Thursday 18th Aug, 2022

What are the most common silo design pitfalls?

Enes Kaya, a Project Engineer at Jenike & Johanson, explains how to avoid some common mistakes in silo design.

From one engineer to another, don’t fall into the same bulk material handling traps that I have in the past. Improper storage silo/bin design can have serious repercussions to a process, and you do not want to be on the receiving end of an irate client, or even worse, be tied up in litigation due to structural failure of a storage silo.

Thankfully, I haven’t been involved in a litigation, but I have received, prior to working at Jenike & Johanson, bad feedback from an irate client regarding a silo design that did not align with their expectations – it wasn’t pleasant.

I am now aware of the pitfalls that lead to improper silo design and can share what to avoid, saving you the grief of going through it yourself.

To correctly design a silo or storage bin, it helps to first highlight some common pitfalls that can result from poor design.

Common issues:

  1. Inadequate outlet size to overcome cohesive arching (bridging) or ratholing
  2. Hopper walls that are too shallow to promote flow
  3. Poor interface design between silo/hopper and the feeder
  4. Inadequate live capacity
  5. Flow rate limitations (often due to a fine powder)
  6. Erratic flow (either insufficient or uncontrolled)
  7. Non-homogeneous flow due to particle size segregation

Often, vendors will recommend methods to activate/encourage flow from silos, whether it be with the use of vibratory dischargers, air cannons, sonic horns, or other. Effectively, the purpose of these is to ‘shock’ the contents of the silo and promote flow. Fluidisers, common with fine powders, is another flow promotion method. While using fluidisers is not necessarily poor design, it’s important to ensure that the material characteristics allow for their use as the powder’s flow behaviour may be transformed into liquid flow.   

Flow patterns

Jenike originally defined two flow patterns, funnel flow and mass flow, in the 1960’s. In funnel flow, the material is stagnant along the walls, and the material moves in the centre flow channel, i.e. it funnels within itself.  Unfortunately, with most bulk materials in funnel flow, this flow mode has the propensity to encourage the formation of stable ratholes and/or cohesive arching, exacerbate segregation, cause erratic or limited flow, and potentially flooding with fine powders. In contrast, in mass flow all material is in motion during discharge provided that the material flows on the walls due to a combination of low friction and sufficiently steep hopper slope.

In funnel flow, the hopper is usually less steep than mass flow, therefore you can achieve the required storage capacity with a shorter overall silo height often resulting in economic savings.  Be cautious here however, as ‘live’ capacity may not be the same as storage capacity. With funnel flow and a cohesive material that induces ratholing, the ‘live’ capacity may only be 10-20 per cent of the total storage capacity yielding a disadvantage. The selection criteria for a funnel flow silo are straight forward. If the material is coarse, free-flowing, nondegradable, and if segregation is not an issue in your process, then the material can be handled in a funnel flow silo. The savings in head room has allowed you to reap the benefits of the cost savings. 

If, however, the material does not satisfy all of the requirements above, you must handle the material in a mass flow silo to ensure reliable and controlled flow. Often the costs of rectifying the design later, or the continued/prolonged reduction in throughput rate, are immensely higher than the capital expense incurred at the onset. Recall the true engineering success is in silo discharge reliability and not silo “storage” or holding capacity, which is elementary volume calculations. 

The discharge equipment (i.e., feeder or gate) is critical to mass flow. It needs to draw material uniformly by activating the entire cross section of the outlet. If the hopper is designed for mass flow but you do not have a correctly designed feeder, you may still suffer discharge issues as the feeder may only be drawing from a portion of the hopper outlet. In fact, you may convert the mass flow bin into a funnel flow one!

Storage and operating requirements

Now that we are aware of the common design flaws and we understand silo flow patterns, we need to consider the storage and operating requirements; essentially, the basis of design (BoD).

Generally, design engineers place all their focus on the design criteria. This includes consideration of the required storage capacity, discharge rate, feeding method, fabrication materials, safety and environmental considerations, and any local design standards and specifications. What remains unclear to most, as it was to me, is that all of the design criteria are linked directly to the bulk material flow characteristics. It’s like trying to design a liquid pipeline without knowing the specific gravity, viscosity, or required head pressure. Without understanding a bulk material’s flow properties, you will not achieve an effective and reliable flow system. In short, the bulk material characteristics give you the parameters to achieve the BoD.

The caveat, however, is that unlike liquid applications, bulk material flow characteristics can be vastly variable and can depend on common factors such as particle size, shape, moisture, as well as external environmental conditions of temperature, humidity, pressure, and storage time at rest. Thus, there can be significant risk using data from a standard library or a similar process you experienced elsewhere.

Bulk material characteristics

Material flow properties must be measured prior to any design to predict and understand how the material will behave in the process and equipment. If you don’t have the characteristics, you are effectively designing blind; as an aside, this is frequently the reason Jenike & Johanson are engaged to develop retrofit solutions to a materials handling issue. In fact, this happened to me. The critical parameters for bin/silo design are:

• Particle size distribution, shape, and bulk density range

• Operating conditions such as temperature and relative humidity

• Storage time at rest

• Moisture content range (most materials become more difficult to handle as this increases)

• Cohesive strength (to define the dimensions required to overcome arching and ratholing)

• Wall friction (to determine the required hopper angles and surface needed for mass flow)

• Permeability (to understand the possible effects to flow rate in fine materials)

• Abrasiveness (to understand if special liners or materials are needed to avoid wear)

• Friability (to determine if particles are sensitive to breakage during handling)

Knowing what I know now, how would I have approached the design process and BoD differently?  For starters, one of the first steps would have been to request a flow properties test report, ensuring we can be in full compliance to the BoD. As the saying goes, “knowing is more than half the battle.”

Determining the material properties under representative handling conditions will provide you with the critical information needed to design handling equipment to not only store material, but to also reliably discharge it. That is how you can avoid a common material handling “trap.” 

Do you have a bulk solids handling question? Jenike & Johanson has developed the science of bulk solids flow and specialises in applying it to solving the most challenging bulk solids handling problems. So why not put them to the test with your question? 

Note: The advice here is of a general nature. Specific solutions are very sensitive to their circumstances; therefore, you should consult with a specialist in the area before proceeding.