenance-free partially fluidized floor system will have the greatest effect on your dome s overall cost. Using a floor design with a cross tunnel and central vault to house fluidizing air components and conveying equipment below the floor saves piping and electrical costs, and selecting a fluidizing air system with less piping and fewer connections reduces the floor s installation cost and the energy the system requires to withdraw your powder. To help you evaluate available dome design and component options and determine which will provide the most cost-effective solution for your storage requirements, work with a consulting engineering firm experienced in concrete dome design. This firm will collaborate with all parties involved in the project, including those supplying the dome s pneumatic floor system; the systems for ship

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