Material Flow Archives - Control Concepts, Inc.

What Causes Material Bridging in Hoppers and How AirSweep Can Help

December 12, 2022 5:54 pm Comments Off

material bridging inside of hopper

Material bridging in a hopper can slow down production and reduce bulk storage space. The stagnant material can spoil or become contaminated, and workers have to clear the vessel and throw out compromised batches.

What is material bridging?

Bridging is the material block that forms above the discharge outlet of a hopper, silo, or any bulk material handling equipment.

The bridge is caused by friction on the vessel walls which slows down the flow of material. Eventually, the material particles start to bind and interlock, until it partially or completely blocks the discharge outlet.

The bridge is so strong that it can support the weight of material above it. New material can condense the trapped material further and strengthen the bonds.

Types of bulk material bridging

Mechanical bridging. Particles mechanically interlock, like jigsaw puzzles. This usually happens for particles that are larger than 3mm.
Cohesive bridging. Particles compress until they bind together. This usually happens for smaller particles, especially those that attract moisture.

What causes material bridging in the hopper?

Material bridging can be caused by the hopper design, the bulk material’s flow properties, and the conditions in the plant.

Hopper Design

Ideally, the hopper design should reduce friction and encourage material flow. However, there is no one-size-fits-all solution. Solids and liquids behave differently, and materials have unique properties that can also vary in different conditions. These factors have to be considered while choosing the hopper’s angle, and the outlet’s shape and diameter.

Bulk Material Properties

Some bulk materials are simply more prone to bridging and arching. This includes:

Moist and sticky materials
Materials that tend to settle during storage
Materials with a high cohesive strength
Hygroscopic powders that attract moisture
Pellets that soften and form soft clumps
Fibers that can entangle
Irregular particles that can interlock
Heavy materials that can push down on stagnant material and cause it to compress

Plant Conditions

Fluctuating temperatures can affect the stress on the hopper walls and the material flow properties. Moisture in the air can cause materials to expand and compress. Gas in the air can also permeate through powders and fill empty spaces.

The best solution for material bridging in the hopper

Material flow aids solve bridging and other material blocks by breaking up the cohesive bonds in the bulk material. However, not all flow aids can do this safely or effectively.

Manual cleaning—where workers hammer the vessel or enter the hopper to clear the build-up with axes and shovels—is slow and dangerous. Some bulk materials release flammable or toxic gases during storage. Workers are also at risk for falls, sprains, health problems from direct exposure to the material, or being hit by falling material.

Vibrators can damage the vessel and can cause some materials to become more compact. Fluidizers can only activate light powders; air cannons are expensive and still leave material residue on walls.

AirSweep is the most cost-effective solution for material bridging in hoppers. It releases powerful air pulses that break up bridging and sweep stagnant material back into the flow stream.

AirSweep material flow engineers customize the system so the units are installed at key areas on the hopper to stimulate first-in/first-out flow. They select the right model for the bulk material and the vessel size. The AirSweep USDA-accepted model is also specially designed for sanitary applications like food, beverage, and pharmaceuticals.

The AirSweep material flow system is energy-efficient. It uses less air than air cannons, and the units pulse in sequence instead of running continuously like vibrators and fluidizers. The units also have a large area of activation and move more material with every pulse.

Find more information about AirSweep, or get a customized quote.

Material flow: How to improve it in silos and hoppers

May 14, 2021 6:38 am Comments Off

Promoting material flow

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

AirSweep Material Flow Aid

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.

Tag Archive: Material Flow