- Type
- Hopper-bottom / cone-bottom silo
- Identify
- Hopper surface, outlet and feeder
- Key distinction
- Geometry does not establish the flow pattern
Hopper geometry and outlet equipment
A hopper is the inclined section that feeds a discharge point. The cone, transition pieces, outlet valve and downstream feeder all affect where material can remain and which surfaces need cleaning.
A conical hopper does not by itself establish mass flow. Bulk-solid properties, wall friction, geometry and the outlet arrangement all influence the way material moves through the vessel.
Bridging and ratholing
Bridging is an arch-shaped obstruction over the outlet. Ratholing describes a flow channel through otherwise stationary material. They are different flow behaviours and should not be treated as the same type of blockage.
Changes in moisture, product grade or discharge equipment can alter flow. Cleaning may remove retained material without resolving an underlying feeder or flow-design problem.
Internal surfaces and cleaning method
Hopper components may be steel, coated, composite or fabric, and each has its own limits. Cleaning pressure, tools and agents need to suit the internal finish, its condition and the stored residue.
Food and feed powders may also bring contamination and combustible-dust concerns. The dust assessment and the cleanliness standard for the next product still apply where residue remains.
Inspection and return to use
Before the hopper returns to operation, it should be clear which areas were available for inspection, what deposits remain inaccessible and what condition is required. An open outlet does not make a silo safe to enter, and any entry requires its own confined-space assessment.
The cleaning method needs to reflect the hopper geometry, outlet and feeder condition, product behaviour and internal finish.
Sources and further reading
- Symaga: hopper silos — Manufacturer hopper and discharge arrangements.
- Jenike & Johanson: poor bulk-solids flow — Arching, ratholing and mass/funnel flow; no universal clean or design setting.
- HSE: dust explosions in the food industry — Food-dust hazard assessment and control principles.
- HSE: introduction to confined spaces — Great Britain confined-space assessment and avoiding entry where possible.