Ratholing in silos and hoppers can halt production, waste materials, and increase maintenance costs—affecting up to 30% of bulk powder operations in food, pharma, and chemical manufacturing.
Key Takeaways
- What causes ratholing in silos and hoppers? Ratholing occurs when cohesive or fine powders stick to container walls, forming a central flow channel. Factors include particle size, moisture content, hopper angle, and funnel flow patterns.
- How can ratholing and bridging be prevented? Use mass flow hopper designs, appropriately angled walls, and flow aids like acoustic cleaners, AirSweep systems, or cone valves. Regular maintenance and material conditioning further reduce blockages.
- What are the operational impacts of ratholing? Ratholing reduces efficiency, increases downtime, and risks material contamination. Proactive flow solutions minimise manual clearing, lower maintenance costs, and maintain consistent production rates.
Introduction
Ratholing happens when material clings to the sides of a silo or hopper, restricting flow to a narrow channel above the outlet. If left unaddressed, it can lead to production delays, product degradation, safety hazards, and expensive downtime. In this guide, we’ll explain exactly what causes ratholing, why it’s so disruptive, and how you can prevent it with modern material flow solutions that protect your equipment and maximise efficiency.
Key Challenges:
- Ratholing: Occurs when a stable flow channel forms in a bin, leaving material stuck along the walls.
- Bridging: Forms when material creates an arch above the outlet, preventing flow entirely.
Common Causes:
- Material Properties: Fine powders, cohesive substances, and moisture content increase susceptibility to blockages. Find out more about particle properties here.
- Equipment Design: Incorrect hopper angles, poor bin design, and insufficient flow aids exacerbate ratholing and bridging.
Impacts:
- Production Downtime: Frequent blockages result in lost hours, missed targets, and costly shutdowns.
- Maintenance Costs: Manual clearing and equipment repairs increase operational expenses.
- Procurement Pressures: Equipment selection becomes critical to long-term operational efficiency.
To understand the science behind these issues and explore practical solutions for your industry, read the full article.
Read More About Powder Handling- Dust Hazards
What Is Ratholing?
Ratholing definition in silos, hoppers, and material containers
Ratholing occurs when a narrow flow channel forms within a bulk material, leaving large portions of material stagnant against the walls of the container. This phenomenon typically happens in bins, silos, or hoppers handling fine or cohesive powders. Material flows through the central core, while the rest remains immobile.
Ratholing results in reduced material flow and inefficient emptying of the container. It forces operators to intervene, often through vibration or manual clearing, to maintain proper flow rates. Over time, the accumulation of stagnant material can cause significant production delays and maintenance issues.
- Definition: Formation of a flow channel through stagnant bulk material, leaving material along the walls of the container.
- Synonyms: Channeling, core flow.
Ratholing Vs Bridging
Bridging is a common material flow issue where an arch forms across the outlet of a hopper, bin, or silo. This bridge prevents material from flowing freely, creating a complete blockage. Bridging is often triggered by the cohesion of fine or moist powders, which allows particles to bond together and form structural arches. This problem can also occur due to poor equipment design, especially if the hopper’s outlet or walls are not appropriately angled to encourage flow.
When bridging occurs, material flow stops entirely, halting production until the blockage is cleared. This can involve costly downtime, equipment adjustments, or manual intervention to break the arch and restore normal flow.
- Definition: The creation of an arch or bridge within the material that prevents flow.
- Synonyms: Arching, vaulting.
Both ratholing and bridging severely impact material handling systems, especially in industries reliant on bulk powder feeding. Addressing these issues through proper design, material conditioning, and flow aids is crucial to maintaining consistent production and reducing operational costs. Continue reading to explore the science behind these problems and learn how to effectively manage them.
Why Does Ratholing Happen?
Cohesive powders and fine particles are more prone to ratholing due to their tendency to stick together. Materials with a small particle size and low density often form compact masses, preventing free movement. The force needed to break this compaction is rarely achieved through gravity alone, resulting in a central flow channel with material remaining stuck along the walls.
- Material Properties: Cohesion, particle size, and density determine how likely a material is to form ratholes. Fine powders like flour, cement, and pharmaceuticals are at high risk.
- Bin Design and Hopper Angle: The geometry of the container plays a significant role. Hoppers with shallow angles allow material to adhere to the sides, promoting ratholing. Steeper hopper angles (generally greater than 60 degrees) reduce the likelihood of stagnant material, encouraging more uniform flow.
- Flow Dynamics: Flow behaviour within the container is critical. Ratholing typically occurs in funnel flow patterns, where material moves through a narrow channel in the centre, leaving the surrounding material undisturbed.
Why Does Bridging Happen?
Bridging occurs when bulk material forms a stable arch above the outlet, preventing further flow. Cohesive forces and particle size are critical in determining whether a material will form a bridge. Fine powders or materials with high moisture content are more likely to bond together, forming rigid structures capable of stopping flow. In some cases, electrostatic properties also contribute, as particles attract and bind together.
