Powders and granules cause more trouble in chemical plants than almost any other part of the process. Silos stop discharging, hoppers form arches, feeders drift, pneumatic lines plug and screw conveyors jam. A classic RAND Corporation study of solids-processing plants found they reached only about two thirds of design capacity in their first year, mostly because of mechanical and physical problems rather than chemistry. Most of those problems give warning in data that plants already collect: discharge rates, feeder load cells, line pressures and motor currents. This guide covers how bulk solids flow, why bridging and ratholing happen, how pneumatic conveying fails, what limits feeder accuracy and how continuous monitoring catches problems before they stop production. To see your solids handling data analyzed, book a short walkthrough.
Solids Handling and Conveying in Chemical Plants: Prevent Bridging, Ratholing and Line Chokes
Silos, hoppers, feeders and conveying lines monitored from the signals you already have, so flow problems are seen while they are forming, not after the line stops.
Why Solids Handling Limits Plant Capacity
The best-known evidence comes from a RAND Corporation study led by Edward Merrow. Reviewing new plants, it found that those processing solids operated at only about 64% of design capacity in their first year, against 90–95% for plants handling only liquids and gases. A later account in Powder and Bulk Solids summarized it more starkly: two thirds of the solids plants operated below 80% of nameplate at the end of their first year, and a quarter failed to reach 40%.
RAND’s own summary is that the key problems were mechanical and physical, not chemical. Among the plants studied, 52% reported solids transfer problems, 48% mechanical equipment failures and 45% plugging of equipment by solids. Those numbers are decades old, but anyone who has worked in a powder plant will recognize them.
The problems persist after start-up. Raw material changes, humidity, temperature and wear all change how solids flow. A silo that discharged well with one supplier’s material may rathole with another’s. A conveying line that ran for years may start plugging after a blower loses efficiency.
Monitoring makes those changes visible early. We can review your solids handling data on a call.
Mass Flow, Funnel Flow and Why It Matters
Jenike and Johanson, whose methods underpin most bin and hopper design, describe two basic flow patterns.
- Only a central channel moves
- Material near walls stays stagnant
- Ratholing likely with cohesive powders
- Last-in, first-out flow
- Segregation enhanced
- Fine powders can flood when channels collapse
- All material moves when any is withdrawn
- No stagnant zones near walls
- Ratholing not possible
- First-in, first-out flow
- Segregation minimized
- Powders deaerate, reducing flooding
Whether a hopper achieves mass flow depends on wall angle, wall friction and the material’s properties. Powder and Bulk Solids explains that flow reports use measured wall friction to set the hopper angle from vertical needed for mass flow, and that the angle varies with wall material.
Many existing silos are funnel-flow by design or by accident, for example after a liner wears or a new material is introduced. Monitoring helps because funnel-flow problems show recognizable patterns in level and discharge data.
Knowing which flow pattern each silo has is the starting point. Our engineers can review your vessel list.
Arching, Ratholing and Their Data Signatures
The two most common silo problems have different causes and different signals.
Large particles lock together across the outlet. More common with coarse, irregular material.
Moisture, fines, particle shape or temperature bond particles into a stable arch.
Only material above the outlet flows; the rest stays in place and may cake.
Discharge rate falls to zero while level stays constant; feeder starves suddenly.
Discharge continues but level falls only in a narrow zone, then flow stops when the channel empties.
A rathole collapses and aerated powder floods the feeder, causing a surge.
Jenike shear testing measures the properties that cause these problems. The flow function relates a material’s cohesive strength to the consolidation pressure it has seen, and from it engineers calculate the minimum outlet size to prevent arching and the critical rathole diameter.
In operation, combining level readings, discharge rate and feeder load-cell data shows when a silo is moving toward one of these states. Storage time matters too: many cohesive materials gain strength the longer they sit, so a silo that discharges well after an hour may arch after a weekend.
Linking flow problems to material lots and storage time often reveals the cause. See it in a demo.
Pneumatic Conveying: Dilute Phase, Dense Phase and Plugging
Pneumatic conveying moves powders through pipes with air or nitrogen. The two main modes behave very differently.
| Mode | Typical gas velocity | How solids move | Main risks |
|---|---|---|---|
| Dilute phase | Above about 20 m/s (over 4,000 ft/min) | Fully suspended in the gas | Wear, particle breakage, high energy use |
| Dense phase | About 1–3 m/s at pickup (200–600 ft/min) | Slugs or plugs pushed below saltation velocity | Plugging if pressure or gas flow is wrong |
| Saltation | Velocity at which solids fall out in horizontal pipe | Particles settle on the pipe bottom | Gradual line blockage |
| Choking | Velocity at which a vertical line blocks | Solids accumulate in risers | Sudden blockage |
These figures come from Processing Magazine’s overview of pneumatic conveying; academic references give similar values, such as a minimum of about 15 m/s for dilute phase. The safe operating window for any given material and line comes from testing and design.
