Glossary
Hoppers and silos
Bridging (bulk-solids)
Also known as arching, arch formation, hopper bridging, silo bridging.
Bridging (also arching or arch formation) is the formation of a stable mechanical arch of bulk-solid material above the discharge outlet of a hopper or silo. Once a bridge forms, no material flows out of the outlet even though the vessel above is full. Bridging is the universal failure mode of bulk-solids storage.
How a bridge forms
Cohesive forces between particles — moisture films, electrostatic charge, chemical bonding — combine with the converging-flow geometry to lock particles into an arch shape. The arch is self-supporting against the load above. Cohesion increases with:
- Fine particle size (especially Geldart-C powders — see Geldart classification)
- Moisture
- Hygroscopic chemistry (urea, ammonium nitrate, lime)
- Long residence time (consolidation under sustained load)
- Temperature cycling
Diagnosing a bridge
- Outlet flow stops while the level above remains high
- Mass-flow indicators report no movement
- A simple tap on the hopper outside the discharge cone produces a hollow sound
- Borescope inspection from the inlet shows the arch directly
Remedies
| Technique | Notes |
|---|---|
| Sonic horn | Continuous prevention; non-contact; minimal infrastructure |
| Air cannon | High-intensity periodic; effective on hard bridges; structural stress |
| Bin vibrator | Continuous vibration; can compact powder further if poorly sized |
| Whip hammer | Manual; legacy; HSE concerns |
| Fluidisation pad | Aerates the lower vessel; not suitable for wet material |
| Mechanical screw extractor | Bypasses the bridge entirely; high capex |
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.
- Rat-holingRat-holing is a flow pattern in which material discharges through a narrow vertical channel above the outlet, while the surrounding material remains stagnant and consolidates.
- 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.