- Material Factors: Moisture content increases cohesion, making materials like powders and granules susceptible to bridging. Particle shape and size also play a role—irregular particles with rough surfaces can interlock and form stronger bridges.
- Electrostatic Properties: When particles become charged, they attract each other, leading to higher cohesion and a greater risk of arch formation.
Flow Patterns
Material flow patterns are a key factor in both ratholing and bridging. Funnel flow and mass flow are the two primary patterns observed in silos, bins, and hoppers:
- Funnel Flow: Material moves through the centre, leaving stagnant zones along the walls. This pattern is prone to both ratholing and bridging, as the uneven flow creates opportunities for blockages.
- Mass Flow: Material moves uniformly across the entire cross-section of the container, preventing stagnant areas. This flow pattern helps reduce the risk of both ratholing and bridging, though it requires carefully designed hopper angles and surfaces.
What Causes Ratholing and Bridging?
Material properties (cohesiveness, particle size, moisture)
Material properties play a significant role in the occurrence of ratholing and bridging, for example fine, cohesive powders. Flour, cement, and gypsum are particularly prone to both issues, as the cohesive forces between particles are very strong, increasing the chances of forming ratholes or arches.
- Particle size: Materials with smaller particle sizes (fine powders) are more prone to ratholing or bridging than materials with larger particle sizes (coarse powder). Read our article on particle size here.
- Cohesion and Moisture: High cohesion levels holds particles together and restricts flow, this effect is more pronounced in fine powders with high cohesion forces or materials with higher moisture levels.
- Wide Particle Size Range: A wide distribution of particle sizes within a material can lead to by irregular flow patterns and the formation of ratholes and bridges. Milling materials to a narrow particle
Environmental Conditions
Environmental factors impact material flow patterns: high humidity and static electricity can increase the occurrence of ratholing and bridging.
- High Humidity: Moisture from the surrounding environment make materials clump together and form blockages.
- Static Electricity: In dry environments, static electricity builds up on particles, causing them to attract and bind together. This effect is particularly common in non-conductive materials like plastics.
Hopper and silo design factors
Design flaws in bins, hoppers, and silos contribute significantly to ratholing and bridging. In many cases, poorly designed equipment creates conditions that encourage these flow blockages.
- Incorrect Hopper Angles and Outlet Sizes: The angle of the hopper walls affects how material flows out of the container. Shallow angles allow material to form stable arches or ratholes, while angles over 60 degrees encourage smoother flow. Similarly, if the outlet size is too small, material may form bridges, cutting off flow completely.
What Materials Rathole?
The Impact of Ratholing on Production
Reduced efficiency and downtime
Ratholing and bridging directly cause production stoppages. When bulk powder flow is obstructed, operations must halt to clear blockages manually or by using mechanical aids. Every minute of downtime translates into lost production, which can quickly accumulate.
- Time Lost: Clearing these blockages can lead to significant downtime and repeated interruptions disrupts the production schedule, lowering overall efficiency.
Quality risks (waste)
Uneven material flow, caused by ratholing and bridging, causes inconsistent feeding rates to other equipment. This can negatively affect product quality, resulting in rejects, reworks, or even contamination.
- Inconsistent Flow: Uneven material flow results in inconsistent mixing, that may compromise the quality of the final product.
- Material Wastage: Blockages can lead to spoilage, especially of sensitive products. For example, stagnant material in silos may become contaminated and unusable if left blocked for longer periods.
Increased maintenance and safety risks
The wear and tear on equipment due to recurring blockages increases maintenance costs.
- Increased Wear: Commonly used methods of manually clearing blockages (hitting the container/equipment inlets) can cause damage, driving up repair and replacement costs.
- Reactive Maintenance: Unplanned maintenance is costly. Delaying investing in a solution and relying on short-term fixes raises operational costs and can lead to more expensive issues later.
Traditional Ratholing Solutions And Their Limitations
Vibrators: Benefits And Structural Risks
Vibrators attach to storage vessels to shake material loose, these vibrations break the cohesive bonds between particles and restore flow.
Benefits:
- Breaks material compaction quickly
- Reduces manual intervention
- Offers a relatively low-cost installation
Structural Risks:
- Frequent vibration stresses weld seams and joints
- Long-term use causes metal fatigue and structural cracking
- Excessive vibration can worsen ratholing by compacting material around the hopper walls
- Vibration-induced noise impacts working conditions
Air Cannons And Blasters: Coverage Limits
Air cannons release high-pressure bursts to disrupt ratholes. They target specific points inside bins or silos with sudden jets of compressed air.
