Bridging in Silos and Hoppers is a result of friction on the inner walls slowing the rate of flow and restricting the material from moving freely. Particles then bind together to form an arch above the outlet area. This either severely limits the flow or stops it completely. To overcome the issue, the wall friction must be reduced.Silo bridging (also called arching) issues can occur on a regular basis, but the arch usually yields. It is only a problem if the arch does not collapse when flow is required.

There are two types of arches that occur depending on what type of material is stored inside the structure. The first is called a mechanical arch, formed by relatively large particles (above 3 mm) interlocking. The second is formed when smaller particles like powders bind together under compression, called a cohesive arch. The resulting arch can support the weight of the material above it and restrict the motion, leading to poor Silo efficiency.

Poor Silo flow rate is a great inconvenience for a site of any size. At worst, it can result in plant shutdown and loss of valuable bulk storage space – leading to loss of income.

Several issues, including a change in climate and poor maintenance, are factors that can disrupt a site’s production. Troublesome materials, such as polyester pellets, can sometimes form soft clumps which causes silo bridging over the outlet area. Primasonics Acoustic Cleaners are widely used to improve the use and efficiency of Silos and Hoppers, ensuring continuous production in the short and long term.

Weather & moisture issues

Extreme climate changes and moisture effects are issues that need to be considered carefully when designing and maintaining silos and hoppers. Temperature fluctuations, such as the change from daytime to nighttime, can lead to much higher wall stresses than normal. If the flow problems are not dealt with immediately, the consequences could lead to devastating results.

Moisture is another factor than can disrupt silo and hopper efficiency. If the bulk material is slightly wet, it will have a tendency to expand with increased moisture content. Changes in temperature may cause moisture to migrate between stagnant particles, causing some to expand.

An example of this would be if a silo on a brewer’s site (storing damp grain, corn and other ingredients) was left to sit for a few days without discharge. It’s possible for moisture migration to cause the material to swell. This could lead to increased pressure on the silo wall and the possibility of the metal splitting.

Some traditional Silo cleaning techniques require dangerous procedures such as lowering a worker on a rope to manually remove the build-up. This is dangerous due to the release of material and the possible presence of gases.

In cases of bridging, an additional danger exists as the exit hole also needs to be rodded from underneath, exposing the worker to significant risk from falling material when the blockage is cleared. Other methods include:

Vibrators. By their very nature and location, vibrators have to first transmit all their energy through the wall of the structure. This can firstly result in severe loss of power and secondly propagate cracks and signs of wear, resulting in long lasting damage to the metal. Vibrators can also compact the material and further restrict material flow.

Air Cannons and Blasters: Used in groups, these devices employ an unidirectional blast of compressed air to try and remove blockages. The purchase of multiple units means that the initial cost is high as is the installation cost and the increased cost to satisfy their compressed air requirements. Furthermore, they do not provide a preventative or long-term solution to build-up.

Aeration Pads: At first glance, Aeration Pads seem to be a cheap and easy Silo Bridging Solution, yet they provide a multitude of inefficiencies. Similar to the air cannons and blasters, they require multiple unit installations in each Silo Hopper. Secondly, once the material being discharged comes into contact with the aeration pads, they are prone to erosion and abrasive wear. It is therefore highly likely that an investment in regular replacements and installations will be needed.

Not only does an Acoustic Cleaner use the latest and safest technologies, it’s also a preventative solution and can decrease the risk of future Silo blockages. The service does not require down time on site and will operate during normal working hours. Primasonics Acoustic Cleaners are able to operate from the top of a Silo through in situ material to clear the bridging at the base of the structure. Our systems are also non-invasive and do not require the Silos to be emptied.

So, what does an Acoustic Cleaner do exactly to relieve blockage in Silo and Hoppers?

Well, sound waves are transmitted into the particulate matter or bonded material causing the solid particles to resonate and dislodge from the inner walls. Once dislodged, the Silo buildup will then fall away due to gravity.

Powered by a solenoid/timer and compressed nitrogen, an Acoustic Cleaner will activate the titanium diaphragm to create the correct sound frequency. The timer is set to sound the Acoustic Cleaner for a few seconds at periodic intervals during either filling or discharging from the silo.

From a 20 tonne capacity silo right through to a 100,000 tonne capacity one, Primasonics Acoustic Cleaners are guaranteed to eliminate flow problems and improve silo efficiency. 100% of the de-bonding power goes into the material without risk of damage to any structure. We can even supply a technical research report that has been prepared jointly with the University of Liverpool, one of the UK’s leading acoustic academic institutions, to prove that our Acoustic Cleaning Systems will not cause any structural damage.

When considering how to clean a silo, there are many benefits to Acoustic Cleaning, including:-

• Health and Safety- Traditional methods of silo cleaning required a person to manually clean the structure either by dangling from a rope or from underneath the silo. This introduced significant risk from falling material and from the possible presence of gases. By using the latest and safest remote cleaning technologies, there is no risk to a person’s safety.
• Efficiency– Bridging can be likened to creating a ‘funnel effect’, meaning silos and hoppers cannot be used to their full storage capacity. Acoustic cleaners eliminate this issue, ensuring mass flow of material is maintained and production is achieved at maximum efficiency.
• Prevention- Rather than dealing with just one problem area, Acoustic Cleaners are designed to take a holistic approach and decrease the risk of future silo blockages. This will prevent vertical wall build up by consistently dislodging the presence of any compacted materials before a further issue arises.
• No down time- The service does not require down time on site and will operate during normal working hours. Primasonics Acoustic Cleaners are able to operate from the top of a silo through in situ material to clear the bridging at the base of the structure.
• Non-invasive- Our systems are non-invasive and do not require the silos to be emptied. Unlike vibrators and explosive cleaning methods, Acoustic Cleaners are designed to reflect sound from solid surfaces therefore no damage due or wear from intense vibrations will be caused.
• Quality- By employing acoustic cleaners, correct blending is achieved and bulk storage is maximised with the product remaining fresh. Acoustic Cleaners will also ensure that material flows smoothly on the “first in – first out” principle to ensure consistent blend quality.