Silo Failures: Case Histories and Lessons Learned by Dr. John W. Carson

March 23, 2023 | By Jenike & Johanson

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Silos and bins fail with a frequency which is much higher than almost any other industrial equipment. Sometimes the failure only involves distortion or deformation which, while unsightly, does not pose a safety or operational hazard. In other cases, failure involves complete collapse of the structure with accompanying loss of use and even loss of life.

Presented are numerous case histories involving structural failure which illustrate common mistakes as well as limits of design.

Although statistics are not available, hundreds of industrial and farm silos, bins, and hoppers experience some degree of failure each year. [1-3] Sometimes the failure is a complete and dramatic structural collapse. Other times the failure is not as dramatic or as obvious. For example, cracks may form in a concrete wall, or dents in a steel shell, either of which might appear harmless to the casual observer. Nevertheless, these are danger signals which indicate that corrective measures are probably required.

The economic cost of a silo failure is never small. The owner faces the immediate costs of lost production and repairs, personnel in the vicinity are exposed to significant danger, and the designer and builder face possible litigation because of their liability exposure.

The major causes of silo failures are due to shortcomings in one or more of four categories: design, construction, usage, and maintenance. Each of these is explored below, with examples and lessons learned.

Silo design requires specialized knowledge. The designer must first establish the material's flow properties [4], then consider such items as flow channel geometry, flow and static pressure development, and dynamic effects. Problems such as ratholing and self-induced silo vibration have to be prevented, while assuring reliable discharge at the required rate. Non-uniform loads, thermal loads, and the effects of non-standard fabrication details must be considered. Above all, the designer must know when to be cautious in the face of incomplete or misleading information, or recommendations that come from handbooks, or from people with the "it's always been done this way" syndrome.

Fig. 1, Non-uniform pressures caused by eccentric withdrawal

Having established the design criteria, a competent design has to follow. Here the designer must have a full appreciation of load combinations, load paths, primary and secondary effects on structural elements, and the relative flexibility of the elements. [5,6] Special attention must be given to how the most critical details in the structure will be constructed so that the full requirements and intent of the design will be realized.

Five of the most common problems which designers often ignore are described below, along with a few examples of each.

This is one of the most common causes of silo structural problems, since it is so often overlooked. It results when the withdrawal point from the hopper is not located on the vertical centerline of a circular silo [7,8], and is particularly common when using silos with multiple hoppers in which only one or two of the hopper outlets are used at a time. If the resulting flow channel intersects the silo wall, non-uniform pressures will develop around the circumference of the silo leading to horizontal and vertical bending moments. See Figure 1. Many silo designers incorrectly account for these non-uniform pressures by only increasing hoop tension. [9,10]

Some examples:

Fig. 2, Constant pitch screw feeder caused eccentric withdrawal

A blending silo utilized 24 external tubes to withdraw plastic pellets at various elevations from the cylinder and cone sections. Significant wrinkles developed in the cylinder section above several of the tubes. The lessons to be learned here are:

Support beams, inverted cones, blend tubes, and other types of internals can impose large concentrated loads and/or non-symmetric pressures on a silo wall leading to unacceptable bending stresses.

Two examples:

Lessons learned:

Fig. 3, Comparison of wall normal pressures due to assumed funnel flow and actual mass flow

Sometimes mass flow develops in silos, which were structurally designed for funnel flow. [4] Even if this doesn't occur, the local pressure peak, which develops where a funnel flow channel intersects a silo wall, can be devastating. [6]

In some circumstances, ignoring the properties of the bulk solid to be stored can be worse than assuming an incorrect flow pattern. Consider, for example, designing a steel silo to store coal. Lacking a sample of coal which could be tested to form the design basis, the designer may resort to an often quoted design code [12] which lists the wall friction angle for "coal on steel," with no consideration as to the type of coal, its moisture, particle size, ash content, or the type of steel, its surface finish, etc. Flow and structural problems are common when this approach to design is taken.

Two examples:

Lessons learned:

Fig. 4, Comparison of wall normal pressures due to assumed high wall friction and actual low friction

Many silos are constructed of bolted metal panels (usually steel or aluminum), while others are constructed of reinforced concrete. Both types of construction have specific design requirements.

