Understanding saltation velocity, pickup velocity (how much material moves compared to how much air is exchanged), and terminal velocity (the velocity of air/material impacting the silo) are all critical factors in properly designing a silo for the type of fill system utilized. Never run fill lines at an angle, though many concrete plants do; always run them vertically, if possible, or horizontally and then vertically when needed to offset fill positions. Angled lines will move less product and require many times the additional air due to line sloughing in poor dilute phase filling in what is known as the “unstable conveying zone” (Figure 6).

Tank pressure is one area of silo design that most producers relate to easily. In filling a silo, a particular amount of product is moved, displacing a corresponding amount of air. When filling a silo with a bucket elevator — as often done at cement terminals — the air exchange is much less than that for silos filled pneumatically. Although dense phase filling involves less air exchange than dilute phase filling, in both cases, all of the air placed in the silo must be allowed to escape and the cubic feet of air (air space) displaced by the fill product.

Exhaust considerations

The air that escapes is filtered in baghouses or filter cartridges in dust collector systems. Making sure these air filters are sized large enough to allow the correct amount of air to escape is vital. With too little filter capacity, a silo becomes a huge pressure tank — a very dangerous situation. Where states do not permit over-pressure pop-off valves due to air pollution concerns, maintaining a properly sized filter system is especially important. Just a few pounds per square inch of pressure on a 12-ft. diameter silo roof can create enough force to lift hundreds of thousands of pounds of dead weight. Over-pressure alarms and careful attention by users are in order.

Currently becoming more popular are scavenger systems comprising vacuum, i.e., negative draft, equipment that pulls from various silos, mixers, or dry-batch truck loading points. Some are quite effective; however, many of these systems as adapted for the concrete producer marketplace are not properly designed. While an over-pressure silo is dangerous, an under-pressure silo constitutes an even greater hazard. Never use a scavenger system without a means of letting air into the silo for pressure stabilization or without an under-pressure valve or alarm system. More silos are damaged by under-pressure, by vacuum, than any other means. Collapse, buckling, and implosion can occur. When investigating scavenger systems, note how these systems are integrated into the entire silo air-exchange and balance scheme. At concrete plants, some designers have long warned of pulling too much air out of a silo, causing expensive product to get pulled into the scavenger system rather than deposited into the storage silo. Occasionally, plant manufacturers place a baghouse atop scavenger-equipped silos simply to make sure the silo can breathe. Uncapped fill pipes typically do not offer adequate balance air for pressure equalization.

Concrete producers, too, might breathe a bit easier simply knowing a little more about their silos' capabilities, and what can and cannot be taken for granted where these ‘simple’ storage tanks are concerned.

Robert Ober is Vice president and General Manager of Besser Company's material handling and batch plant facility located in San Antonio, Texas. A leading cement terminal and silo manufacturer in North America, offers a complete line of batch plants and mixing equipment for ready-mix, pipe, and block, prestress and precast production. Mr. Ober can be reached at 210/333-1111 or rober@besser.com

REFERENCES

1. AISC Standard for Steel Construction - Allowable Stress Design, Ninth Edition, American Institute of Steel Construction, Inc., 1990.

2. John Carson, Jenike & Johanson, Inc., “Why Silos Fail,” Powder & Bulk Engineering, 2001.

3. ACI Standard 313-91 - Standard Practice for Design and Construction of Concrete Silos and Stacking Tubes for Storing Granular Materials, American Concrete Institute, 1991.

4. A.G. McLean, University of Wollongong, Australia, The Design of Silos for Safe and Reliable Operation, 1985.