Hopper & Silo Design

3 commonly flow problems “Rat-holing”, Mechanical arching and Cohesive arching.

Problems That Are Frequently Encountered Involve:

1. Erratic flow
2. Flooding
3. “Rat-holing” & blockage 
4. Mechanical & cohesive arching
5. Segregation
6. Degradation
7. Lack of storage capacity

Flow Mass

In a Mass Flow silo, the walls are steep and smooth enough to ensure flow of all particles.

Mass Flow Key Features

1. “first in – first out”
2. All storage capacity is “live”
3. Minimize segregation
4. Hopper half angle [Θ] is typically < 25° (from vertical)
5. Even flow

Core Flow

It occurs when the silo is not steep or  smooth enough to force the bulk material to slide along the walls. It also occurs when the outlet of the silo is not large enough or fully effective.

Core Flow Key Features

1. “first in – last out”
2. “dead” regions of product
3. Exaggerates segregation effects on particles
4. Hopper half angle [Θ] is typically > 25° (from vertical)
5. Erratic flow

Core Flow Can

It can result in “rat-holing” when sloughing off of material from the top does not occur, due to the cohesive strength of the material.

“Rat-Holing”

It occurs when the central core has been discharged, whilst leaving the majority of the product supporting itself around the inside of the silo.

Bulk Material Analysis

“design a mass flow bin, hopper or silo”

Objective Of Bulk Material Analysis:

• Determination of:
  
  • 1. Standard properties of bulk material
    • Density {packed & unpacked}
    • Angle of repose {static & dynamic}
  • Critical values of moisture
  • Wall friction angle [φ_x ]
  • Angle of inner friction [φ_e ]
  • Angle of slide [Θ]
  • Minimum silo outlet opening required for mass flow
  • Considering various liner materials
  • Considering time consolidation

Critical Moisture

Moisture Measurement

Bulk Material Testing

Shear Cell

Ring Shear Cell

Time Consolidation

Time Consolidation Bench

Critical Dimensions For Mass Flow Bins

Plan Asymmetrical Design

Various Silo Designs

Theoretical Design

Bulk Materials that are very sensitive to compression and very sticky

Design Without An “Undercut”

Bulk Material can build up in corners and compact at the floor

Easy Flow

Bulk Materials e.g. Mined Gypsum

Sticky

Bulk Materials e.g. FGD Gypsum, Marl, Shale

Very Sticky

Limits

1. The analysis cannot be made with bulk materials having a high surface moisture like sewerage sludge.
2. The particle size for the analysis has to be smaller than 1 mm.
3. So the real situation inside a large silo cannot be really simulated, but the results are very conservative.