Benefits:
- Clears blockages at targeted points
- Reduces need for manual clearing
- Works in high-temperature and abrasive environments
Coverage Limits:
- Single cannon affects a limited area (usually within a 1–2 metre radius)
- Multiple cannons required for large silos, raising costs and complexity
- Incorrect placement leads to persistent dead zones
- High-pressure blasts accelerate wear on vessel linings
Aeration Pads: Wear And Inefficiencies
Aeration pads introduce low-pressure air through vessel walls to reduce friction between powder particles.
Benefits:
- Lowers the effective internal friction angle
- Improves flow by fluidising powders
- Easy to retrofit onto existing vessels
Wear And Inefficiencies:
- Continuous airflow erodes pad surfaces and vessel walls
- Pads clog over time, reducing effectiveness
- High air usage increases operational costs
- Fluidisation is ineffective for coarse, sticky, or damp materials
Modern Solutions To Prevent Ratholing
Acoustic Cleaners: How They Work
Acoustic cleaners emit high-energy sound waves at low frequencies to dislodge material inside vessels.
How They Work:
- Emit sound waves (typically between 60–350 Hz) to break up material
- Sound Waves vibrate particle surfaces, overcoming the cohesive forces between particles
- Continuous or pulsed operation keeps material flowing
Acoustic cleaners: pros and cons
- ✅ Reduces need for manual cleaning
- 🚫 Loud, may exceed noise limits
- 🚫 Maintenance though infrequent is very costly
- 🚫 Can’t be uses for IBCs
AirSweep Technology: How It Works
Flow aid systems use focused bursts of compressed air through nozzles installed inside vessels e.g. AirSweep technology
How AirSweep Works:
- Nozzles embedded into the vessel wall fire short air blasts
- Pulses lift and move stuck material without compaction
- Designed to activate only when needed, reducing air consumption
AirSweep: pros and cons
- ✅Longer nozzle life compared to aeration pads
- 🚫 Very localised effect, multiple nozzles needed
- 🚫 High cost, can’t be uses for IBCs
- 🚫 Requires constant compressed air supply
Cone Valve Technology: How It Works
Cone valve technology uses a moveable cone inside the container outlet to control powder discharge.
How It Works:
- A vertical lift exposes an annular gap for even powder discharge
- Gentle vibration during lift reduces bridging
- Self-clearing design prevents stagnation zones
Cone valve: pros and cons
- ✅ Can be used in IBCs
- 🚫 Almost impossible to clean due to space inside hollow cone
- 🚫 Long drying time when cleaned
- 🚫 Incomplete closing causes mess
| Solution | How It Works | Advantages | Disadvantages | |
|---|---|---|---|---|
| Vibrators | Transmit vibrations through silo walls to dislodge material | Easy to install, low upfront cost | Risk of structural damage, not suitable for thin-walled silos | |
| Air Cannons/Blasters | Release blasts of compressed air to break blockages | Effective on existing blockages, quick action | Limited coverage, expensive to install multiple units | |
| Aeration Pads | Inject air directly into material to promote flow | Inexpensive, simple concept | Prone to wear, requires multiple units, needs frequent replacement | |
| Acoustic Cleaners | Emit sound waves that vibrate and debond compacted materials | Non-intrusive, prevents ratholing, low maintenance | Higher initial investment | |
| Flow Aid Systems (e.g., AirSweep) | Release timed pulses of air along vessel walls | Highly efficient, energy-saving, works on sticky solids | Requires installation at multiple points | |
| Cone Valve Technology | Controls powder flow via a moving cone mechanism | Provides mass flow, reduces segregation, future-proofs systems | Higher cost, limited to specific container types |
How to Choose the Best Ratholing Solution
Factors To Consider: Material Type and Equipment Size
Material properties directly affect the effectiveness of a flow solution. Highly cohesive powders like flour, cement, or pharmaceutical ingredients behave differently compared to free-flowing granules or pellets.
Key considerations include:
- Material Cohesiveness: Sticky or moisture-prone powders need stronger flow interventions like acoustic cleaners or air pulse systems.
- Particle Size and Shape: Fine, irregular particles form tighter bridges and ratholes than coarse, uniform materials.
- Electrostatic Behaviour: Materials that carry static charges may cling to hopper walls, requiring specialised dislodging techniques.
Equipment size also plays a critical role:
- Hopper Dimensions: Large silos demand low-frequency acoustic cleaners for deeper material penetration.
- Outlet Design: Narrow discharge points benefit from precision flow aids like cone valve systems to ensure mass flow.
Always match the solution’s strength and technology to both the material and container dimensions.
Maintenance and Safety Considerations
Maintenance frequency and ease determine operational reliability. Safety concerns also influence the choice of system.
Maintenance aspects to examine:
- Ease of Access: Systems like AirSweep nozzles are accessible for maintenance, while internal vibrators often require disassembly.
- Durability: Components exposed to abrasive materials must resist wear to prevent frequent replacements.
