When considering the options available to meet a need for more grain storage capacity, the decision may seem fairly simple. After picking steel or concrete, vertical or horizontal, flat or hopper bottomed, the hardest choices remaining are selecting the accessories and the manufacturer.
But there may be another alternative to the traditional grain storing methods. Farmers Cooperative Elevator & Mercantile, Dighton, Kansas, U.S., stumbled upon another solution: dome storage.
Jim Fox, grain merchandiser for Farmers Coop, explained that it took a local reporter and an attentive salesman before his company knew dome storage was an option.
It happened after a local radio station came by to interview the elevator manager about the need for more grain storage. A salesman from Salina, Kansas, who was representing Yeadon Fabric Domes, a dome manufacturer based in Guelph, Ontario, Canada, heard the broadcast interview and contacted Farmers Coop.
After a few proposals and some independent research, Farmers Coop decided to build storage for 1 million bushels of wheat in the form of a dome rather than a silo or a bin.
FABRIC-COVERED DOMES. There were two major reasons why Farmers Coop chose dome storage. "We needed something in a hurry, and this thing was about half the price of steel bins," Mr. Fox said.
But underlying the issues of construction time and price was a bigger issue for the cooperative: efficiency. "Many elevators ship wheat that they are storing — but don't own yet — down to a terminal elevator and pay to have the grain shipped and stored there," Mr. Fox explained. "And we figured we might as well pay ourselves."
"It's a long-term storage unit," he said. "It doesn't unload fast. It's made to leave the commodity in there for a while. But we always have on hand 1 million bushels of farm-stored wheat, so this is great."
Overall, it cost the elevator 85 cents per bushel (U.S. currency) to build the dome, a price that includes the leg, conveyor and all other equipment. Mr. Fox estimates that the facility will be paid for in three to four years.
Construction of the dome began in April 1999 and was finished just two months later. Now, the dome storage unit is filled to capacity with last year's wheat. Mr. Fox said Farmers Coop will probably empty the dome this year and fill it with fresh wheat next summer.
The dome is 252 feet in diameter and 72 feet tall. The base is made up of an 8-foot concrete stem wall and concrete floor. The dome's cover — a rubberized nylon mesh with a life expectancy of about 20 years — must be constantly inflated to keep its shape. The cover is bolted and sealed to the outside of the concrete wall stems, "like stretching a balloon over a bottle," Mr. Fox said.
Fans maintain the air pressure of about 1.5 pounds per square inch inside the dome to keep it inflated. In case of an electric power outage, there are two backup systems, one powered by natural gas and one with propane.
"It is not airtight," Mr. Fox said, "but the air that does come in is high volume and low pressure. And the dome's aeration system is excellent. It is very calm in there, so dust isn't suspended in the air."
Due to the type of cover, the dome is not loaded from the top. Rather, grain is transferred to and from the concrete elevator with a system of conveyor drags and elevator legs. Grain drops directly from the conveyor onto an elevation drag. Once the grain is inside the dome, the elevation drag takes the grain up to the middle of the dome's ceiling, where the grain is dropped into a pile 70 feet below. In addition, a dump pit and leg was installed to dump trucks on site so farmers can unload their grain directly into the dome instead of the elevator. After dumping the truck load into the pit, the leg conveyor transports the grain up to top of the dome, where it drops in the same place as if it came from the elevator.
The only downside to this type of storage is that it takes a while to unload, Mr. Fox said. A tunnel runs through the middle of the dome, from one end to the other. Gravity will flow out about two-thirds of the grain, he estimates. "But the last 300,000 bushels will have to be pushed to the tunnel conveyor."
CONCRETE AND METAL DOMES. Although Farmers Coop found a fabric-covered dome to be most efficient, concrete and aluminum domes also are common. While these domes tend to be more expensive, they also have much longer life spans and enormous strength.
David South, president of Monolithic Dome Institute and Monolithic Constructors, Inc., Italy, Texas, U.S., has built some concrete domes to store grain, although he is more active in the fertilizer industry.
"But concrete domes are fantastic for grain storage," he said. Because they are so well insulated with inches of polyurethane foam, he explained, dome storage keeps the grain cooler and there is very little condensation within the dome.
"And they are just a lot stronger," Mr. South added. For example, an accidental loading of 75,000 tonnes of fertilizer on top of a concrete dome in Huron, Ohio, did not cause any structural damage.
"Domes won't fall over either, like traditional silos if they are unloaded incorrectly, and they can withstand up to F-5 tornadoes (winds above 261 mph), hurricanes and earthquakes," he said. Domes have also proved strong enough to contain an explosion. In 1988, a 1-million-bushel capacity dome built by Monolithic Construction six years earlier for an Alabama grain company exploded, blowing a 100-foot diameter hole in the dome's roof. Amazingly, only the dome was damaged. The dome's top acted like a relief valve, Mr. South said, and escaping gas sucked the debris back into the dome so no lives or property were damaged.
"Domes will keep grain better and longer, especially for larger storage of 1 to 2 million bushels," Mr. South said. "I've done research, and domes are very cost competitve." Too close in cost, he said, to get people to break away from the traditional silo or bins. Mr. South also said that the dome's life span of 150 years or longer — keep in mind the Pantheon dome in Rome, the only structure built in 126 A.D. that is still functional and in use — was not an incentive for the grain industry professionals with whom he has spoken.
