Bridging, hang-ups and rat-holes: what actually breaks them

Material stops leaving a bunker for two different reasons, and they are cured differently. An arch is a bridge standing over the outlet and carrying the whole column above it. A rat-hole is a narrow channel through which only the material directly above the outlet leaves, while the rest stands against the walls. There is no universal remedy: what breaks an arch can compact the material around a rat-hole and make things worse.

Two different failures, not one

Jenike & Johanson, an engineering firm that has done nothing else for half a century, defines them as briefly as it gets. Arching (bridging) is when “an arch-shaped obstruction forms above the hopper outlet and stops flow”. Ratholing is when “discharge takes place only in a flow channel located above the outlet”.

The difference is practical. With an arch nothing comes out at all: the full volume sits above, the outlet is empty. With a rat-hole discharge continues but is partial: a tenth of the contents leaves, the scale readings disagree with what the operator sees, and material against the walls stands for weeks and cakes. The first is noticed within the shift. The second is discovered a month later, when the whole vessel has to be cleaned out.

Why the sledgehammer is not a method

The standard answer to a hang-up is a sledgehammer. Joseph Marinelli, a bulk solids specialist, puts the state of affairs plainly: gravity flow is desired, “however, flow aid devices, such as sledgehammers, vibrators, and air blasters, may be required to assist gravity”.

The operative word is assist. A sledgehammer assists exactly once and leaves a mark: Jenike & Johanson have a name for it — bin rash, the dents left by years of hammering. A dent changes the wall geometry for the worse: material clings to it more readily than to smooth steel, and the next arch forms sooner. Every blow makes tomorrow's problem slightly worse than today's.

Why vibration sometimes makes it worse

With fine, cohesive materials — cement, lime, chalk, concentrates — continuous vibration may not help but harm: it drives air out of the mass, the material loses mobility and packs tighter than before. Silo discharge handbooks state this as a plain limitation: vibrating bottoms are “not to be used with powder that have a high compressibility index”, and air injection “should only be used during the discharge to avoid further consolidation of the powder”.

Hence a simple rule. Continuous vibration “just in case” is not prevention, it is a way of tamping down the contents. The action has to be short, strong and applied only when flow has actually stopped.

What this means in practice: a weak permanent vibrator on cohesive material is the worst option of all — it fails to break the arch and compacts the mass while doing so. Rare powerful impacts on a signal beat continuous humming.

A person inside the vessel is not an option

The most common way to clear a hang-up is to send someone in to break it from above. That is exactly the case the American bulk storage standard prohibits in a clause of its own. OSHA 1910.272(g)(6): “Employees shall not enter bins, silos, or tanks underneath a bridging condition, or where a buildup of grain products on the sides could fall and bury them.”

The same standard covers walking on the material to stir it: “walking down grain” and similar practices, where an employee walks on the material to make it flow or is on moving material, are prohibited (1910.272(g)(1)(iv)).

The wording is that hard for a reason. An arch collapses instantly and completely, with the person standing on top of it. Europe has its own confined-space rules, but the physics does not depend on jurisdiction.

What actually breaks a hang-up

Impulse works: a short powerful blow on the wall at the point where the arch stands, then a pause. That is how the Rockhammer RH-200 impactor is built: up to 280 J per impact, a series on command rather than permanent humming. It is welded on at the critical points of steel bunkers, silos, transfer chutes and conveyor troughs.

The second half of the solution is automation. Thirty-two preset programs and an input for a blockage sensor: the system sees the flow stop by itself, runs a series of impacts and restores discharge. No standby man with a sledgehammer — and he is the source of both the dents and the injuries.

Power supply 230 V, consumption up to 160 W, air at 7 bar, 7 impacts per second, unit mass 70 kg, up to two units per control box.

An honest limit: the RH-200 mounts on steel structures only — it is not used on concrete silos. Because of its power we always assess the structure strength and wall thickness before installation: that is part of the engineering selection, not a formality.

When the problem is the geometry, not the impactor

If an arch forms every shift on the same material, the cause is most likely the outlet size. It can be calculated: the critical diameter at which neither an arch nor a rat-hole forms follows from the cohesive strength of the material, its bulk density and the cone geometry — the classic Jenike method. An impactor in that situation will work honestly every hour and mask a design error.

So the question “what do we hit it with” properly comes after the question “why does it stop”. If the material has changed — wetter, finer, caking when it stands — that is one task. If the vessel has never released its contents since commissioning, that is another one, and it belongs to the designer.

How to tell what is happening in your case