Martin Engineering’s global air cannon product manager Brad Pronschinske explains how new air cannon technologies can minimise lost production, reduced revenue, employee morale issues in cement production.
Raising the temperature of the cement meal to ensure it doesn’t cool the kiln when it arrives saves on energy consumption, reduces the calcining time and promotes plant efficiency.
Within seconds before entering the kiln, material fed into the preheater at a temperature as high as 93ºC is heated to a scorching 815ºC.
However, no matter how many stages the preheater has, material can quickly adhere to the coarse refractory wall, build up and clog the system. Bottlenecks in the flow leads to expensive unscheduled downtime.
Many operators mitigate clogs by implementing regular cleaning schedules, assigning workers with water lances extended through access holes.
A worker ascends the tower and dons a suit of high-heat personal protective equipment (PPE). Lancing the material with high-pressure water clears the blockage, and the workers restore proper flow.
Unfortunately, this procedure is always done while the preheater is still in operation, causing a tremendous amount of heat and some molten material to blow back.
The PPE, the heat and the safety issues make preheater cleaning one of the most unpleasant jobs in a cement plant.
“To improve safety and increase efficiency, virtually all cement plants in the USA have air cannons installed on the preheaters,” Martin Engineering US air cannon business development manager Mike Moody said.
“Over the years, we’ve worked to improve the technology to the point where both installation and maintenance can be performed safely without a shutdown or exposure to intense heat.”
Preheater flow
Today’s preheater tower designs can have as many as six stages in towers up to 12 storeys tall. The meal flows down a chute to a splash box, proceeds to the airflow of the riser duct, where it gets heated and progresses to the next stage, and the procedure happens all over again.
One of the biggest contributors to material buildup is high heat and material velocity. As material gets hotter, it gets stickier, clinging to the sides of the flow chutes and splash box, as well as in the riser duct. If left unchecked, clogs can form quickly, stopping the material flow, which leads to unscheduled downtime and lost production. Large buildups can even completely block the outlet of the cyclone.
Whenever operators open the access door, this lowers operating temps. Cold water also reduces temperature, and injecting water into the hot tower creates steam, which can result in dangerous steam.
A better alternative is a series of low-pressure air cannons.
Air cannon nozzles are strategically positioned in the tower, riser duct or cyclone. As adhered material is dislodged, it returns to the flow, with the pressurised shot facilitating the flow and enhancing efficiency.
New technology has even 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.
Sequenced shots
The basic components of the air cannon include an air reservoir, fast-acting valve with a trigger mechanism and a nozzle to distribute the air in the desired pattern to most effectively clear the accumulation.
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.
Tanks and valves
To facilitate maintenance without a process shutdown, air reservoirs with volumes typically ranging from 35L up to 150L are now outfitted with a rear-facing valve that can be removed without dismounting the tank.
Today’s fast-acting valves can release the tank volume in a fraction of a second, creating a high-magnitude force at the exit nozzle that’s installed through the wall of the vessel or duct.
The new designs feature a hybrid valve concept that provides more force, uses less air and simplifies maintenance in challenging applications with limited budgets.
Positive-firing valves that respond to an air pressure surge delivered by a solenoid have also made air cannons safer. Unlike negative pressure-firing designs, a cannon equipped with this type of valve will not discharge accidentally in response to a drop in pressure, so an air supply failure or broken line cannot trigger it suddenly.
The high-speed valve design is mounted on a smaller air reservoir, delivering higher discharge forces than less efficient valves on larger tanks.
The new generation of valves produces about twice the blast force output of previous designs, saving energy by using about half the compressed air volume.
Nozzles and service
Another innovation has been the Y-shaped assembly that allows the nozzle to be maintained or replaced without removing the tank or disrupting the refractory.
The system allows technicians to mount the units on furnaces, preheaters, clinker coolers and in other high-temperature locations while production continues uninterrupted.
It dramatically reduces downtime associated with traditional approaches to installation, service and replacement, which require that high-heat processes be halted to allow core drilling and mounting of the cannons.
“Avoiding unplanned shutdowns is key to a plant’s profitability,” Moody said. “Any shutdown other than scheduled maintenance is production time that can’t be recovered”.
“Once that production time is lost, it can’t be made up again. With the core drilling option and Y-pipe, installation and maintenance can be done during normal production.”
The nozzle is the component that takes the stored energy in the form of compressed air and directs it to do work, so it has a dramatic effect on the performance of any air cannon.
The application dictates the type of performance needed, including the shape of the air blast or length of the plume. A primary goal in every application is to use the stored energy as efficiently as possible.
A series of retractable air cannon nozzles was developed specifically for high-temperature applications, extending into the material stream only during the firing cycle to protect the nozzles from extreme temperatures and abrasion.
These “smart” nozzles allow the cannon and nozzle to be installed independently, so the nozzle can be accessed for inspection or service during production, without stopping the process or removing the cannon.
The new design solves two common industry problems: dislodging accumulations in hard-to-reach areas without shutdown or manual labour, while significantly extending nozzle life.
Further, the units can be serviced from outside the vessel without disturbing the refractory, reducing potential damage while minimising service time and risk of injury.
Conclusion
Flow through the preheater tower takes only seconds, but a clog can cause hours or days of downtime that can’t be made up, resulting in significant losses (production time that can’t be recovered).
Prevention and maintenance can be a gruelling task that leads to increased labour costs. Thanks to new air cannon technologies, the lost production, reduced revenue, workplace safety implications and employee morale issues can be effectively minimised.
“After calculating the costs saved by avoiding downtime, this technology pays for itself quickly,” Moody said.
“The savings are compounded by reduced maintenance, improved safety, lower labour costs and greater production efficiency.”