- Energy Use: Pneumatic and acoustic systems typically consume less energy compared to mechanical alternatives like vibrators.
Safety factors to prioritise:
- Non-Intrusive Systems: Acoustic cleaners operate without mechanical impact, reducing risk to structural integrity.
- Air-Only Solutions: Systems powered by compressed air, rather than electricity, lower the risk of sparks and explosions in sensitive environments.
- Reduction of Manual Intervention: Flow aids should eliminate the need for operators to manually dislodge material, preventing injury and contamination.
Cost Versus Long-Term Benefits
Initial equipment costs represent only a portion of the investment. Ongoing performance, reliability, and savings from reduced downtime must guide purchasing decisions.
Key financial points include:
- Installation Costs: Systems like aeration pads may seem low-cost but require multiple units and frequent maintenance.
- Operational Costs: Energy-efficient systems lower operating expenses over time.
- Longevity and Reliability: High-quality systems prevent recurring problems, reducing maintenance interventions and avoiding lost production.
Prioritise solutions that offer preventive capabilities rather than reactive fixes.
Summary
Ratholing isn’t just a minor inconvenience—it’s a serious obstacle to efficient, safe, and profitable production. Understanding its causes and consequences is the first step; investing in the right solutions is the next. Modern flow aids and preventive technologies can make a significant difference. Improve Your Material Flow Today and discover how you can eliminate ratholing for good and keep your operations running smoothly.
About S3 Process
S3 Process is a UK-based specialist in process equipment for the food, beverage, chemical, nutraceutical, battery, and pharmaceutical manufacturing sectors. Our directors bring a combined 60 years of international application experience, working with manufacturers worldwide to solve complex process challenges and improve operational efficiency.
Ratholing FAQs
Why Does Ratholing Keep Occurring Despite Installing Flow Aids?
Ratholing can persist even with flow aids due to a mismatch between equipment and material properties.
- Material Cohesion: Highly cohesive materials, such as fine powders, tend to cling to surfaces, forming ratholes even if flow aids like vibration systems are installed.
- Improper Hopper Design: The hopper angle may not be steep enough to promote mass flow. Adjusting the hopper design is often necessary.
- Flow Aid Insufficiency: Air cannons or vibrators may not be optimally positioned or calibrated. Re-assessing flow aid positioning and power is crucial.
How to Prevent Bridging in High-Humidity Environments?
High humidity can increase material cohesion, leading to bridging.
- Pre-treat Materials: Use dehumidifiers or pre-treat materials to reduce moisture content.
- Use of Flow Aids: Air blasters or fluidisation systems can counteract the increased stickiness caused by humidity.
- Storage Design Adjustments: Ensure that hoppers are designed to prevent moisture build-up and include a steep enough angle for proper flow.
How Do I Minimise Production Downtime Caused by Ratholing or Bridging?
To minimise downtime, it’s important to address the root cause of ratholing and develop a proactive maintenance plan.
- Preventive Maintenance: Regular inspection of flow aids and hoppers helps identify issues early.
- Material Monitoring: Real-time flow sensors can detect ratholing before it leads to a complete blockage, allowing operators to address it without a full shutdown.
- Optimised Bin Design: Implementing mass flow bin designs ensures consistent material movement, reducing blockages.
How Can I Ensure My Flow Aid Devices Are Operating at Maximum Efficiency?
Flow aids may not always function at full capacity due to misalignment, wear, or improper calibration.
- Routine Calibration: Regularly check the calibration of air cannons or vibrators to ensure they’re delivering the right force to move the material.
- Check for Wear: Over time, devices like vibrators can experience wear, reducing their effectiveness. Maintenance and timely replacement can prevent failures.
- Proper Positioning: Devices need to be strategically placed to target areas prone to blockages, such as near the outlet or high-cohesion zones.
Further Reading and Research
Recommended Articles
-
Ensuring Effective Material Flow from Bulk Solids Storage Vessels – Covers the differences between funnel flow and mass flow, and explains how design choices influence ratholing risk.
Suggested Books
-
Bulk Solids Handling: An Introduction to the Practice and Technology by C. R. Woodcock and J. S. Mason (Springer, 1987)
-
A practical reference that covers storage bin design, flow properties, mechanical conveying and dust control. Useful for process, design and maintenance engineers.
-
-
Powders and Bulk Solids: Behavior, Characterization, Storage and Flow (2nd Edition, Dietmar Schulze, Springer 2021)
-
Offers detailed explanations of powder properties, measurement techniques, silo design, feeder systems and discharge behaviours. Includes worked examples.
-
Recommended Websites
-
Bulk Solids Portal – Silo Design: Avoiding Flow Problems Through Adequate Design
-
Explains flow patterns (mass versus funnel), with advantages and limitations of each, including tips on avoiding ratholing and segregation.
-