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Solve or Prevent Poor Flow

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Overview

What is Poor Flow?

Two costly flow problems experienced in a silo, bin, or hopper are arching and ratholing.

Arching (bridging) occurs when an arch-shaped obstruction forms above the hopper outlet and stops flow. It can be an interlocking arch, where large particles mechanically interlock to form an obstruction, or a cohesive arch. A cohesive arch occurs when particles bond together due to effects of moisture, fines concentration, particle shape, temperature, etc.

Ratholing occurs when discharge takes place only in a flow channel located above the outlet. If the material being handled is cohesive, the material outside of this channel will not flow into it and may cake or agglomerate. Once the central flow channel has emptied, all flow from the silo stops.

A rathole has formed in a stockpile.

A rathole within a hopper that for sawdust.

Hammering against a hopper to get built-up material to flow again

A hammer is used in an attempt to get built-up material within a hopper to flow again.

Damage from using a hammer on this vessel. Commonly called "bin rash".

Funnel flow is a flow pattern where solids flow in a channel formed within stagnant material

Funnel flow is a flow pattern in which solids flow in a channel formed within stagnant material.

Mass flow is a flow pattern in which all solid in a bin is in motion whenever any of it is withdrawn.

Expanded flow discharge to prevent ratholing

Expanded flow discharge can be used to prevent ratholing.

Challenge

Why is Poor Flow a Problem?

Limited “live” capacity
Segregation of fines or dust
Idle equipment or process
Erratic or stopped flow

Powder flooding or flushing
Structure vibrations, quaking
Missed shipments or delays
Caking, agglomeration, wasted material

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Factors

When & Where Can Poor Flow Occur?

These problems routinely occur in silos discharging in a funnel flow pattern, in which some material moves while the rest remains stationary. Funnel flow occurs when the sloping hopper walls of a silo are not steep enough and sufficiently low in friction for material to flow along them. Particles slide on themselves rather than the hopper walls.

Funnel flow:

Ratholing, if material is cohesive
Caking and material spoilage
Flooding of fine powders
First-in, last-out flow sequence
Sifting segregation effects worsened

The alternative silo discharge pattern is called mass flow, where upon withdrawal of any material, all of the material in the silo moves. Ratholing and stagnant material zones are not possible in mass flow hoppers.

Mass flow:

No ratholing
Uniform flow
No stagnant material
First-in, first-out flow sequence
Sifting segregation effects reduced

In mass flow, the hopper outlet must be sized to both prevent the formation of an arch and to allow the required discharge rate. See below for a video demonstration of funnel and mass flow patterns.

Expanded flow can develop when a mass flow hopper is placed beneath a funnel flow hopper. The mass flow hopper is designed to activate a large enough flow channel in the funnel flow hopper to overcome ratholing. This flow pattern can be effective for large diameter silos, as well as for gravity reclaim stockpiles.