What could have happened? The scenario described here actually occurred and was caused by something silo design engineers refer to as thermal ratcheting (Figure 3, page 24).

During daylight hours as the ambient temperature rises, the silo enlarges mostly in circumference. The degree of expansion is determined by the specified hoop strength, i.e., tensile wall strength, and/or by incidental geometries, that is, the shape of the silo and the existence of rigid areas in the structure. The stored material at rest is allowed to settle as the wall expands. When night comes and the temperature drops, the silo wall contracts. The near full silo cannot push the material back up the silo wall without a substantial increase in tensile stresses. This thermal ratcheting effect is compounded for every day the temperature rises, as the sun warms the silo, followed by a cool night when a near-full silo sits idle. For this reason, bolted construction silos are often welded on the interior after assembly, and rivets and bolts must meet exacting specifications. Clearly, in the case of a silo of bolted-construction design, the entire structure is only as good as the fasteners used — and the personnel who are responsible for tightening them.

Thermal ratcheting and tensile stresses are primary reasons that so few smaller-diameter silos are constructed of concrete. Reinforcing the concrete sufficiently requires a wall thickness and a heavy steel component involving considerable expense. In most markets, heavily reinforced concrete becomes cost-effective for silos starting at about a 45-ft. diameter.

Humidity considerations

Where any bulk material is concerned, humidity must be factored into the silo design. At concrete plants, powders and both fine and coarse aggregate are normally found; however, material handling engineers and silo designers typically deal with a much wider variety of bulk material including meal, flours, plastics, food stuffs, pharmaceuticals and pigments, to name just a few. All types of bulk storage require design criteria that take into account the varying percent of relative humidity.

Imagine a silo full of cement where aeration has been miscalibrated and contains free water from condensation or is totally inoperative. The cement has settled and compacted creating tensile stresses. The material may arch or rathole, producing extraordinary loads on the silo wall as the full load of the material is transferred to the silo wall just below the obstruction when the silo is mass-flow emptied below. The silo wall may then buckle below the obstruction level. Should the ratholed or arched product suddenly break loose, the high dynamic loads imposed can cause a hopper or cone failure. As neither of these scenarios is desirable, the need for proper material assessment, silo design, and environmental considerations is once again emphasized. Humidity is not something to be taken lightly by bulk storage design engineers or by concrete producers (Figure 4).

Filling considerations

The means by which a silo is filled constitutes another important consideration in its design. Filling accomplished pneumatically using a vertical 4-in. pipe with a close 90 degree bend to a diffuser comprises dense phase loading. Were the same bulk tanker truck fit with a 5-in. line without increasing the blower size — not an uncommon practice — the Saltation velocity may be surpassed. Passing the Saltation velocity point, a dilute phase conveying method is used. Since air always flows from a higher pressure to a lower pressure and thereby expands, the velocity in a fill pipe is always increasing. Clearly, velocity, pressures, air exchange ratios, dust collection, and vacuum are all important factors directly related to the silo structure. Keep in mind that a silo must be filled under all conditions: when in use, when idle, when almost full, and when almost empty. Again, a properly designed silo must perform across the entire spectrum. Requesting a silo manufacturer simply place a 5-in. line on the silo so it can be filled faster may instigate further problems, least of which will be dust control (Figure 5, page 26).