Ing. Roberto D. Hajnal, APOSGRAN board member and owner of Juan O. Hajnal S.A., Buenos Aires, Argentina, an engineering and supplier company to the grain storage and processing industry, said dome storage can sometimes be cheaper than silos. "Six years ago, for a tender of a 100,000-tonne facility, we studied several alternatives," he said. The first option was two 25,000-tonne domes and one 50,000-tonne dome, with total costs of U.S.$54 per tonne. The second option was three horizontal silos with similar dimensions, for a total of U.S.$45 per tonne. However, the dome storage cost was cheaper than the third option of three 33,000-tonne vertical concrete silos at U.S.$70 per tonne.
"Cost-wise, the horizontal silo cannot be beaten, but if you don't have enough space, dome storage is a very interesting solution," Mr. Hajnal said. "For example, compared to the horizontal silo, the handling equipment for the 100,000-tonne facility was only 5% more expensive for the dome alternative."
Although Mr. Hajnal has seen few dome storage units for grain throughout Argentina or Brazil, he said that the lack of corrosion in domes due to the concrete and insulation is an advantage.
According to Mr. South, the overall cost might be 50% higher to build a concrete dome, but the cost per bushel is still comparative and construction can be completed, on average, in two to three months, depending on the dome size.
"The disadvantage is that, in the end, we produce one large unit, rather than many smaller bins," he said. "In the U.S., concrete is close in cost to metal, which has such a strong tradition-hold. However, the concrete dome is the building of choice in the third world countries."
In fact, Iraq was commissioning many such concrete domes for food storage before the Persian Gulf War began in 1991. Mr. South's company constructed 30 domes of 10,000 tonnes capacity in Iraq in only six months.
According to Mr. Hajnal, another disadvantage of domes is the inability for visual inspection from above, similar to a catwalk inside a horizontal silo. Also, domes usually have only one access point to its interior, complicating matters in a time of emergency, such as heated material or a fire, he said.
The new grain import terminal Comercializadora La Junta (C.L.J.), located in Manzanillo, Colima, Mexico, is one of the rare grain facilities with dome storage. Jointly owned by the Saskatch-ewan Wheat Pool and members of the Gomez family, the terminal opened in June 1999 with all grain storage in two 25,000-tonne concrete storage domes. At its peak operations, the terminal handles 2 million tonnes of grain annually.
Joaquin Fernandez, director of operations at C.L.J., said the very high risk of earthquakes and hurricanes in that region are the primary reasons that concrete storage domes were used rather than traditional methods. The domes, he said, can support winds up to 200 miles per hour.
Cari Kauppi, vice-president of sales for Temcor Bulk Storage, Carson, California, U.S., said Temcor's aluminum domes might be a good option for grain storage at port facilities because the aluminum oxidizes and creates its own protective layer.
Dome Technology, Idaho Falls, Idaho, U.S., specializes in thin shell concrete domes and constructed the two 86.5-foot-high domes at the C.L.J. terminal.
Dome Technology normally covers cement, cement clinker, gypsum and fertilizer, and has built dome schools, homes and churches. Although the company is not actively involved in the grain industry, domes can be a good solution for grain storage, in the right situation, according to Mike Huntsman, assistant to the general manager.
"If you need to store large quantities of grain, it offers a strong, waterproof structure," he said. Domes are not a good option when routine mixing or blending is needed, he added.
To construct a dome, Dome Technology uses modern architectural fabrics welded into dome shapes, which are inflated and used as the form to which polyurethane foam and concrete are sprayed. The spray-applied concrete in conjunction with steel rebar and polyurethane foam make a dome that is strong, durable, economical and energy efficient.
LIMITED INTEREST. Perhaps because domes are not perceived as having the traditional cost advantages of silos, many dome companies are not actively pursuing the grain industry. "We will let the grain industry come to us," seems to be the consensus among many dome constructors.
Geometrica, Inc., Houston, Texas, U.S., a specialist in large-span domed roofs and storage covers for bins and silos, is one company seeking the grain industry's business. Vellor Sunder, vice-president of industrial products, said Geometrica's dome covers, available in galvanized steel or aluminum, are well suited to cover freestanding piles of grain or material within a silo, and can easily accommodate aeration systems.
"Clients in cement, power, mining and other industrial markets are quite aware of our capabilities and achievements," Mr. Sunder said. "We now need to reach the grain industry with our structural systems and economies."
With the ability to span 150 feet to 900 feet, Geometrica's domes are cost-effective and are generating interest in South America, the Middle East and Southeast Asia, in addition to the U.S., he added.
"Geometrica's structures could be ideal for centralized grain terminals, port discharge and loading facilities, and could be a very good solution even for large warehouses to store grain in bags or bulk bags," he said. "We believe domes will become popular as economical solutions for large diameter bin or silo covers and for enclosing freestanding piles of grain and other agro-materials."
On the whole, however, dome use for grain storage is still relatively small. Cost and practicality will likely prevent domes from replacing elevators, but there are some instances where a dome can fit a grain storage need, mostly for long-term storage of a large amount of grain. There is a greatly varying range of dome types, from fabric to metal covers to aluminum and insulated concrete, all in equally varying prices.
Jim Fox at Farmers Cooperative Elevator & Mercantile realizes his company is in the minority. "Dome storage is rare," he said, "but it does exist."
As he researched dome storage, he found more dome units in Iowa and Minnesota. Mr. Fox said he came across one corn producer in Iowa who has been storing corn in a dome storage unit for almost ten years.
"So far, we're pretty happy with the dome, although we've only had it for a year," he said. "We've had a lot of people stop and look — it's quite a conversation piece."