Silo failure is not an uncommon occurrence in the bulk solids industry. Numerous silos that have failed due to incorrect design, poor construction/installation, improper operation, or lack of maintenance are reported every year. Failures can range from a small deformation in the silo shell to catastrophic rupture and complete collapse of the structure. Regardless of the degree of failure, indications of structural damage should not be ignored, and proper design and construction practices must be enforced. A failed silo can result in large economic costs, injury, and in some cases, loss of life.
Causes of Silo Failures
A key aspect of silo design is to understand how the material’s flow properties and the resulting flow pattern affect the silo structure. The silo designer must select a silo geometry and materials of construction to not only provide a specific flow pattern but also to ensure that the silo can withstand the material-induced loads. The pressure distribution for mass flow is different than that for funnel flow. If a silo designed structurally for funnel flow experiences mass flow, there will be a local pressure peak near the top of the hopper section. This significant increase in pressure can cause a radial tear to develop in the hopper section, resulting in failure of the silo.
One of the most common causes for structural problems in circular silos is bending of the walls due to eccentric discharge of material. This occurs when the outlet is not located along the vertical centerline of the silo. It is commonly found in silos with multiple outlets when only one outlet is active at a time. It can also occur in silos with elongated outlets when part of the outlet is blocked, perhaps because of interlocking of large agglomerates or frozen lumps or because the feeder interface has not been designed to allow uniform withdrawal of material across the entire outlet length. In this situation, an eccentric flow channel develops over the active region of the outlet and intersects the silo wall. Non-uniform pressures develop around the circumference of the silo resulting in horizontal and vertical bending moments on the walls.
Whenever possible, a silo should be center-filled and center-discharged. If eccentric discharge is required or has the potential to occur, a structural analysis should always be performed to ensure that the silo can withstand the non-uniform loading and resulting bending moments.
Poor construction practices and use of incorrect materials may cause a silo to fail. It is important for the design engineer to inspect the contractor’s work to ensure that design specifications are being followed. This includes checking bolt sizes and strength, steel placement, rebar size and spacing, thickness of silo walls, etc.
The design of the silo foundation is equally important. Although uneven settlement is uncommon, the consequences are often catastrophic since the center of gravity of most silos is well above the ground.
Design specifications must be clear and closely followed, and qualified contractors must be hired to ensure high-quality workmanship.
Silo failures can usually be prevented or minimized with periodic inspections and repairs. Routine inspections and repair of silo walls and/or liners are important for preserving the structural integrity and ensuring that the silo is in proper working condition. Other preventative measures include looking for signs of distress such as distortions or cracks in the walls.
When handling abrasive materials or using carbon steel construction in a moist environment, there will be an increased tendency for corrosion and erosion to occur. Walls that have deteriorated or thinned are less able to resist applied loads compared to when they were new.
Ageing steels present special challenges. Abrasive wear removes the protective surface layer of the steel and exposes new material. This accelerates the ageing process and significantly reduces the strength of the structure.
It is important to prevent stagnant material that could trap moisture on the interior or exterior walls of a silo.
Example #1: Deterioration of Two Reinforced Concrete Coal Silos
Two reinforced concrete silos, each 70 m tall and 20 m in diameter, storing about 12,000 tons of coal, deteriorated gradually over a period of about three decades from the time they were erected and put into service. Significant vertical and horizontal cracks, concrete spalling and delamination, as well as rebar corrosion, were observed. Figure 1 shows examples of the deterioration. Both silos are critical for the plant operation since all the coal mined at the site passes through them.
After a thorough investigation of the problem, it was determined that the combination of non-uniform pressures along the silo wall circumference because of eccentric discharge and the flexure at the junction of each silo wall with the common wall between them was the main cause of the silos’ deterioration.
(see Figure 1)
The solution was to design a structural concrete liner to take the loads (eccentric and concentric) imposed by the stored coal, as well as to increase the rigidity of the connection between the silo walls and the common wall.
The silos have been in successful operation with no sign of cracking for almost five years since the concrete liners were installed.
Example #2: Corrosion and Flow Problems in a Potash Bin
A 50-year-old potash product bin had been plagued with flow problems, wear, and corrosion issues for its entire working life. The hopper had been replaced five times in the last 10 years. When the hopper once again reached the end of its life, the owner decided it was time to investigate ways to improve the bin’s operation and extend the hopper’s life.
The old bin consisted of a circular cylinder with a cone-shaped hopper that had not been designed with the potash’s flow characteristics in mind. Potash from an upstream dryer entered this bin at between 140ºC and 165ºC, but because the flow pattern in the bin was funnel flow, the potash outside the active flow channel was able to cool down. This caused moisture to migrate toward the outer regions and condense on the walls, which resulted in corrosion of the walls, particularly in the structurally critical region near the bottom of the cylinder and top of the hopper. It also reduced the live capacity of the bin because the potash in this area hardened and built up on the walls over time. The bin experienced product hang-ups, clumping on the walls and plugging at the hopper outlet. The flow issues in the bin also contributed to a number of mill shutdowns. These problems increased maintenance costs, limited production capacity and degraded product quality.
Converting the bin’s flow pattern to mass flow was the ideal solution because it eliminated ratholes and stagnant material. The existing cone and the bottom portion of the cylinder were replaced with four stacked hopper sections (see Figure 2), and the entire bin was thermally insulated.
(see Figure 2)
Since the changes were implemented, bin operation and maintenance issues have been nearly eliminated. The new hopper is predicted to last over 30 years virtually free of maintenance and product-loss problems, thereby paying for itself many times over.
Example #3: Hopper Dropped off Steel Silo
Several silos storing limestone rock are located above ground at a mine in the Midwestern U.S. Each silo has a capacity of about 400 tons and consists of a 20-ft-diam circular cylinder, below which is a conical hopper section. The silos are leg-supported, and one silo used to be located above a building that was used as a laboratory. One day -- suddenly and without warning -- the hopper section of that silo separated from the cylinder section (see Figure 3), killing two workers in the lab beneath.
(see Figure 3)
The problem was found to have been the result of buildup of lime dust on exposed flanges of the silo’s ring support steel. Moisture was trapped in the buildup, which corroded the silo’s cylinder section (see Figure 4). Eventually the silo wall thinned to the point that it could no longer support the hopper.
(see Figure 4)
This silo failure points out the importance of routine inspection of an entire silo, both inside and out. Providing a means of access to make this inspection easier is often a good idea. This failure also demonstrates the importance of designs to prevent buildup.
Silos can have a long life span and operate reliably if they are properly designed, constructed, and maintained. The designer is responsible for complying with silo design codes at a minimum, but the designer must also ensure that the design meets all the probable loading combinations. To do this, the properties of the bulk solids to be stored, the potential flow patterns, and the silo’s intended purpose must be fully understood. The designer, builder, and owner must agree that the construction and intended performance are satisfactory. Once it is fully operational, it is the owner’s responsibility to properly maintain and service the silo as required.
Tracy Holmes is president, Jenike & Johanson Ltd, Mississauga, Ontario, Canada, and Dr. John W. Carson is president, Jenike & Johanson, Tyngsboro, MA. For more information, visit www.jenike.com.