Silo and Hopper Ratholing

Silo and hopper ratholing is an issue which causes the compaction of materials on silo and hopper side walls, which results in a severely restricted material flow. Silos and hoppers are particularly prone to this issue if the materials they are storing are cohesive.

In this case, if the silo walls are not smooth or steep enough and an insufficient amount of friction is created for the cohesive materials to slide along the side walls, meaning particles will slide on themselves instead. As a result, the material in question will become cakey and build-up around the side walls, until it eventually becomes stagnant.

A rat hole is thus formed as the stagnant material forms an internal holding around the inner walls of the silo or hopper channel, resulting in material discharge only taking place in the flow channel directly above the outlet.

In comparison to “mass flow”, which occurs when all of the materials are in motion during discharge, Ratholing results in a “funnel flow” effect. This means that once the central core of materials are discharged, flow comes to a halt and the stagnant material remains stationary.

Ratholing in silos and hoppers can be the cause of an extensive range of problems affecting production, namely the compromisation of efficiency and quality.

Efficiency: The poor flow that is created, as a result of a rat hole, means that it restricts the silo and hopper from being used to their full storage capacity. A decrease in the efficiency of flow and production, carries the risk of causing delays in shipments and deliveries, which could ultimately cost you valuable time and money.

Quality: If the stagnant material that accumulates against the silo and hopper side walls is left stationary for a significant period of time, then the material is prone to degrading and could produce dangerous gases. Not only does this pose risks in the management of quality control, but stagnant material is effectively wasted product and resources.

Further ratholing problems: If a rathole collapses, it poses the risk of creating an environment that will promote the formation of material bridging and further ratholes. In this case, the collapsing ratholes will produce a high rate of incoming powder, which will entrain pressurised air and becomes fluidised as a result.

Once fluidised, the pressurised powder will rush through cracks, poorly fitted valves and crevices in worn equipment. This behaviour is called fine powder flooding or flushing and can cause a lot of dust and product wastage. Once depressurised, the fine powder will then cling to silo and hopper side walls and plug outlets, thus causing silo blockages and further ratholes.

Traditional ratholing solutions offer vast and varying methods of intensity, as well as success. However, one major flaw that they have in common, is that there solutions are short-term.

An effective silo cleaning service, is a process that should maximise the efficiency of silo’s and maintain the quality of their working form into the future. Yet, traditional ratholing solutions only target specific and current problems, meaning they do not prevent problems from reoccurring in the future, and in some cases can exasperate additional issues.

• Vibrators offer a high impact method, in which vibrations are transmitted through silo walls to promote mass flow. However, whilst vibrators are easy to fit into empty silos and can provide a time and cost efficient solution, the structural damage they can cause is long-lasting. Unless silo’s are built to withstand powerful vibrations, vibrators risk working to propagate cracks and signs of wear, resulting in metal fractures and ultimately mechanical failure.
• Air Cannons and Blasters are a group of permanent de-clogging devices which deliver localised blasts of compressed air. The impact of the compressed air, is used to clear blockages and buildup on interior walls, but limited coverage requires the purchase of multiple units. Furthermore, because air cannons only treat blockages that have already occured, cost and maintenance will remain high, as they do not provide a preventative or long-term solution to ratholes.
• Aeration Pads are quiet, inexpensive and seemingly easy to install in empty silo’s, yet provide a multitude of inefficiencies. Firsty, in comparison to your single Acoustic Cleaner, they require multiple unit installations in each Silo Hopper. Secondly, because the materials being discharged come into contact with the aeration pads, they are prone to erosion and abrasive wear. It is therefore highly likely that investment in regular replacements and installations will be needed.

Acoustic Cleaning is a non intrusive technology, which provides a long-term silo & hopper ratholing solution. Not only do Acoustic Cleaners efficiently remove ratholes, but they also prevent them from occurring again- thus proving to be both an effective and economical Silo Maintenance investment.

Primasonics Acoustic cleaners create a series of high energy- low frequency sound waves, which penetrate materials adhering to side walls with rapid pressure fluctuations. By using alternating frequencies, acoustic cleaners are able to target particles that have different masses and which therefore travel at different speeds. As a result, the compacted materials will vibrate at different rates to their surrounding environment, causing them to debond from each other and dislodge from surfaces.

Unlike traditional hopper and silo solutions, acoustic cleaners can tailor their frequency to deal appropriately with different silo and hopper issues. The appropriate frequency is determined by the amount of product build up, its adhesiveness and the natural vibration frequency of the compacted material

For ratholing it is recommended that a low frequency is used, in order to reach longer distances, for example in large silos. The acoustic cleaner will sound for approximately 10 seconds, before sounding again around every 8 minutes, preventing ratholes from developing in the first place and ensuring that maximum flow is maintained.

• Non Intrusive– Unlike vibrators and explosive cleaning methods, Acoustic Cleaners are designed to reflect sound from solid surfaces. Therefore, the powerful sound waves that they transmit will only penetrate targeted materials and not the silo and hopper side walls. This will prevent structural damage and the propagation of wear due to intense vibrations.
• Prevention– Rather than dealing with specific and localised issues, Acoustic Cleaners are designed to take a holistic and long-term approach to combatting silo blockages. Not only do they target ratholes, but they also eliminate and prevent vertical wall build up and material bridging, by consistently dislodging the presence of any compacted materials before issues can arise.
• Efficiency– As explained earlier ratholing powder flow 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.
• Minimal Sound Pollution– The sound waves created by acoustic cleaners only penetrate the targeted material. This means that sound is only increased within the vessel, where cleaning is required and therefore prevents sound pollution in the local area.
• Health & Safety– In addition to the elimination of costly manual work and cleaning regimes, acoustic cleaners also eliminate the danger of sparking, as they are powered by compressed air rather than electricity.