Glossary
Hoppers and silos
Rat-holing
Also known as rat holing, rathole, piping (silos).
Rat-holing is a bulk-solids flow pattern in which material discharges through a narrow vertical channel directly above the hopper or silo outlet, while the surrounding material remains stagnant and progressively consolidates. The result is a funnel-flow condition gone to the extreme: most of the silo contents never move.
Why rat-holing matters
- Effective storage volume collapses — only the narrow flowing column is usable
- Stagnant material consolidates and ages — eventually hardens beyond recovery without manual cleanout
- First-in, last-out becomes never-out — older material is trapped indefinitely
- Catastrophic collapse risk — when the rat-hole eventually breaks open under load it can release tonnes of compacted material suddenly into downstream equipment
Causes
- Narrow outlet relative to silo diameter
- Steep but insufficiently steep cone angle
- Cohesive material below its mass-flow threshold
- Failure of a discharge aid (vibrator, aeration) that previously prevented funnel flow
Prevention
The structural remedy is to redesign the cone for mass flow — steeper angle, larger outlet, smoother wall finish. Where that is not feasible, sonic horns mounted on the cone wall continuously vibrate the stagnant material and break the rat-hole pattern, restoring closer-to-mass-flow behaviour.
Related terms
Related terms
- HopperA hopper is an inverted-pyramid or conical vessel for storing and discharging bulk solids. Bridging and rat-holing are the universal failure modes; sonic horns are a clean, low-maintenance remedy.
- SiloA silo is a large vertical bulk-solids storage vessel. Cement, fly-ash, lime, biomass, fertilizer and food-powder silos all bridge and rat-hole; sonic horns are the leading flow aid.
- Bridging (bulk-solids)Bridging (also arching) is the formation of a stable arch of bulk solids above the discharge outlet of a hopper or silo, stopping material flow. The universal failure mode of bulk-solids storage.
- Mass flow and funnel flowMass flow is first-in-first-out: all material moves uniformly. Funnel flow is first-in-last-out: a central column moves while surrounding material stagnates.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.