Brad Pronschinske, Global Flow Aids Manager for Martin Engineering, explains how different flow aids can be used to resolve a number of bulk material handling issues.
To achieve a consistent flow, conveyors, chutes and vessels handling large volumes of bulk material must be designed to accommodate the cargo they will be handling.
Designing a conveyor and chutework that can handle every material situation is virtually impossible. Materials with high moisture content can adhere to walls or even freeze during winter temperatures. Continuous operation can compress the material, and physical properties often change due to natural variations in the source deposits. At times, the system can become completely blocked by just a small change in these parameters.
To overcome these issues, a variety of devices collectively known as flow aids can be employed.
What are flow aids?
Flow aids are components or systems installed to promote the transport of materials through a chute or vessel. Because they 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. This is because the operation of these devices can create potentially damaging stress on the structure.
The best practice is to use the flow aid as a preventative solution to be controlled by timers or sensors to prevent material buildup, rather than waiting until material builds up and restricts the flow. Using a flow aid device in this way saves energy, reduces noise and improves safety, since the flow aid runs only when needed while still reducing buildup and plugging.
Air cannons
One solution for managing material accumulation in chutes and vessels is the low-pressure air cannon. Also known as an ‘air blaster’, it uses compressed air to deliver an abrupt discharge to dislodge the buildup. 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.
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.
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Air cannons can be customised for specific service environments, with different air blast characteristics created by manipulating the operating pressure, tank volume, valve design and nozzle shape.
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.
The 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.
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 application of engineered vibration, which supplies energy precisely where needed. This reduces friction and breaks up the material to keep it moving to the discharge opening, without damaging the chute or vessel.
Vibrators reduce the cohesion between the material particles, and the adhesion between the particles and the container wall to increase the material flow. The devices 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.
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.
Vibration can induce stress into metal structures, and the walls may need to be reinforced at the point(s) of application. Vibrators 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.
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 buildup 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.