Martin Engineering global flow aids manager Brad Pronschinske gives Australian Mining insights into how engineered flow can solve problems at materials handling operations.
In order to achieve confined and consistent flow on conveyors handling large volumes of bulk material such as coal, aggregate or biomass, transfer chutes and vessels must be designed to accommodate and facilitate the flow of the cargo they will be handling.
But even if the operating conditions are expected to be ideal, many engineers include flow aid devices in new designs to ensure delivery of the specified results and deal with changes in bulk material properties.
Designing a conveyor and chutework that would handle every material situation is virtually impossible. Materials with high moisture content can adhere to chute or vessel walls or even freeze during winter temperatures.
Continuous operation can serve to compress the material, and physical properties often change due to natural variations in the source deposits, changes in suppliers or specifications or if the material remains stagnant in storage for a long period of time.
At times, the system can become completely blocked by just a small change in any of these parameters. To overcome these issues, a variety of devices collectively known as flow aids can be employed.
Where are flow aids?
As the term implies, flow aids are components or systems installed to promote the transport of materials through a chute or vessel. Because they will affect a conveyor’s loading, flow aid devices can also impact spillage and dust.
By designing active flow aids into a conveying system, the operation gains a level of control over the material that cannot be obtained with static approaches (such as low-friction liners) alone.
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 chute 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 the discharge is open and material can flow as intended.
If used when the discharge is closed, the energy of the flow aid may pack the material more tightly, making flow more problematic when the discharge is opened and potentially causing damage to the chute or bin.
The best practice is to use the flow aid as a preventative solution to be controlled by timers or sensors to prevent material build-up, rather than waiting until material builds up and restricts the flow.
Using a flow aid device in a preventive mode saves energy, reduces noise and improves safety, since the flow aid runs only when needed while still reducing build-up and plugging.
One solution for managing material accumulation in chutes and vessels is the low-pressure air cannon, originally developed and patented by Martin Engineering in 1974.
Also known as an “air blaster,” it uses plant compressed air to deliver an abrupt discharge to dislodge the build-up.
Cannons can be mounted on metallic, concrete, wood or rubber surfaces. The basic components include an air reservoir, fast-acting valve with trigger mechanism and a nozzle to distribute the air in the desired pattern to most effectively clear the accumulation.
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, tower, duct, cyclone or other location.
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.
In many applications, an engineered firing sequence will relieve the build-up problems. The air blasts help break down material accumulations and clear blocked pathways, allowing solids and/or gases to resume normal flow.
In order 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 an issue, processors 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 proactively include the mountings in new designs so that future retrofit can be done without hot work permits or extended downtime.
A new process has been developed for installing air cannons in high-temperature applications without a processing shutdown, allowing specially-trained technicians to mount the units on furnaces, preheaters, clinker coolers and in other high-temperature locations while production continues uninterrupted.
The patent-pending technology is designed to dramatically reduce expensive downtime associated with traditional installation methods, which require that high-heat processes be halted to allow core drilling and mounting of the cannons.
This new approach allows technicians to add air cannons and nozzles to an operation while it’s in full swing, without disrupting the process.
It’s been proven in dozens of installations to date, helping high-temperature processes maintain effective material flow and minimise shutdowns, improving efficiency while reducing lost production time.
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.
However, 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.
A better solution is the application of engineered vibration, which supplies energy precisely where needed to reduce friction and break up the 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, bunkers, screens, feeders, cyclones and heat exchangers.
Vibrators perform the same function as thumping on the outside of a bottle of ketchup: they reduce the cohesion between the material particles and the adhesion between the particles and the container wall to increase the material flow.
The relationship between the bulk material and the optimum vibration frequency to stimulate that material is proportional to particle size.
As a general rule, the smaller the particle, the better it responds to higher vibration frequencies.
Vibrators activate the material inside a chute or bin by energising the outside of the structure’s steel walls and transmitting vibratory waves into the bulk solid.
The earliest form of vibration was a hammer, as the act of pounding on the chute or bin wall overcomes the adhesive force between the material and the wall surface.
However, this hammering on the bin or chute wall often leads to damage of the surface. Eventually, the small indentations that result – sometimes called “hammer rash” – will exacerbate the problem that the hammer blows were intended to overcome.
In addition, the swinging of a heavy hammer poses the risk of potential injury to plant personnel.
Linear vibration is the best solution for sticky, coarse, high-moisture materials. A convenient test is to take a handful of material and squeeze it into a ball.
If the material readily remains in the ball after the fist is opened, linear vibration is probably the best choice.
In contrast to linear designs, rotary vibrators create a vibratory force through the rotation of an eccentric weight, which creates a powerful vibration much as a household washing machine does when its load is off-centre. They supply an energy best suited to moving fine, dry materials.
Rotary vibrators can be powered pneumatically, hydraulically or electrically, with the choice for a given application typically determined by the energy supply most readily available at the point of installation.
Rotary vibrators are available in a wide range of rotational speeds and force outputs to match the specifics of each application. In addition, many rotary vibrators can be adjusted by altering the overlap of the eccentric weights to increase or decrease the amount of unbalance and deliver the desired amount of force or by varying the RPMs.
Vibration can induce stress into metal structures, and the walls may need to be reinforced at the point(s) of application.
They are typically installed on a mounting plate or channel that spreads the vibratory energy (and the weight of the device) over a larger surface area. They can be controlled automatically or manually, allowing use only when needed.
Once installed, a vibrator should be “tuned” by adjusting its force and/or speed to give the optimum effect for each application.
Because flow-aid devices often use compressed air or other energy sources that can create a stored energy hazard, it is critical to follow lockout / tagout / blockout / testout procedures.
Even though build-up in a chute may still be in place, its hold on the chute wall might be weakened to the point that a slight disturbance during maintenance can cause it to fall.
There is also an electrical shock hazard when working on the control systems. To prevent the possibility of remotely energising devices during maintenance and testing, appropriate safety procedures must be in place to prevent unintended actuation.
This article also appears in the October edition of Australian Mining.