What Is Ratholing?
Ratholing—also called piping—occurs in funnel-flow silos handling cohesive bulk materials. In funnel flow, discharge draws material only from a narrow channel directly above the outlet, while the material along the walls stands still. When the bulk solid is cohesive enough to support its own weight, that flow channel simply empties out and leaves behind a stable, roughly vertical cavity: the rathole. The silo may still hold hundreds of tonnes, but its live capacity has collapsed to the volume of that pipe.
Ratholing costs more than lost storage:
- Erratic or interrupted feed. The process downstream starves while inventory sits in plain sight.
- Shrinking usable volume. Each fill/discharge cycle only moves the same narrow core; the rest of the material ages in place.
- Collapse events. A rathole that finally caves in can flood fine powder through the outlet uncontrollably, slam the structure with shock loads, or instantly form a bridge of compacted lumps.
- Structural risk. Off-center flow channels and collapse loads create eccentric wall pressures most silos were never designed for.
- Time makes it worse. The longer stagnant material consolidates, the stronger the rathole wall becomes—yesterday’s sluggish silo is next month’s concrete-hard annulus.
What Causes Ratholing in a Silo?
Two conditions have to be present at the same time:
- A funnel-flow pattern. The hopper walls are too shallow or too rough for material to slide along them, so only the core above the outlet moves. Whether a silo flows in mass flow or funnel flow is determined by wall friction, hopper angle, and outlet size—not by the material alone.
- Cohesive strength. The material can gain enough strength, under the consolidation pressure of its own weight, to support an open vertical channel. Free-flowing materials such as dry sand or plastic pellets rarely rathole; cohesive powders such as fly ash, cement raw meal, hydrated lime, FGD gypsum, or moist coal fines almost always will—if stored in a funnel-flow silo.
Several factors push a marginal silo over the edge: moisture (even 1–3% can multiply cohesive strength in fine powders), high fines content, time consolidation during weekend or seasonal shutdowns, hygroscopic caking, temperature cycles that migrate moisture, and the higher consolidation pressures of tall, large-diameter silos.
If you want the mechanics behind flow patterns, see our upcoming guide on mass flow vs funnel flow; the short version is that the flow pattern is a property of the silo, and the rathole is what a cohesive material does with a funnel-flow silo.
Rathole or Bridge? How to Tell the Difference
Both problems stop discharge, but they form in different places and respond to different fixes—treating one as the other wastes money and can be dangerous.
| Rathole (piping) | Bridge (arching) | |
| Where it forms | Vertical channel above the outlet, through the bin’s core | Across the outlet or hopper cone |
| How flow stops | Gradually declines as the channel empties, then stops with the level still high | Abruptly—flow cuts off at once |
| What a probe finds | Rod drops through an open pipe; material refusal at the annulus | Solid obstruction just above the outlet |
| Typical materials | Cohesive fine powders | Cohesive fines (cohesive arch) or coarse lumps (mechanical interlocking arch) |
| Collapse risk | Flooding of aerated powder, shock loads, eccentric wall loads | Sudden slug load on feeder and structure |
A level indicator that reads high while the feeder runs empty is the classic rathole signature. An abrupt stop with normal level readings usually points to an arch. (Full symptom trees for both are on our silo blockage solutions page.)
How to Prevent Ratholing: Three Levels of Fixes
The options below are ranked by permanence and cost. Most plants land on Level 2 because it retrofits without downtime; Level 1 is the engineering cure; Level 3 costs nothing and helps every option work better.
Level 1 — Redesign for Mass Flow (most permanent, highest cost)
The root-cause fix is converting the silo to mass flow so no stagnant zone exists: steeper hopper angles, low-friction liners (UHMW-PE or 2B-finish stainless), a larger outlet, or engineered inserts that force flow along the walls. The outlet must also be larger than the material’s critical rathole diameter—a property measured in a shear-cell flow test, not guessed. Choose this path for new builds and major retrofits; on an operating silo, the downtime and structural work often make it a last resort.
Level 2 — Flow Aids on the Stagnant Zone (best retrofit value)
If the flow pattern can’t be changed, the stagnant annulus has to be kept moving. Air cannons are the standard tool for cohesive ratholes: nozzles mounted through the wall release a millisecond blast of compressed air that shears incipient buildup off the flow channel before it consolidates into a stable pipe. Three rules make them work as prevention rather than emergency response: start the bottom ring just above the hopper transition where the rathole wall is strongest, work upward in a spiral pattern, and—most importantly—fire on a timed sequence during discharge instead of waiting for a blockage. A sequence controller that fires little and often uses less compressed air than one heroic blast into a hardened rathole.
A caution on industrial vibrators: on genuinely cohesive materials, vibration can compact the annulus and make a rathole stronger. Vibrators earn their keep on arch-prone but less cohesive materials and on curved bin walls; for established ratholing in fine cohesive powders, air cannons are the safer default. (Our air cannon vs vibrator comparison covers the decision in detail.)
For light, dusty buildup—fly ash hoppers, filter hoppers—acoustic cleaners can keep walls clean continuously and pair well with air cannons.
Level 3 — Operating Practices (free, but limited)
Good housekeeping will not cure a funnel-flow silo, but it widens every margin: run the silo first-in-first-out and avoid storing cohesive material untouched over shutdowns; schedule complete empty-outs so consolidation never gets a long run; control moisture upstream (covered storage, dry compressed air); and if the silo has multiple outlets, rotate them so no zone stagnates permanently.
FAQ
Nova, Flow Applications Engineer at DEVONDI.