Plugging gives warning. Line pressure rises and becomes erratic, blower or compressor load changes and cycle times in batch transporters lengthen. Tracking these signals against normal patterns lets operators intervene before a full plug, which often takes hours to clear.
Most conveying systems already measure pressure and flow. We turn them into plugging warnings in every rollout.
Feeder Accuracy and What Disturbs It
Loss-in-weight and gravimetric feeders set the ratio of ingredients in continuous processes. Their accuracy decides product quality.
Illustrative. Separating refill periods from normal running shows where the variation comes from.
Coperion describes high accuracy for loss-in-weight feeders as 0.1–0.5% at two sigma, based on 60-second samples. Its technical paper lists the main sources of error: vibration, hopper refill with its change in bulk density, pressure pulses lifting the hopper and density changes as the hopper level falls.
Each error source leaves a signature. Refill errors line up with refill events; vibration errors line up with nearby equipment; pressure pulses line up with downstream process changes. Monitoring that aligns feeder data with these events points straight to the fix.
Feeder accuracy problems are often blamed on the feeder when the cause is upstream. Ask our team to check yours.
Combustible Dust and Safe Operation
Many chemical powders are combustible. Solids handling monitoring must support, not replace, the plant’s dust safety program.
Monitoring helps by flagging conditions that raise risk, such as dust collector differential pressure falling sharply, which can indicate a torn filter, or temperatures rising in dryers and silos. All safety functions stay in the plant’s certified systems.
Our specialists can map which existing signals support your dust hazard analysis.
Solids Handling Checklist
Use this checklist to set up monitoring across a solids handling system.
Linking material lots to flow events is often the most revealing step. Start it in a solids review.
What Better Solids Handling Is Worth
Solids problems cost capacity, quality and labor.
The RAND findings show how much capacity solids problems can take from a plant. Even a mature plant typically loses hours each month to flow problems, and each intervention carries safety risk as well as lost output.
A review of last quarter’s solids handling stops usually shows the largest causes. Book one with our advisors.
How iFactory Delivers Solids Flow Monitoring
Arching and ratholing signatures detected.
Line pressure and gas flow patterns watched.
Two-sigma accuracy by feeder and by refill state.
Flow events tied to supplier, lot and storage time.
Screw, rotary valve and blower condition tracked.
Dust collector and temperature anomalies flagged.
It runs on premises and reads your existing PLC and historian data. Share a month of solids handling data and we will show your flow events in a working session.
Find the Flow Problems Costing You Capacity
Share level, feeder and conveying data for a month. We find arching, ratholing, plugging and feeder drift events and link them to materials and conditions.
Discharge rate is falling while the level reading barely moves. That pattern points to a rathole: only a central channel is flowing and the rest of the silo is stagnant.
A Rathole Spotted Before the Feeder Starved
This exchange shows how a solids process engineer might use iFactory.
iFactory ships as a pre-configured NVIDIA AI server, racked and ready with the solids flow monitoring models loaded. Rack it, plug in power and Ethernet, and the AI is live on your network. Our scope covers data connections across silos, feeders, conveying lines and dust collection, DCS, PLC/SCADA, historian, LIMS and CMMS integration, cabling and network setup, operator and engineer training, and 24×7 remote monitoring. Recommendations run in advisory mode first, and nothing writes to your control system without your management of change approval.
Server installed, DCS and historian links live, historical process, lab and maintenance data loaded.
Models calibrated on your own unit data, then run in advisory mode on one unit with your process engineers reviewing every recommendation.
Rollout to the agreed units under your management of change, operator and engineer training, and 24×7 remote monitoring in place.
Software, server and integration come as one package. For pricing on your site, contact our sales team.
Frequently Asked Questions
A RAND study found solids plants averaged about 64% of design capacity in their first year, against 90–95% for liquid and gas plants, mainly because of mechanical and physical problems such as solids transfer and plugging.
Arching is a stable arch over the outlet that stops flow. Ratholing is a stable empty channel above the outlet, with the material around it not moving.
A flow pattern in which all material moves whenever any is withdrawn, avoiding stagnant zones and ratholes. It depends on hopper angle, wall friction and material properties.
Gas velocity falling below what the solids need, often because of blower wear, leaks, higher solids loading or changes in the material.
Coperion defines high accuracy as 0.1–0.5% at two sigma on 60-second samples. Refill, vibration, pressure pulses and density changes are the main error sources.
A solids handling area can typically be monitored within a 6–12 week rollout, using existing signals. Plan it with our engineers.
Keep Powders Moving Through Your Plant
iFactory recognizes arching, ratholing, plugging and feeder drift as they form, links them to materials and conditions and helps your team act before production stops.
Illustrative. Lower is better. F-3 drifts during hopper refill, which points to refill and density effects rather than the screw.