Bolted connections transfer loads through various load paths, and can fail in at least four different modes: bolt shear, net section tension, hole tear-out, and piling around bolt holes. Which mode results in the lowest failure load depends on specifics of the metal (e.g., its yield and ultimate strengths, thickness), the bolts (e.g., size, strength, whether or not fully threaded, how highly torqued), spacing between bolt holes, number of rows of bolts, etc. [14-16]

Compressive buckling must also be considered, particularly if the bolted silo has corrugated walls or is constructed from aluminum.

Reinforced concrete construction presents different problems [17,18]. Concrete is strong in compression but very weak in tension. Thus, reinforcing steel is used to provide resistance to tensile stresses. A silo that has only a single layer of horizontal reinforcing steel is capable of resisting hoop tension, but has very little bending resistance; therefore if non-uniform pressures occur (e.g., due to an eccentric flow channel), the silo is likely to crack. Unfortunately, the inside face of the silo wall, where cracks are difficult to detect, is where the maximum tensile stresses due to bending are most likely to occur. Undetected cracks can continue to grow until the silo is in danger of imminent collapse.

An example:

Lessons learned:

The walls of outdoor metal silos can expand during the day and contract at night as the temperature drops. If there is no discharge taking place and the material inside the silo is free flowing, it will settle as the silo expands. However, it cannot be pushed back up when the silo walls contract, so it resists the contraction, which in turn causes increased tensile stresses in the wall. This phenomenon, which is repeated each day the material sits at rest, is called thermal ratcheting. [19-23]

Another unusual loading condition can occur when moisture migrates between stagnant particles, or masses of stagnant particles, which expand when moisture is added to them. If this occurs while material is not being withdrawn, upward expansion is greatly restrained. Therefore, most of the expansion must occur in the horizontal plane, which will result in significantly increased lateral pressures on, and hoop stresses in, the silo walls.

Two examples:

Lessons learned:

In the construction phase, there are two ways in which problems can be created. The more common of these is poor workmanship. Faulty construction, such as using the wrong materials or not using adequate reinforcement, and uneven foundation settlement are but two examples of such a problem.

The other cause of construction problems is the introduction of badly chosen, or even unauthorized, changes during construction in order to expedite the work or reduce costs.

Close inspection of contractors’ work is important in order to ensure that design specifications are being followed. This includes checking for use of correct bolts (size, strength, etc.), correct size and spacing of rebar, specified type and thickness of silo walls, etc.

An example:

Lessons learned:

Foundation design for silos is not appreciably different than for other structures. As a result, uneven settlement is rare. However, when it does occur, the consequences can be catastrophic since usually the center of gravity of the mass is well above the ground.

Example:

Lessons learned:

Unauthorized changes during construction can put a silo structure at risk. Seemingly minor details are often important in ensuring a particular type of flow pattern (especially mass flow), or in allowing the structure to resist the applied loads.

Example:

Lessons learned:

A properly designed and properly constructed silo should have a long life. Unfortunately, this is not always the case. Problems can arise when the flow properties of the material change, the structure changes because of wear, or an explosive condition arises.

If a different bulk material is placed in a silo than the one for which the silo was designed, obstructions such as arches and ratholes may form, and the flow pattern and loads may be completely different than expected. The load distribution can also be radically changed if alterations to the outlet geometry are made, if a side outlet is put in a center discharge silo, or if a flow-controlling insert or constriction is added. The designer or a silo expert should be consulted regarding the effects of such changes before they are implemented.

When a poorly flowing material is placed in a silo which was not designed to store and handle it, flow stoppages due to arching or ratholing are likely. Sometimes these obstructions will clear by themselves, but, more often, operators will have to resort to various (sometimes drastic) means to clear them. No matter which method is used, the resulting dynamic loads when an arch or rathole fails can collapse the silo. [26]

Self-induced silo vibrations can also result in significant dynamic loads for which most silos are not designed to withstand. [27,28] In addition, few if any silos can withstand the loads imposed by an explosion — either internal or external.

