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Silo design

Concrete producers are constantly faced with changing technology. Computer control systems,

reclamation equipment, mixing techniques, chemical admix mechanics, and material handling

advancements are but a few of the current technologies in the ever-evolving concrete

production industry. At least some things never change: take storage silos, for example. These

vertical tanks are about as simple as it gets, right? Dead wrong and dead may prove the

critical term if a concrete producer or his plant engineer is careless in the design or use of a

silo.

It may surprise some that storage silos are not just tanks but, in fact, structures that have

function criteria including how the silo is to fill, empty, and store a given bulk product; flow

pattern, i.e., mass or funnel flow; structural geometry comprising the shape and materials

incorporated in silo design; and how these are affected in full or empty operations and undervaried ambient conditions.

Understanding the material to be stored

A common mistake on the part of concrete plant owners arises from the assumption that a

cement silo design can be used for storing any bulk product around the concrete plant. The

fact, however, is that cement storage design criteria are distinct; it is not safe to assume that

slag, fly ash, silica fume or silica sand, lime, or other bulk fines can be contained in a silo

originally designed for cement storage. Producers need to understand the characteristics,

properties and flow patterns of any material to be stored in a silo.

To determine a material property and flow pattern, contacting the material supplier and

asking for complete product data worksheets is recommended. Such measures have become

increasingly important as replacement ingredients in bulk are being added with far greater

frequency to concrete and concrete products. Self Compacting Concrete (SCC) and High

Performance Concrete (HPC), often calling for various bulk powder or fines additives, are

frequently purchased without consideration of their respective silo storage requirements. For

instance, cement is aerated to make it flowable or fluid-like. Although weighing in some cases

as little as 60 percent of certain cements (by volume), fly ash can be handled much like cement,as the latter has higher design criteria but not always (Figure 1).

Design criteria

Consider, for example, a conical or round silo constructed of steel containing a product that

has a lower friction coefficient (wall friction angle) than a referenced designer's cement on

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steel. Assume a producer selects a product, such as fly ash (60-74 lb. per cu. ft.), and places

the product in a storage silo without consulting the silo designer. Because the material is

lighter than the cement product (85-94 lb. per cu. ft.), the producer presumably feels safe in

doing so. A problem, however, arises in that the cement silo was designed to accommodate the

properties of normal weight cement. The silo designer, assuming a mass flow discharge of a

fluidized product that may typify fly ash as well, was also taking into account a wall friction

angle creating a structural vertical transfer of weight to the cylinder wall of the silo. With its

round surface area compared to the coarse surface area of cement, the lighter fly ash has a

lower friction angle; consequently, a greater amount of the silo product weight is now creating

pressure on the silo hopper or bottom cone section far greater than specified in the original

design. A complete hopper or cone section failure could result. Should the friction angle be

greater, higher compressive loads will be transferred to the cylinder wall, which might cause

buckling or the silo's collapse (Figure 2).

Thus, informed producers recognizes that avoiding errors in silo use contributes to a saferenvironment and workplace. Silo failure can be catastrophic to workers and their businesses.

Contacting your silo designer before placing any product in the vessel is always advisable.

Multi-compartment silos

Whenever a conical silo is split into separate compartments, careful design calculations are

required. Various design pitfalls must be avoided that can lead to asymmetric pressures

imposed on internal components, given certain product, ambient and load conditions (wind,

rain, snowfall, and seismic). These pressures can cause uneven emptying and further loading

or pressure peaks. The silo designer must be knowledgeable regarding inserts involved in a

properly constructed split or multi-sectional storage silo and their effect in relation to pressure

peaks resulting from erratic flow properties. Stiffening supports and multiple openings in silo

hoppers or cone bottoms must be incorporated as needed.

Other considerations are the structural ramifications of a multi-compartment silo, which may

be full on one side and completely or partially empty on the other(s) under high wind loads or

in severe weather conditions, such as tornado (vacuum conditions) or hurricane (severe uplift

and external wind-load pressures). Even in the absence of pressure peaks due to external

adverse factors, nonuniform pressures created by eccentric withdraw from multi-compartmentsilos can cause total structural failure. Designing a multi-compartment silo by splitting a

simple, single-compartment storage tank is a recipe for potential disaster.

The same design criteria apply even more stringently to rectangular or square silos. Placing a

mass-flow product like cement or fly ash into a bin-style silo that may have been designed for

funnel-flow material properties imagine how sand is drawn down can create an

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underdesign situation. If mass flow develops, the pressure may be much greater than design

criteria would accommodate in a funnel flow bin. Consequences could include silo collapse.

While the reverse situation may occur, the ramifications are not as great. Clearly, if your

storage bin or silo is designed as a rectangular structure for bulk powders, it must be fit for the

storage and discharge of mass-flow products (if that is what you plan to store) and not funnel-

flow products.

Temperature considerations

Observing that the belts are not running upon viewing a plant, a concrete producer may

assume the plant is idle. Yet, the plant is always in motion, especially where storage silos are

concerned. Silos are like huge sails when subjected to wind loads; they deflect, sway, and even

rotate. However, the silo moves differently depending on how full or how empty it is at a

given time. Think of the silo's movement relative to its contents as comparable to the position

of the sail to the wind the effect is that great. The variables are seemingly inexhaustible, butcertainly, load combinations for wind and uplift in silos that are full, partially full, and empty

are important design criteria. Empty silos, for example, are not necessarily creating lower

loads.

Picture a cool evening about ten o'clock on a Saturday. A fully exposed silo (any shape) has

been standing in the sun all day long. Now, consider that the plant is idle, and the silo is nearly

full, having been partially filled last at two o'clock on the previous afternoon. Suddenly, a

huge implosion occurs; cement (or any given powder) rains down on everything. Fortunately,

all plant personnel had left the yard for the weekend.

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 tensilestresses. 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.

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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 waterfrom 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, dustcollection, 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).

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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.

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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.