How to prevent material clogging

Air cannons are engineered to safely clear and prevent clogging, promote material flow and avoid costly downtime. To know if the technology will work best for a specific application, the first step is understanding how, where, when and why clogs happen in any given silo. The second step is removing any worker involvement in clearing the clog, aside from pushing a button to activate the cannon if it is not automated or controlled by logistical software.

Silo clogging and safety

Regardless of the cause, the types of clogs in a hopper can pose unique challenges for discharge, as well as very serious safety issues. There are several unsafe practices around silos that too often result in serious worker injuries or fatalities, mainly sudden discharge of adhered material and entrapment. 

Poking or lancing from beneath the clog at the spout can result in a sudden surge of falling material, burying or crushing the worker(s) below and seriously damaging the receiving belt. Beating the vessel walls with mallets or other objects to loosen adhered material is common. Operators who engage in this method find this worsens the situation over time as the divots and ripples left from hammer strikes provide places for additional material accumulations to start. 

If a worker enters the vessel and stands on the volatile bridge, a sudden discharge could pull the worker into the cavity. Sometimes material buildup on the sides of the vessel reaches higher than the worker and falls from above, causing serious injury or burial.

Material churn

Silos are designed to hold a certain volume of a particular material, so awareness of the maximum load is important. Repeatedly filling and emptying them makes load requirements especially important in those cases, since capacity is reached repeatedly under many conditions. When working with bulk solids, environments with high moisture and freezing regularly experience clogging. Wide variations in the size and shape of the material can also affect the flow characteristics, leading to buildup and clogs [1].

Arches form when material consistency changes during loading or the top material contains more moisture. It can also just be caused by gravity. This is very dangerous since material discharge has a long fall distance. The flat top surface can give workers a false sense of stable ground, so vessel entry is ill-advised. Air cannons placed at the upper point of where material begins to adhere keep material flowing toward the discharge spout.

Plugs are generally caused by compacted moist material or the contents that have been left for long periods. Strategically placed air cannons can help loosen material to get it flowing. Sometimes the contents have hardened to such a degree that a silo cleaning service is needed, which utilizes the support of the air cannon system to resolve the issue faster, lowering the cost of the service.

Buildups can be caused by several factors: weather, silo design, the way the silo is loaded, a horizontal grain of the metal on the side of the silo, the silica content of the material, etc. Buildups can be economically mitigated using strategically placed air cannons at common collection points to keep material flowing toward the discharge spout.

Ratholes often form over time and reduced the capacity of the silo. Since the material is flowing, they are often ignored by operators, but can severely impact on production. The significant weight put on the thin walls of the silo and structural supports can pose a serious safety issue if buckling or a collapse occurs.

Air cannons

Pioneered and patented by Martin Engineering, low-pressure air cannon technology has progressed exponentially over the 50 years since its conception. They use a plant’s compressed air system to deliver a powerful discharge to dislodge material buildup. Mounted on a pipe assembly, the basic components include an air reservoir, a fast-acting valve with a trigger mechanism and a nozzle to distribute the air in the desired pattern to clear the accumulation most effectively. 

Strategically positioned on the silo, when compressed air (or inert gas) in the tank is suddenly released by the valve, it is directed through an engineered nozzle toward the specific or general location depending on the design of the nozzle. The air blasts help break down material accumulations and clear blocked pathways, allowing solids and/or gases to resume normal flow.

Often installed in a series and precisely sequenced for maximum effect, the network can be timed to best suit individual process conditions or material characteristics. Specific air blast characteristics can be achieved by manipulating the operating pressure, tank volume, valve design and nozzle shape to customize the air cannon installation to the service environment. 

Valve replacement

The valve in an air cannon is considered a wear part, but it is common practice to refurbish them rather than replace them with new ones. Since clearances and fits are critical to proper operation, valves should be rebuilt and repaired by the manufacturer, or specifically trained plant maintenance personnel. 

Martin Engineering created a program to supply factory-rebuilt air cannon. Customers can receive a standard pallet-sized container with six refurbished valves, so there’s no need for users to rebuild worn-out components. The changeout can be accomplished in just ten minutes, at less than half the cost of new valves. The used valves are shipped back to the company, where the units are rebuilt to as-new condition by factory-trained technicians. Customers save time and money, with no need to stock repair parts or provide the training/labor to rebuild.

Silo flow

Air cannons are not new, but the technology has progressed to the point that the footprint on the compressed air system of a plant is considerably lower. Efficient silo flow is essential to plant production, so justifying the expense of installation and operation are easy once calculated against the cost of downtime and the importance of workplace safety.