Solve or Prevent Poor Flow
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.
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.
- 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.
- 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. Click here 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.
Why is Poor Flow a Problem?
Poor flow can have many implications, including:
- 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
How can Jenike & Johanson Help Solve or Prevent Poor Flow?
The key to avoiding poor flow is to evaluate the flow properties of the material and to engineer an appropriate bulk material handling system.
We are prepared to assist you. Our team of experienced engineers and laboratory technicians have the knowledge, skills, and state-of-the-art tools and bulk material testing laboratories available for meeting your bulk material handling needs.