Two examples:

Lessons learned:

Changing material properties or polishing of the inside surface of the silo may cause mass flow to develop in a silo which was structurally designed for funnel flow. (The opposite can also occur – funnel flow in a silo designed structurally for mass flow – but this generally is not as serious a problem.) Mass flow will result in a dramatically different wall pressure loading than with funnel flow, particularly at the top of the hopper section.

Two examples:

Fig. 5, End-result of mass flow developing in a silo designed structurally for funnel flow

Lessons learned:

A pressurized cylinder is more resistant to compressive buckling than an unpressurized one. [9] In addition, if this pressure is caused by a bulk solid (as opposed to a liquid or gas), it is even more resistant. The reason is as follows: Gas or liquid pressure is constant around a silo's circumference and remains unchanged as the silo starts to deform. On the other hand, the pressure exerted by a bulk solid against a silo's wall increases in areas where the walls are deforming inward, and decreases where the walls are expanding. This provides a significant restraining effect once buckling begins.

Now consider what happens if an arch forms across a silo's cylinder section, and material below it is withdrawn. Not only is the restraining effect of the bulk solid lost, but the full weight of the silo contents above the arch are transferred to the now unsupported region of the silo walls. Buckling failure is likely when this occurs.

Example:

Lessons learned:

Maintenance of a silo comes in the owner's or user's domain, and must not be neglected. Two types of maintenance work are required. The first is the regular preventative work, such as the periodic inspection and repair of the walls and/or liner used to promote flow, protect the structure, or both. Loss of a liner may be unavoidable with an abrasive or corrosive product, yet maintaining a liner in proper working condition is necessary if the silo is to operate as designed. Other examples of preventative maintenance items include roof vents, level probes, feeders, dischargers, and gates.

The second area of maintenance involves looking for signs of distress (e.g., cracks, wall distortion, tilting of the structure) and reacting to them. [29] If evidence of a problem appears, expert help should be immediately summoned. An inappropriate response to a sign that something is going wrong, including the common instinct to lower the silo fill level, can cause a failure to occur with greater speed and perhaps greater severity.

Silo walls thinned by corrosion or erosion are less able to resist applied loads than when they were new. This is a particular problem when handling abrasive materials or when using carbon steel construction in moist or otherwise corrosive environments. Combining the effects of abrasion with corrosion significantly accelerates the problem. This can occur, for example, with special aging steels. Abrasive wear causes the surface layer to be removed, thereby exposing new material and speeding up the aging process which significantly weakens the structure.

Three examples:

Lessons learned:

Silo failures often cause significant damage and sometimes result in death. Often these failures could have been prevented or the damage could have been minimized with information that could have been gained through routine inspection.

Example:

Lessons learned:

A common reaction to signs of silo distress is to ignore them, often because personnel are unaware of both the meaning and consequences of doing so. Another common reaction is curiosity. People have lost lives because, due to their curiosity, they were in the wrong place at the wrong time. Even if danger signs are understood, it is common for inappropriate action to be taken in an attempt to "reduce" the chance of failure. In some extreme cases, catastrophic failure has been induced where, with appropriate action, the damage could have been relatively minor.

Two examples:

Lessons learned:

Silos that are designed, built, operated, and maintained properly, will provide long life. Each of the case histories given above illustrates the effects of one or more of the shortcomings possible in design, construction, usage, and maintenance. In each example, the cost of repairs or rebuilding, the cost of litigation, and the cost of insurance added up to several times the cost of doing the job properly in the first place.

The best approach to the design of a silo, bin, or hopper for bulk materials is one that is reasoned, thorough, conservative, and based on measured parameters. Design engineers are not legally protected by sticking to a code of practice. Compliance with the locally applicable code is, of course, necessary, but it should never be regarded, by itself, as a sufficient condition to the performance of a satisfactory design.

It is the responsibility of the designer to ensure that the design is based on sound, complete knowledge of the materials being handled, that the design is competent, and that it covers all foreseeable loading combinations. It is the joint responsibility of the designer, builder, and owner that construction is of an acceptable standard, and fulfills the intent of the design. It is then the responsibility of the owner to properly maintain the structural and mechanical components. It is also the responsibility of the owner to ensure that any intended alteration in usage, discharge geometry or hardware, liner material, or any other specified parameter, is preceded by a design review with strengthening applied as required.