Martin Engineering global air cannon product manager Brad Pronschinske explains how flow aids can help avoid expensive delays when working with silos.
Humidity, material type, or settling can cause serious material flow issues like clogging and expensive delays.
Once material adheres, the buildup is generally fast and dense, introducing a workplace hazard to remove from a silo.
When operators notice material is no longer flowing, spilling over the edge of the vessel or backing up on conveyors, they know a clog has formed.
Seeking ways to address it quickly without the proper tools or training can also be the moment when workplace safety degrades.
Flow aids are engineered to safely clear and prevent clogging, promote material flow and avoid costly downtime.
To know what technology will work best for a specific application, the first step is understanding how, where, when and why clogs happen in any given vessel or transfer point. The second step is removing any worker involvement in clearing the clog, aside from pushing a button to activate the flow aid if it isn’t automated or controlled by logistical software.
Discharge point geometries
Discharge channels come in varying shapes, depending on the vessel and the material flow characteristics.
Spouts that are narrow, such as those found on conical or pyramidal shapes, direct flow in a vertical column either into a chute or specific loading area. Slotted spouts, like those found on the wedge or transition shapes, distribute material in a narrowly defined line for loading onto conveyors or into containers (trains, trucks, etc.)
The geometry of a vessel must match that of the discharge point or it will be prone to clogging. These discharge points can feature gates or grates that stop or separate the material. Grates can be used to slow or direct the flow of material when loading onto a conveyor or filling of transport bins on a train or truck.
Either way, operators find that they can exacerbate clogging by stopping or slowing material at a structural choke point.
Unsafe practices
Once a clog has been detected, there are several unsafe practices that at the time may seem harmless, but frequently cause serious worker injuries or fatalities year after year. The two main causes of injury are sudden discharge of adhered material and entrapment.
One unsafe method is beating the vessel walls with mallets or other objects to loosen adhered material. Over time, the more the walls are pounded, the worse the situation becomes, as the bumps and ridges left in the wall from the hammer strikes will form ledges that provide a place for additional material accumulations to start.
Another hazardous practice is poking or lancing underneath the clog at the discharge point. This can result in a sudden surge of falling material, burying or crushing the workers below.
Perhaps the most prevalent cause of worker injuries and fatalities is confined space entry of the vessel. Along with potentially sinking into the material in the centre, the material could be bridging and suddenly release. If a worker enters the vessel and stands on the volatile bridge, a sudden discharge could pull the worker into the cavity. Another serious hazard of confined space entry is material buildup on the sides of the vessel, reaching higher than the worker. This buildup could fall from above, causing serious injury or burial.
Air lancing the clog from the mouth of the vessel at the top is an option many operations have chosen, but guardrails are highly recommended. The reach of the lance and the pressurised air stream must match the size of the vessel. Workers can fall in trying to reach the lance down to the clog, even if guardrails are present. All these hazards can be avoided by introducing flow aids to the vessel to mitigate clogs, promote material flow and reduce downtime.
Greater safety with less downtime
As the term implies, flow aids are components or systems installed to promote the transport of materials.
Flow aids come in a variety of forms, including rotary and linear vibrators, high- and low-pressure air cannons and aeration devices, as well as low-friction linings and special chute designs, to promote the efficient flow of bulk materials.
These modular systems can be combined in any number of ways to complement one another and improve performance.
The components can be used for virtually any bulk material or environment, including hazardous duty and extreme temperatures.
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One of their primary advantages is that an operation can obtain a level of control over the material flow that is not possible any other way.
When employing flow aids, it’s critical that the chute and support components are sound and the flow aid be properly sized and mounted, because the operation of these devices can create potentially damaging stress on the structure. A properly designed and maintained vessel will not be damaged by the addition of correctly sized and mounted flow aids.
It’s also important that any flow aid device be used only when discharges are open and material can flow as intended. The best practice is to use flow aids as a preventive solution to be controlled by timers or sensors to avoid material buildup, rather than waiting until material accumulates and restricts the flow.
Using flow aid devices in a preventive mode improves safety and saves energy, since flow aids can be programmed to run only as needed to control buildup and clogging.
Engineered vibration
The age-old solution for breaking loose blockages and removing accumulations from chutes and storage vessels was to pound the outside of the walls with a hammer or other heavy object.
A better solution is the use of engineered vibration, which supplies energy precisely where needed to reduce friction and break up a bulk material to keep it moving to the discharge opening, without damaging the chute or vessel.
The technology is often found on conveyor loading and discharge chutes but can also be applied to other process and storage vessels, including silos, bins, hoppers, railroad cars, screens, feeders, cyclones and heat exchangers.
Air cannons
One solution for managing material accumulation in chutes and vessels is the low-pressure air cannon. It uses a plant’s compressed air to deliver a powerful discharge to dislodge the buildup.
Cannons can be mounted on metallic, concrete or wood surfaces. The basic components include an air reservoir, fast-acting valve with a trigger mechanism and a nozzle to distribute the air in the desired pattern to clear the accumulation most effectively.
The device performs work when compressed air (or some other inert gas) in the tank is suddenly released by the valve and directed through an engineered nozzle, which is strategically positioned in the chute.
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. The air blasts help break down material accumulations and clear blocked pathways, allowing solids and/or gases to resume normal flow.
To customise the air cannon installation to the service environment, specific air blast characteristics can be achieved by manipulating the operating pressure, tank volume, valve design and nozzle shape.
In the past, when material accumulation problems became a recurring issue, operators would have to either limp along until the next scheduled shutdown or endure expensive downtime to install an air cannon network. That could cost a business hundreds of thousands of dollars per day in lost production.
Many designers now proactively include the mountings in new designs so that a future retrofit can be done without hot work permits or extended downtime.
Valve replacement
Over time, the valve in an air cannon will wear and it is common practice to refurbish them rather than replace with new ones.
Because clearances and fits are critical to proper operation, it is recommended that flow aid devices be rebuilt and repaired by the manufacturer, or that the manufacturer specifically train plant maintenance personnel to properly refurbish the equipment.
To simplify the process and avoid system downtime, one manufacturer has created a program to supply factory-rebuilt air cannon valves that carry the same warranty as new valves. 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 to rebuild.
Let the silo flow
The number of bulk handling operations with centralised automated systems that monitor and support production is growing and becoming more sophisticated.
Flow aids can be added to the central control logistical system to be fired remotely. They are also accompanied by a separate solenoid box mounted a safe distance away that can be fired manually if need be.
Having these redundancies available allows operators full control over bulk material flow and produce a safer workplace with less downtime from clogging resulting in a lower cost of operation.
