Understanding ratholes

April 1, 2005
Diamondback Technology supplies ChemicalProcessing.com with this exclusive piece on rathole formation in plant hoppers.

A rathole is a circular, cylinder-shaped hole that develops in a mass of bulk solids and, left undisturbed, can remain to form an integral part of the bin structure.  A rathole usually forms above a bin outlet in that portion of a hopper that is not steep enough to maintain flow at the walls.

Ratholes can be costly.  In addition to robbing vital live-bin capacity, a rathole can suddenly collapse with such force that it can rip the bin structure.  At the very least, a collapsing rathole can severely compact material that leads to arching, a condition discussed in the [month] issue of Chemical Processing.  In addition, ratholes often require poke bars or sledge hammers to dislodge them, which can further damage a bin’s structure.

Flow and potential rathole formation in conical hoppers

Ratholes develop most often in conical hoppers that are not steep enough to produce flow at the walls.  Generally, flow is limited to a central flow pattern.  If the material is even slightly cohesive, material may stick to the walls and eventually become part of the bin structure.

Rathole stability is affected by the material’s strength and by the hopper’s outlet diameter.  If the outlet diameter exceeds the critical rathole diameter (or the diameter at which the material is no longer stable), material along the hopper walls peels off the top of the hopper and fills the central region as the bin level drops.  When the level at the center drops below the knuckle between the vertical cylinder and the cone, flow may occur at the hopper walls.  However, if the hopper outlet is large relative to the bin diameter, uniform flow may occur along hopper walls only when the bin level is high.  If the material level drops and the vertical pressures diminish, dead regions can form on the sides.

Flow and potential rathole formation in flow outlet hoppers

A slot outlet extending to the entire diameter of a cylindrical bin converges flow in one direction only.  If the material level in the hopper is one diameter or greater above the outlet, flow expands to the vertical walls at a very flat angle.  This pattern occurs even when the hopper walls are rough of if the material flows on itself.  As the solids level within the bin drops, the flow channel narrows and the non-flowing region extends up the side walls.  Decreasing the material level at the center of the hopper may again expand flow to the cylinder walls.

If the slot outlet does not extend across the entire bin diameter, flow will not expand to the walls.  As a result, flow converges both at the side walls and end walls.  This two-dimensional convergence occurs at very steep angles and, as a result, creates a flow channel with a diameter slightly larger than the diagonal of the outlet.  If this expanded flow channel diameter exceeds the critical rathole diameter, material will slough off into the central flow channel and empty the entire bin without hangups.  On the other hand, if the material’s critical diameter is larger than the flow channel, the non-flowing material at the sides form a stable rathole and the bin’s capacity is then limited to that central flow channel.

Flow and potential rathole formation in pyramidal hoppers

Pyramidal hoppers are popular due to their simple, flat-plate construction.  Funnel-flow patterns can develop in a pyramidal hopper with a slot outlet if the slot length is greater than the critical rathole diameter and the width is greater than 40% of the length.  These hoppers also tend to promote stagnant, non-flowing material in the valleys where the angle is shallower, which can result in spoiled material.

Flow and potential rathole formation in Diamondback Hoppers

A fourth hopper design, a Diamondback Hopper, combines the circular outlet of a conical hopper with the flow expansion and rathole-breaking capabilities of a long slot.  The Diamondback converges in one direction only (plane strain) and the walls, which are 90-degrees from the converging direction, are vertical or slightly diverging.  This geometry offers minimal arch or rathole support.  The circular cross-sections have no sharp corners that can trap solids.  Because the converging walls take all the support, the Diamondback works effectively with flatter walls while retaining the same arch- and rathole-breaking ability as a long slot.

As with the flow hopper, the top diameter of an Arch-breaking Diamondback Hopper must exceed the critical rathole diameter to empty the entire bin without hangups.


Flow patterns in conical, slot-outlet, pyramidal and Diamondback Hoppers illustrate the importance of hopper design in preventing flow problems. 

If headroom allows, a conical hopper that is steep enough to create flow at the walls will reduce rathole potential. 

Likewise, a slot-outlet hopper than converges material in one-dimension only and extends flow to the hopper walls will prevent ratholes and will save headroom because the hopper requires flatter angles than required when material flows in two dimensions. Usually, slot-outlet hoppers require special screw designs with variable pitch and shaft diameters along the entire slot length to process material effectively. 

Pyramidal hoppers often require substantial structural reinforcement of the flat sides that can increase the cost of fabrication, but can be effective if there are no issues with material spoilage from stagnant, non-flowing areas in the valleys.

Because of its complex design geometry, a Diamondback Hopper will be more expensive than conical or pyramidal hoppers, but because of its circular outlet, it can use a low-power feeder, making it significantly less expensive than a long slot-outlet hopper that requites a custom screw design.

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