Bulk Engineering, Bulk Equipment, Silos

Ask an engineer: How can I build a better silo?

The causes of many silo failures are incorrect design, poor construction or installation, improper operation, or lack of maintenance. Corin Holmes, general manager for Jenike & Johanson, explains.

Bins and silos are a common sight at most industrial plants around the world. The smallest of these, may contain only a few kilograms of material while the largest may have a capacity of tens of thousands of tons.

A common function of bins and silos is to provide storage or surge capacity for the material being handled. Failures of silo is not an uncommon occurrence in the bulk solids industry. Failures can range from a small deformation in the silo shell to catastrophic rupture and complete collapse of the structure. Indications of structural damage should not be ignored, and proper design and construction practices must be enforced. 

Design for flow

To predict and therefore control how a material will flow in a given container, you must determine the material’s flow properties. These can be measured in a bulk solids testing lab under conditions that accurately simulate the handling process and environment. 

Observe the images in Figure 1 showing what we term “hammer rash”. This is always a result of operators “encouraging” flow with some sort of mechanical aid and is a direct result of a mismatch between hopper/silo geometry and the materials being handled. 

A key aspect of silo design is understanding how the material’s flow properties and the resulting flow pattern affect the silo structure itself. 

Matching test work to silo design

Reliable feed of bulk solids into and out of the silo can be achieved by considering “flowability”.  Defined as the ability of a bulk solid to flow through a given piece of equipment reliably. It considers the relationship of the material itself to the equipment in which it is to be handled. A poor-flowing, difficult-to-handle material may be reliably handled in properly designed equipment while an easy-flowing material may exhibit flow problems in incorrectly designed equipment. The most critical flow properties to aid assessing a material’s flowability in relation to the handling equipment are:

  • Bulk density (changes in density as a function of consolidation). 
  • Cohesive strength (the ability of particles to pack together and form arches and ratholes), 
  • Wall friction (measure of friction between particles and flow surfaces), 
  • Chute angle (critical angles in a chute to maintain flow), and 
  • Abrasive wear (progressive loss of a solid surface caused by sliding contact)

Cohesive strength, wall friction, and compressibility are commonly measured at bench scale using a Jenike direct shear tester. These flow properties are affected by many material and operational characteristics and parameters so it is critical to incorporate these into the test program. Reliable flow is contingent upon the equipment design being matched to the bulk material(s) it will handle over the full range of conditions (moisture content, temperature, chemical composition and particle size, shape, and distribution).

Consider the following case study:

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 over a ten-year period. When the hopper 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.

The first step to solution in this example was to measure and determine the Potash material’s flow properties. Selecting the appropriate flow pattern was the next step. Funnel flow bins have stagnant material along hopper walls that extends into some portion of the cylinder or vertical sided portion of the bin. Mass flow occurs when all the material is in motion whenever any is withdrawn. Thus, selection of geometry, and materials of construction, to not only provide for a specific flow pattern but also to ensure that the silo can withstand the material-induced loads is of paramount importance. 

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.

A common cause 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 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 centre-filled and centre-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.

In our case study, converting the bin’s flow pattern to mass flow was the ideal solution as it eliminated ratholes and stagnant material. The existing cone and the bottom portion of the cylinder were replaced with four stacked hopper sections (Figure 2), and the entire bin was thermally insulated.  

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.


It is critical, for successful design of material handling systems, to consider the flow properties of the bulk material(s) to be stored, under the conditions that will be present in the system. Standards exist for measuring these properties and design methods exist that provide guidelines for their use in the design of storage systems to prevent flow problems and ensure reliable discharge. 

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 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. 


Jenike, A.W. 1964.  Storage and Flow of Solids, Bulletin No. 123, Salt Lake City: University of Utah Engineering Experiment Station.

ASTM D6128-16. 2016. Standard test method for shear testing of bulk solids using the Jenike shear cell. West Conshohocken, PA: ASTM International. Available from www.astm.org.

Carson J. W., and Marinelli J., 1994. Characterize Bulk Solids to Ensure Smooth Flow, Chemical Engineering, Vol. 101, no. 4, April, pp 78-90.

Carson, J.W. and T. Holmes: Why Silos Fail, Powder and Bulk Engineering, November 2001, pp. 31- 43.

Barnum R.A., (2009). Ebb and Flow – Understanding Powder Flow Behavior. Pharmaceutical Processing. pp. 18-21. Los Gatos, CA: Netline Corporation.

Schulze, D. (2007). “Powders and Bulk Solids – Behavior, Characterization, Storage, and Flow”, Berlin: Springer.

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