Bridging, Ratholing, and Other Problems In Material Handling

hand in hopper with material experiencing problems

All manufacturing processes need efficient material flow for smooth operations and cost-efficiency. Sluggish and inconsistent flow can cause production delays and compromise product quality and safety.

For example, in the food and pharmaceutical industry, stagnant material can cause cross-contamination between different materials or batches. This can lead to the presence of foreign substances, allergens, or contaminants in the final products, posing significant health risks to consumers.

Other industries that use precise or patented formulas – such as chemicals, cement, pharmaceuticals, electronics, and manufacturing – may also lose product integrity and performance if materials aren’t mixed in the correct proportions.

The two most common problems in material handling are “bridging” and “ratholing.” Let’s look at their definitions, causes, and what you can do to prevent these flow problems.

What is bridging?

Bridging, also known as arching, occurs when bulk materials form a stable, compacted bridge across a hopper or silo outlet. This restricts – and eventually stops – material flow.

Bridging is common in fine powders, fibrous substances, or materials with high moisture content. These have a tendency to bind together and become even more compact as more material is poured into the vessel.

Bridging can become even more challenging when materials stick to the hopper walls. This often happens with moist, sticky solids like cement mixes or food ingredients like whey protein.

What causes bridging?

Bridging and other problems in material handling are caused by two major factors: the material properties and the shape of the vessel.

Material properties include particle size and shape, tendency to attract moisture, and natural cohesion and adhesion. Cohesion is the tendency of particles to stick together, and adhesion is the tendency to stick to vessel walls and other surfaces.

The geometry of the hopper or silo outlet also impacts the potential for bridging. A funnel-shaped outlet with a steep angle and narrow opening can be more prone to blocks. External factors such as vibrations or uneven flow rates can also disturb the material flow.

What are some materials that are prone to bridging?

These materials are more likely to cause material handling problems.

  • Fine powders like cement, fly ash, powdered sugar, cocoa powder, detergents and cleaning agents, or some additives and preservatives
  • Fibrous materials like wood chips, tobacco, straw, and certain types of grain
  • Hygroscopic materials like sugar, salt, fertilizers, and other powders that absorb moisture from the air
  • Thin flakes or fluffy materials like certain cereals, animal feed, and plastic flakes
  • Sticky or viscous materials like adhesives, resins, or certain chemicals
  • High-fat or oily materials like nuts, seeds, and certain grains
  • Resins and polymers with high cohesive properties
  • Chemicals and salts with electrostatic properties

What is Ratholing?

Ratholing is also one of the most common problems in material handling. It occurs when material cakes on the vessel walls.

Ratholing causes two problems: it narrows the space for the material to flow out (also called funnel flow) and leaves stagnant material that can spoil or contaminate the formula.

Ratholing can also limit the bin’s total capacity.

What causes ratholing?

Like bridging, ratholing is caused by material properties like cohesion and particle size distribution, and vessel geometry. It tends to happen in cone-shaped hoppers with walls that aren’t steep enough to encourage flow. If the material is even a little cohesive, it sticks to the walls.

What materials are more prone to ratholing?

Ratholing is a flow issue that affects specific materials, often characterized by their cohesive and interlocking properties. Some common materials that are prone to ratholing include:

  • Cohesive powders like flour, cement, coal, pharmaceuticals, or food ingredients like coffee
  • Granular materials such as fertilizer granules or prilled chemicals
  • Minerals, ores, and fine sands with high cohesive strengths and fine particle sizes
  • Plastic pellets with strong cohesive properties
  • Chemical powders with strong cohesive properties
  • Flaky or elongated particles can form preferential flow paths
  • Animal feeds with pellets or ground materials

Solve problems in material handling with AirSweep

Thousands of plants around the world use AirSweep flow aid systems to solve bridging, ratholing, and other material flow problems. The nozzle releases highly pressurized air pulses to promote on-demand, first in and first out flow.

AirSweep breaks up material blocks and sweeps the vessel walls clean. It is the only flow aid that can handle problematic powders and even sticky and moist solids without damaging the vessel or consuming a lot of energy.

AirSweep can even be used on thin or small vessels, or installed in hard-to-reach areas.

Tell us about your problems in material handling and get a customized proposal for your process.

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