Hoppers Archives - Control Concepts, Inc.

How to Prevent Bridging in Material Hoppers (aka The Ketchup Effect)

July 28, 2023 10:39 pm Comments Off

bulk material Ketchup in a factory getting packaged

Have you ever struggled to get ketchup or any thick sauce out of a bottle? That’s the same issue that production facilities face when they try to move solids or powders through bulk material hoppers. The bulk material gets stuck, and production stops.

Unlike liquids, bulk solids and powders will not flow readily without material handling intervention. Even some thick liquids like ketchup are notoriously difficult to handle. The bulk material clings to the vessel walls, or becomes more compact – also known as bridging.

What is bridging?

Bridging in material hoppers refers to the phenomenon where bulk materials, such as powders, granules, or pellets form an arch-like structure over the outlet. It stops the bulk material from flowing freely, which leads to several material production problems:

Need for manual intervention to loosen the material
Lower productivity and machine downtime
Inconsistent product formulas
Stagnant bulk material that can spoil or contaminate the next batch
Bulk material waste from throwing away spoiled material or out-of-spec batches

What causes bridging in material hoppers?

Know your bulk material’s flow properties. Choose non-cohesive materials whenever possible. If cohesive bulk materials are necessary, consider additives or treatments to reduce their cohesive properties.
Control moisture. Implement proper moisture control measures to ensure the bulk material remains within the acceptable range for smooth flow. This might include using desiccants, heaters, or dehumidifiers.
Select the right design of material hoppers. Select an appropriate hopper design based on the bulk material’s characteristics. Conical material hoppers with an angle between 50 to 60 degrees, and the right outlet size, can help promote steady material flow.
Consider hopper liners. Using hopper liners made of low-friction materials like Teflon can help reduce bulk material adhesion and bridging.
Use the right flow aid. Many manufacturers use industrial material flow aids to break up bulk material blocks and encourage regular, reliable flow. Aside from speeding up production, it prevents stagnant or spoiled material and the safety risks from manual intervention and cleaning.

Solve bridging in material hoppers with AirSweep

Preventing bridging in material hoppers is essential to ensure the continuous and efficient flow in industrial processes.

Both global brands and SMEs use AirSweep material flow aid to get smooth, on-demand flow. AirSweep is a pneumatic material flow aid that releases powerful, high-pressure air pulses that break up bridging and sweep the vessel walls clean. It is energy-efficient, requires very little maintenance, and has proven effective for even the toughest flow problems.

AirSweep can be installed on material hoppers as well as other process equipment like silos, chutes, conveyor belts, or any place where material tends to hang up.

Since AirSweep uses aeration rather than vibration, it does not cause metal fatigue and damage small or thin material hoppers. AirSweep is also certified safe to use in hazardous and flammable environments, and there are models that are specifically designed for sanitary applications.

Material flow: How to improve it in silos and hoppers

May 14, 2021 6:38 am Comments Off

Engineers and plant managers are often tasked to design or recommend the right container for storing and handling bulk solids. They have to weigh several factors — capacity, storage duration, installation cost, and how well it protects the material from moisture and pests.

But there’s another factor that greatly affects productivity and can lead to expensive hidden costs: the material flow. Some containers are more prone to bridging and arching, particularly above the discharge. This can affect productivity, product quality, and profit.

Let’s look at the two most common containers – silos and hoppers – and how to promote reliable material flow in them.

Overview: Material flow: How to improve it in silos and hoppers

Silos and hoppers are prone to different material flow issues. In the following slide presentation, you’ll learn the difference between the two and how material flow can help improve their performance.

What’s the difference between a silo and a hopper?

Silos are the oldest storage vessel used by man. Credit: Michael Trolove / Grain Silos at Manor Farm

Silos are used for bulk storage. While the earliest silos were primarily used to store extra grain, modern silos can hold any kind of material: cement, black carbon, plastic resins, wood chips, and more.

Common types include tower silos, bunker silos, and bag silos.

Hoppers are used for temporary storage or measured feeding systems. They come in different shapes: cones, wedges, pyramids, or a combination of these.

Common material flow problems

Ideally, any material in silos and hoppers can be discharged quickly and consistently to prevent production delays or accumulation of stagnant material that can spoil or contaminate the next batch. However, many plant managers are plagued with material blocks in their production line.

Ratholing

Material builds up on the vessel walls, restricting the flow and the vessel’s full storage capacity. When the flow channel empties and material flow ceases, it can form a stable rathole that may collapse and block the opening.

Bridging or arching

Materials interlock or bond together to form an arch above the outlet, blocking any flow. The strength of this arch depends on the cohesive strength and internal friction of the particles. Hammering or blasting with an air cannon can break the arch, but this can overload and damage the vessel.

Segregation

If the vessel is filled with different particle sizes, it’s natural for finer particles to collect at the center of the bin, while coarser particles stick to the slope. The variable, inconsistent material discharge can affect product uniformity.

Factors that affect material flow in silos and hoppers

A hopper shows hammer marks from workers who tried to clear material blocks.

Material properties

Some materials are more prone to arching, bridging or segregation. Fine powders, small particles and sticky materials have a higher cohesive strength and tend to bridge or compress along the vessel walls.

Other factors include bulk material density, the pressure applied by its weight in storage, permeability, and tendency to retain moisture. Ideally, these material flow properties are considered while choosing the silo or hopper design.

Vessel angle

If the hopper walls are not steep or smooth enough, the material won’t flow down the vessel walls. It will cake and build up, or if it has enough cohesive strength, it can bridge over the outlet or form a rathole as it empties out.

The hopper angle depends on the friction between the powder and the walls, the friction between powder particles, and the shape of the vessel.

Vessel shape

Generally, square feed hoppers work better for bulk materials with uniform pellets. However, when there is a large variation of particle size — like when virgin material is combined with plastic regrind — circular hoppers allow for a more gradual compression.

Outlet and feeder

The size of the outlet, the discharge rate, and the interface of the feeder can all affect material flow at the bottom of the vessel. This is particularly problematic for materials that have greater cohesive strength and bulk density, or have a tendency to segregate or spoil.

Steep cones or wedge-shaped hoppers may be able to promote mass flow. Flow aids can also break any bridging or arching above the discharge.

How to improve poor material flow in silos and hoppers

Material flow aids can help break through bridging and arching and promote on-demand flow. These can include:

Mechanical flow aids like vibrators or agitators
Pneumatic flow aids like fluidizers and AirSweep
Chemical flow aids like fumed silica

Each flow aid will have its pros and cons. Some are better suited for certain types of bulk solids. For example, vibrators can actually compact moist or dense solids like flax or whey protein, while fluidizers are only effective for light powders like flour. (Get a competitive analysis for different kinds of flow aids.)

Flow aids can be retrofitted into silos and hoppers. This can be more cost-effective than replacing the bins or having them custom-made. Some of them, like the AirSweep, barely require any modification of the silo or the hopper since they are installed on the wall with a mounting bracket.

Finding the “right” flow aid also depends on the type of material, or the size of the silo or hopper. We can provide a customized proposal based on your process.

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