Martin Engineering global air cannon product manager Brad Pronschinske shares his insights into best practices for preventing clogging in silos and hoppers using flow aids.
Despite the enduring desire to keep materials flowing throughout the process, there are always parts of a cement plant where materials must be held, stored, and funnelled into the next stage of production. Even well-designed systems can succumb to the problem of materials clinging to the inside walls of silos, bins, and hoppers.
Once coarse material starts to stick, the build-up is generally fast and dense. This slows the flow of material, causes spillages and secondary blockages, and eventually results in unscheduled downtime.
The temptation to find a quick way to remove the clog- without the proper risk assessment, safe access, tools or training- can create unnecessary risks. This is where flow aids come in. Flow aids are devices specifically engineered and installed to promote material flow, clear buildups, and prevent clogging, avoiding costly downtime and reducing risk.
The first step to selecting the right flow aid is understanding how, where, when and why clogs are happening in a particular vessel.
The next step is designing a system with the right flow aids that removes any worker involvement in clearing the blockages.
Characteristics of clogging
Silos are typically designed to store a certain volume of material, while hoppers generally exist to consolidate and then direct material flow.
In all cases, understanding the function and maximum load of a vessel is important, as is an appreciation of the properties of the materials passing through, including considerations such as potential moisture content and temperature, plus the effect of atmospheric conditions.
Wide variations in the size and shape of particles also affect the flow characteristics. Failure to consider these and other factors can lead to suboptimal designs that increase the likelihood of build-ups and clogging.
If a hopper is not designed to carry the right material load at full capacity, then a sudden surge of material or a clog can lead to potential dangers. And even if a hopper or silo is engineered properly, repeated abrasion from loading over the years can cause the vessel walls to wear thin, which decreases their capacity to contain the weight.
This can also potentially affect the hopper or silo’s structural integrity, which can increase the risk of a serious event.
There are numerous reference guides that explain vessel loading in order for the right design to be determined, such as the American Concrete Institute’s (ACI) structural standards, which describe differing loads in storage vessels:
- Dead load: The total weight of the structure, including attached components and equipment supported by the structure.
- Live load: Forces exerted from the stored material, including high and low pressures caused by flow, plus anything independent of the structure such as snow, positive and negative air pressure, wind or seismic load, and any forces from materials stored against the external wall.
- Thermal load: Caused by temperature differences between the inside and outside faces of the wall.
- Settling load: Forces from uneven settling of the structure.
So the weight, size and type of material, load velocity, plus direction and distribution, structural design of the vessel, as well as factors like weather, can all play a part in clogging and overloading. And the moment the clogged material is discharged, the force of the surge can also overwhelm the structure, gate, skip or conveyor onto which it is flowing, so understanding the weight of the material in the clog itself is important too.
Hopper geometries and discharge points
Discharge points come in varying shapes, depending on the vessel and the material flow characteristics [Fig 1]. Cement plant manufacturers carefully choose discharge point shapes based on a series of load and flow factors.
Spouts that are narrow, such as those found on conical or pyramidal shapes, direct flow in a vertical column either into a chute or a specific loading area. Slotted spouts, like those found on the wedge or transition shapes, typically distribute material in a narrowly defined line for loading onto conveyors or into containers. The shape of a vessel should match that of the discharge point or it will be prone to clogging.
The slope angles in discharge point geometries can also contribute to clogging, depending on material characteristics, the specifications of the application or the placement.
Discharge points can feature gates or grates that stop or help to separate the material. Gates, for example, halt material flow for incremental filling of rail cars or a truck body. Grates can be used to slow or direct the flow of material when loading onto a conveyor. Either way, operators often find that these devices can exacerbate clogging by stopping or slowing material at the structural choke point of the vessel.
Unsafe practices
Once a clog has been detected, there are several unsafe practices that, at the time, may seem like a quick and harmless solution, but frequently cause injuries and are even known to cause fatalities.
One unsafe but common method is beating the vessel walls with mallets or other heavy objects to loosen adhered material. Besides the health and safety risks, 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 yet popular 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 worker(s) below. Physical lancing from above is also common, often with makeshift platforms that allow access but provide little protection should a worker slip and fall into the vessel.
