SOLVING OR PREVENTING POOR SOLIDS FLOW
Two of the most common flow problems experienced in an improperly designed silo (bin, bunker, hopper) are no-flow and erratic flow.
No-flow from a silo can be due to either arching (bridging) or ratholing.
Arching occurs when an obstruction in the shape of an arch or a bridge forms above the outlet of a silo and prevents any further discharge. It can be an interlocking arch, where the particles mechanically lock to form the obstruction, or a cohesive arch. An interlocking arch occurs when the particles are large compared to the outlet size of the hopper. A cohesive arch occurs when particle-to-particle bonds form, allowing the material to pack together to form an obstruction.
Ratholing occurs in a silo when flow takes place in a channel located above the outlet. If the material being handled has sufficient cohesive strength, the stagnant material outside of this channel will not flow into it. Once the flow channel has emptied, all flow from the silo stops.
Erratic flow is often the result of an obstruction alternating between an arch and a rathole. A rathole may fail due to an external force, such as ambient plant vibrations, vibrations created by a passing train, or vibrations from a flow aid device such as an air cannon, vibrator, etc. While some material may discharge as the rathole collapses, falling material often gets compacted over the outlet and forms an arch. This arch may break due to a similar external force, and material flow resumes until the flow channel is emptied and a rathole forms again.
These problems occur in silos where flow takes place through a channel formed within stagnant material. This describes a funnel flow pattern, in which some material moves while the rest remains stationary during discharge from the silo. 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. Under these conditions, particles slide on themselves rather than the hopper walls, and an internal flow channel develops. Mass flow is defined as the flow pattern where upon withdrawal of any material, all of the material in a silo moves.
The key to reliable handling of materials is to design thehandling system equipment based on the measured flow properties of the material to be handled.
The minimum arching (and ratholing) dimensions can be determined by measuring the cohesive strength of the material. The potential for arching (and ratholing) is directly attributable to fines content, moisture content and storage time at rest. Hopper angles for mass flow can be determined by measuring wall friction angles. The minimum outlet size to prevent cohesive arching can be calculated by measuring the cohesive strength of a material. These are two of the major requirements for designing a mass flow silo.
The three methods available for alleviating poor flow of material in an existing silo are: change the material (e.g., reduce moisture, screen to increase particle size, blend different sources), change the operating procedures (e.g., limit storage time at rest, limit silo capacity, use flow aids, use all outlets), or change the equipment (e.g., use less frictional liner, steeper hopper, larger outlet, insert, new feeder).
Jenike & Johanson is prepared to assist you with solving or preventing poor bulk solids flow. Our team of experienced consulting engineers and laboratory technicians have the knowledge, necessary skills, and state-of-the-art tools available for addressing your unique applications.
Jenike & Johanson can apply proven, practical technology to ensure success with your material and handling system. Act now to prevent costly mistakes that may occur later on.
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