Reliable discharge of powders and bulk solids from storage vessels is critical to efficient operations. However, flow from silos, bins, bunkers, and hoppers (referred to collectively as hoppers in this article) is often compromised by bridging (also known as arching/plugging) at the outlet. In some instances, a hopper can operate for months without incident, then suddenly become the focus of maintenance meetings and production delays. Hopper bridging may seem like a straightforward blockage, but it often stems from a complex interaction between the bulk material and the equipment designed to handle it. Determining why flow interruptions occur is the first step toward correcting the underlying cause and producing a lasting solution.
Cohesive Bridging and Particle Interlocking
Cohesive bridging is a common cause of flow stoppages in hoppers. Fine powders, moist materials, and compressible bulk solids can develop enough internal strength to generate a stable bridge across the outlet opening. Once that bridge forms, flow stops completely even though the outlet itself remains physically open. Operators often encounter this problem after environmental conditions shift or material remains stored under pressure for extended periods.
As bulk solids consolidate, particle-to-particle forces increase and allow the material to support itself above the outlet instead of flowing downward under gravity. Temporary solutions such as vibrators or impact hammers may collapse the blockage briefly, but the bridge will frequently reform because the underlying conditions have not changed. In fact, certain materials are pressure sensitive and thus vibration can actually lead to stronger bridges forming within the hopper.
Engineers address such a problem through cohesive strength shear cell testing, which measures how a material behaves under consolidating pressure. Test data allows for the calculation of critical outlet dimensions (BC, BP, or BF) that prevent a stable bridge from forming. The resulting hopper design provides an outlet geometry sized correctly to overcome the material’s cohesive strength throughout normal material handling operations.
Cohesion is not the only driving force that can lead to complete flow stoppages. Particle interlocking is a mechanical mechanism that creates stable bridges at hopper outlets. The mechanical bridges are determined by the particle’s size and shape rather than cohesive bonds. Bulk solids that commonly result in particle interlocking include biomass, aggregates, and recycled material.
Stable Ratholing and Mass Flow Design
Not every flow stoppage prevents flow entirely. In many hoppers, material moves only through a narrow channel above the outlet while stagnant regions remain along the walls. Over time, those stagnant regions can stabilize into a rathole structure that reduces live storage capacity and disrupts discharge consistency.
At first glance, the hopper may appear to function normally because material continues flowing through the center channel. But beneath the surface large quantities of material remain stationary. Older material may stay trapped inside the vessel for extended periods as newly added product flows past it. Due to the stagnant material, level indicators may show the hopper as full, even though material is not coming out. Often, an erratic flow sequence may develop in which a material ratholes, turns to bridging, and then ratholes again. In addition to flow stoppages, ratholes can reduce material quality because of the act of stagnating (by way of caking, oxidizing, or spoiling) or by physically segregating within the bin (which leads to non-uniform product).
There are two effective approaches to eliminating ratholing from a storage vessel. To determine which approach is appropriate, it is important to identify what flow pattern the storage vessel discharges in. The two primary flow patterns are funnel flow and mass flow.
In funnel flow, material moves to the outlet through a funnel-shaped flow channel surrounded by stagnant material. Funnel flow is suitable if the following criteria are met:
- The material is coarse enough to avoid flooding.
- The material is free-flowing, avoiding ratholing.
- The material will not degrade if stagnant.
- Material segregation within the bin is not important for downstream processing.
Should the material not meet these criteria, then the bin must be designed for mass flow. With mass flow, all of the material is in motion whenever any is withdrawn from the hopper. Mass flow design eliminates the stagnant regions that allow stable ratholes to form. To evaluate whether mass flow will occur within the storage vessel, engineers conduct wall friction shear testing to determine how the bulk solid interacts with specific wall materials under actual process conditions. Using the gathered results, they establish the hopper geometry and surface characteristics required for a mass flow pattern.
Time Consolidation and Environmental Conditioning
Some hopper problems only appear after a system sits idle. A material that flowed normally during production may become difficult to discharge after a weekend shutdown or prolonged storage period. Operators often describe these situations as inconsistent since one startup can proceed normally while the next requires extensive intervention to restore flow.
Time consolidation is usually responsible for this behavior. Certain bulk solids gain strength when stored under pressure, particularly if environmental conditions fluctuate. Hygroscopic materials may absorb moisture from the surrounding air, increasing cohesion between particles. To comprehend how a material’s strength changes over time, engineers perform time consolidation shear testing under defined storage periods and pressures. This analysis predicts flow behavior after downtime or environmental exposure and helps identify design modifications that can retain reliable discharge. Material handling solutions may include revised outlet geometries, controlled environmental conditions, or specialized discharge interfaces.
The Impact of The Feeder on Withdrawal Uniformity
It is critical to evaluate the feeder when diagnosing flow stoppages. Should the feeder be improperly designed, it can lead to bridging and/or ratholing in the storage vessel above even if the outlet is sufficiently sized to prevent bridging or ratholing. Ultimately a feeder should:
- Provide reliable and uninterrupted flow of material from the hopper above.
- Control the discharge rate from the hopper above, achieving the required rate while preventing flooding of fine powders (if applicable).
- Remove material across the entire cross section of the hopper outlet to avoid interfering with reliable mass flow discharge from the hopper above.
Need Help Diagnosing Hopper Bridging?
Persistent hopper bridging often points to a deeper design issue involving bulk material flow. Jenike & Johanson helps manufacturers diagnose such challenges through flow properties testing, engineering analysis, and custom hopper and feeder design services. Each of our recommendations is guided by measured material behavior and robust bulk solids handling expertise to support reliable, long-term performance. Contact Jenike & Johanson to speak with an engineer about your flow concerns.