Air lancing the clog from the mouth of the vessel at the top can be an effective option, but only when guardrails are fitted or a safe access platform is used. Of course, the reach of the lance and the pressurised air stream must match the size of the vessel and the clog. And again, unless a harness is worn, workers could still fall in trying to get the lance down to the clog, even if guardrails are present.
Perhaps the most prevalent cause of worker injuries is entry into the vessel.
Along with potentially sinking into the material, especially in the centre, the silo or hopper contents could be ‘bridging’ and suddenly release. If a worker enters the vessel and stands on the fragile bridge or on a buildup to the side, a sudden discharge could pull the worker into the cavity, causing serious injury or engulfment.
These unsafe situations and behaviours can be avoided by introducing flow aids to the vessel to mitigate clogs, promote material flow and reduce downtime.
Productivity
As the term implies, flow aids are components or systems installed to promote the flow of materials through a silo, bin, hopper or chute, whilst controlling buildups, dust and spillage.
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 optimal chute designs, to encourage the most efficient flow of bulk materials.
These solutions can be combined in any number of ways to complement one another in an integrated system to improve overall productivity. Flow aid devices can be used for virtually any bulk material or environment, including hazardous duty and extreme temperatures.
When employing flow aids, it’s critical that the hopper or silo structure is sound and the flow aid device(s) are properly sized and mounted, because their operation can exert additional stresses on the structure.
A well-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 is used only when discharge gates are open, and material can flow as intended. By far 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 begins to restrict the flow. Using flow aid devices in a preventive manner not only reduces the chance of clogging but also enhances safety and can even save energy.
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.
A better solution is the use of “engineered vibration”, which supplies energy precisely where needed to reduce friction and break up potential accumulations to keep material moving to the discharge opening, without damaging the chute or vessel.
The technology is often found on conveyor loading and discharge chutes but can be effectively applied to other processes and storage vessels, including silos, bins, hoppers, screens, rail cars, feeders, cyclones and heat exchangers.
Productivity with a blast
Another highly effective solution for eliminating material accumulation in chutes and vessels is the low-pressure air cannon, pioneered and patented by Martin Engineering in the 1970s. It uses a cement plant’s compressed air system to deliver a powerful and carefully timed blast to dislodge build-ups. Air cannons can be mounted on metal, concrete, or even wooden surfaces, as long as the surfaces are structurally sound.
The basic components include an air tank, a fast-acting valve with a trigger mechanism, and a nozzle that distributes the air in the desired pattern to clear the accumulation most effectively. The sudden blast of air released by the valve on an air cannon is directed through a specially designed nozzle, which is strategically positioned in the chute, tower, duct, cyclone or other location.
Often installed in an array of several air cannons and precisely sequenced for maximum effect, the devices can be timed to best suit individual process conditions or material characteristics. The air blasts help break down material accumulations before they become problematic, allowing materials to resume normal flow.
To customise the air cannon installation to the service environment, specific air-blast characteristics can be achieved by adjusting the operating pressure, tank volume, valve design, and nozzle shape.
In the past, when material accumulation problems became a recurring issue in hard-to-access areas like the pre-heater system, processors would have to either limp along until the next scheduled shutdown or, more likely, endure expensive unplanned downtime to lance the clogged buildups. That used to cost businesses hundreds of thousands of dollars per day in lost production.
Nowadays, such is the importance of keeping material flowing that vessels of all kinds now feature mountings so that devices such as air cannons and vibrators can be fitted safely with relative ease.
Think clean
Given the demands and complexities of cement production, material clogs in silos, hoppers and other vessels should be banished with engineered solutions such as vibration, air cannons and other flow aids. The key to success lies in selecting the appropriate flow aid technology based on material properties, vessel design and operating conditions.
Moreover, the proactive and preventive use of these solutions, rather than reactive measures, has been shown to help maintain optimal performance and can even extend the lifespan of storage and transfer equipment. As cement plants continue to prioritise efficiency and worker safety, investing in well-designed flow aid systems is a necessity. By embracing such innovations, producers can achieve smoother operations, maximise throughput, and create a safer working environment, ensuring that materials move through the process without unnecessary interruptions. AB
