Why material stops flowing: bridging, ratholing and mass flow

A silo that will not empty is rarely a silo that is broken. It is a silo whose geometry lets the material support itself. The hammer marks around the outlet of most old hoppers are the record of a design problem being treated as an operating one. This note explains what is happening inside, which pattern of flow you have, and what actually changes it.

What is happening inside a silo that won't discharge?

Arching, or bridging

Cohesive material can form a self-supporting arch across the outlet. The arch carries the weight of the material above it into the hopper walls, and the outlet — open, with nothing physically blocking it — sees no material at all. Arches form more easily the smaller the outlet, the more cohesive the material and the longer it has sat under load. Wet fines, fibrous biomass and anything that cakes are the usual candidates; coarse, dry, free-flowing granules almost never arch unless a lump jams the opening (a mechanical, not a cohesive, arch).

Ratholing, or piping

In a funnel-flow silo, only a channel of material above the outlet moves; the rest stays put against the walls. If that stationary material is strong enough to stand on its own, the channel empties and a vertical pipe remains — a rathole. The silo then reports "empty" from the feeder's point of view while holding most of its contents. Ratholes are the reason the live capacity of a funnel-flow silo can be a fraction of its geometric volume, and why old material comes out long after it should.

Flushing and flooding

The opposite failure. A fine powder that has been aerated — by pneumatic filling, or by a rathole collapsing into a void — behaves like a liquid for a while and runs uncontrolled through the outlet and past the feeder. Flushing is a flow-rate problem rather than a no-flow problem, but it comes from the same source: material that does not deaerate because it is not moving steadily.

Segregation

When material is dropped into a silo, coarse particles roll to the walls and fines stay in the centre. In funnel flow the centre discharges first, so the process receives fines, then coarse, in a cycle that follows the filling. Mass flow largely removes this by discharging the cross-section together; it is one of the strongest reasons to choose it for blended materials like glass batch or fertiliser mixes.

Mass flow or funnel flow: which pattern do you have?

Neither pattern is wrong in itself. A funnel-flow silo holding dry aggregate is a cheap, sensible silo. The problems start when funnel flow is used for material that consolidates, spoils, segregates or cakes — and that is most of what the process industries store.

What decides the flow pattern?

Hopper half-angle

The angle of the hopper wall from vertical. Steeper favours mass flow. The critical angle is not a fixed number; it depends on the wall friction of this material on this wall surface.

Wall friction

The friction between the material and the actual wall — mill-finish steel, polished stainless, a polymer liner, painted plate. A liner can turn a funnel-flow hopper into a mass-flow one at the same angle; corrosion and build-up can do the reverse.

Outlet dimension

For flow to start, the outlet must be larger than the critical arching dimension for the material at its worst (wettest, most consolidated) state. For a slot outlet the width matters; for a circular one the diameter, and circular outlets need to be substantially larger for the same material.

Cohesion and time consolidation

The strength the material gains under its own weight, and how much more it gains after a weekend at rest. This is what a shear test measures, and what no table of "angle of repose" values can give you.

Hopper shape

Conical hoppers need steeper walls than wedge (plane-flow) hoppers with a slot outlet to achieve mass flow; the slot also allows a smaller minimum outlet width.

The feeder

A feeder that draws from only part of the outlet turns a mass-flow hopper into a funnel-flow one in practice. See below.

How do engineers size the outlet?

The reliable route is measurement. A shear tester determines the material's flow function — how strong it becomes under a given consolidating stress — and its wall friction on candidate surfaces. From those, the standard hopper-design method gives the minimum outlet dimension that will not arch and the wall angle that will give mass flow, with a margin. The test takes a small sample and a few days; the alternative is the plant's own experience, gathered one blockage at a time.

Why the feeder matters as much as the hopper

The feeder under the outlet decides which part of the outlet is actually live. A constant-pitch screw fills at its back end and simply carries that material forward; the front of the slot never draws, the hopper above it stagnates, and the mass-flow hopper you paid for behaves as funnel flow. The same happens with a belt feeder whose interface does not increase its capacity along the outlet.

• Screw feeder — variable pitch or tapered shaft so capacity increases along the slot and the whole outlet draws.
• Belt feeder — a tapered, stepped interface (hopper outlet widening in the direction of belt travel) so the belt picks up material along the whole length.
• Rotary valve — draws evenly across a round outlet but limits the outlet size; usually paired with a small hopper or a bin activator on fine powders.
• Vibratory feeder — good for coarse, free-flowing materials; less predictable on cohesive ones.
• Gates — a slide gate is for isolation, not for rate control; throttling with it makes the live outlet smaller and brings arching back.

When do vibrators, air cannons and fluidisation help?

Flow aids are legitimate tools for the right problem. Air cannons and vibrators break an arch that has already formed; fluidising pads and aeration keep fine powders from consolidating in the cone; bin activators give a small live outlet a larger effective one. What they do not do is turn a funnel-flow hopper into a mass-flow one, and a vibrator run continuously on a cohesive material usually compacts it and makes the arch stronger. The note on safety switches and flow aids goes through each device; the design rule is to use them to recover from an upset, not to make an undersized outlet work every day.

Options when the silo already exists

Most silos we are asked about are already standing, so the question is what can be changed inside the existing shell. In rough order of cost:

1. Change the feeder so the whole outlet is live — often the highest return for the least steel.
2. Line the hopper with a low-friction material to lower the wall friction and move the pattern towards mass flow.
3. Enlarge the outlet, which may mean a new hopper section and a new feeder; check the arching dimension first so it is enlarged enough.
4. Fit an insert (a cone-in-cone or an inverted cone) that changes the stress field in the hopper and enlarges the flowing region.
5. Replace the hopper with a steeper mass-flow section below the existing cylinder; height and headroom decide whether this is possible.
6. Add flow aids as a recovery measure alongside any of the above.

Which of these applies is a small study — a sample, a shear test, a survey of the existing outlet and feeder — rather than a catalogue choice. It is exactly what our storage and flow control engineering covers, and the note on what to send before asking for a quote lists what makes that study quick.