Belt conveyor stops abrasive glass from doing damage

Severe wear and high energy costs prompt a company to replace a pneumatic conveyor with an enclosed belt conveyor.

Gullfiber AB-part of the SaintGobain Group of France- makes glass wool at a factory
near Helsingborg, Sweden. The glass wool is used as insulation in buildings and automobiles.

For many years, Gullfiber made glass wool using virgin minerals, including sand, soda, lime, dolomite, feldspar, and borax. But in the early I 990s, the company reduced its use of virgin minerals in favor of cullet from recycled glass bottles. Today, about 75 percent of Gullfiber's glass wool is recycled bottle cullet.

Using cullet in place of virgin minerals conserves natural resources and reduces waste. It also saves energy because Gullfiber's furnace melts the cullet more easily than it does virgin materials.

This mixture of recycled glass and minerals (top) is melted in a furnace and fiberized to become glass wool (bottom).

Abrasive glass destroys pneumatic conveying lines

When Gullfiber began adding recycled glass to its product, the company used two pair of pneumatic conveying systems. Each pair of conveyors connected one of two batch mixers to short-term storage silos. Material was then fed from the silos to one of two furnaces. Using two pneumatic conveyors to link each mixer and silo provided flexibility. If one conveyor broke down, the other conveyor could operate in its place until repairs were made. Each set of conveying lines was 200 meters long, a total of 800 meters.

Until the company added recycled cullet to the batches, the pneumatic eonveyors worked well. "We didn't have any problems with them," said Magnus Gronborg, the head of process development at Gullfiber. "But then we started using a lot of recycled bottle cullet, and we created a problem."

The problem was abrasion. As the cullet moved at high speed through the conveying lines, it ate away at the pipeline interior, similar to the action of a sandblaster. The result was frequent breakage of the conveying lines, which caused the material to spill out of the conveying lines onto the roofs of buildings and onto the ground. The conveying lines broke so frequently from abrasion, Gronborg said, that the company had to assign two full-time maintenance workers to repair them. Additional maintenance workers were kept on standby over the holidays.

Company fights back with ceramics and new pipe bend

To reduce the frequency of the breaks, the company installed ceramic linings in the pipelines. But the cullet quickly wore through them, too. The company also designed and constructed a special 90-degree bend. The bend was designed to allow the material in the pipeline to form a cushion at the bend. The new design "improved the life of the bends incredibly," Gronborg said, but they were also expensive to make and didn't provide a permanent solution.

The belt conveyor ascends to a storage silo at a 9-degree incline. Tests showed that the conveyor could operate at a 30-degree incline without causing the material to slip backward.

A sure way to reduce the abrasion problem, Gronborg said, was to decrease the amount of cullet in the batches. But that was not an option because the company was committed to recycling waste glass to conserve natural resources and save energy.

In the meantime, maintenance workers struggled to keep up with the breaks in the conveying lines. In 1 year, Gull-fiber replaced 2,400 meters of conveying pipeline due to abrasive wear. That's equal to three times the total length of the factory's conveying lines.

In addition to the breaks, the energy needed to make compressed air for the pneumatic conveyors inflated the company's operating costs. The cost of energy to make compressed air for one pair of pneumatic conveyors for 1 year was 550,000 Swedish krona (about US$71,500). The high energy cost, coupled with the high cost of frequent repairs, was "very frustrating," Gronborg said, and "a big waste of money." It was time to consider other conveying options.

Enclosed belt conveyor offers a solution

At about this time, a Gullfiber maintenance worker told Gronborg about a belt conveyor that he had seen. The equipment was different from most other belt conveyors. Instead of transpoiling material on a flat belt, this conveyor uses a belt folded in half to form a pear-shaped bag. Rubber edges with steel cables running the length of the conveyor belt provide profiles for carrying the belt. And once the belt is folded, one profile is above the other. Thus, the cables have the same radius in curves and the belt can make 180-degree turns.

When loading the belt with material, deflectors open the belt into a U-shape, allowing material to enter. To unload material, the belt opens, folds out, and runs flat over two deflection pulleys. After the material is discharged, the belt refolds itself and starts its return run.

The design of the enclosed belt conveyor appealed to Gronborg, so he invited the belt conveyor supplier to the Helsingborg factory for an assessment. "We asked how they wanted to solve our problem, and they had some good ideas," Gronborg said. Next, Gronborg compared these ideas with proposals that he received from other conveyor suppliers. But these later proposals weren't as good, and Grnborg again contacted the first supplier. He offered them the job of replacing two pneumatic conveying systems with an enclosed belt conveyor.

One factor that favored the first supplier was its plan to install the conveyor without much change to Gullfiber's buildings. In order to preserve the buildings, however, the conveyor would have to operate at an incline because Gullfiber's batch mixers are 5 meters below ground. And the silos are 30 meters above ground. To ensure that the batch wouldn't slip backward during transport, the conveyor supplier conducted tests. The tests showed that Gullfiber's material could be conveyed at a 30-degree angle without slippage. To be absolutely sure no slippage occurred, the maximum incline was specified at 25 degrees.

Next, the conveyor supplier calculated the required size of the conveyor. The main concern in sizing the conveyor was the relatively small capacity of the mixers (2,000 kilograms each). To prevent the mixers from creating a production bottleneck, the conveyor was designed to empty the mixers in 2 minutes, which is equal to 60 tons an hour. That capacity was plenty, Gronborg said, considering that the furnace handles 140 tons per 24-hour period. The other pneumatic conveying systems would continue to operate as before for the time being.

During transport, the conveyor belt folds to create a pear-shaped pouch.

New conveyor cuts costs and improves production

The new enclosed belt conveyor began operating at Gullfiber in July 1996. Conveying starts at the batch mixer, where vibratory feeders discharge the material into the belt's U-shaped pouch. The vibratory feeders keep dust to a minimum. They also resist wear. (In other applications, Gull-fiber has tried using screw conveyors, but abrasion damages them.)

Once loaded with material, the conveyor angles upward at 25 degrees from the batch house. When the conveyor is clear of the batch house, it changes its incline to 9 degrees as it ascends 130 meters toward the storage silo. Once there, the belt opens, discharges the load, and starts its return run.

Since installing the conveyor, Gullfiber has noted several improvements. Reduced maintenance was the biggest improvement. "We just lubricate it every third month. That's it," Grnborg said. "In comparison to the other situation we had, this conveyor is heaven." Another improvement is lower energy costs. Instead of spending 550,000 krona annually to make compressed air, the company now spends only 10,000 krona to power the conveyor drive. Taken together, these lower costs mean that the conveyor will pay for the cost of installation in 5 years.

In addition to cutting costs, the new conveyor improves the process of making glass wool. That's because the belt conveyor doesn't cause the batch ingredients to segregate, or deblend. In pneumatic conveying, the ingredients segregated during transport, disrupting the random mixture of particles. In the new operation, the mixture lies still once placed on the belt. The result is a more predictable mixture of particles, which leads to a more uniform and reliable reaction in the furnace.

The success of the new conveyor has led many people within Saint-Gobain to visit the Helsingborg factory. In fact, some Saint-Gobain factories are already installing identical conveyors. Although Gronborg said that he likes telling company colleagues about the installation, he enjoys the success of the installation even more. That's no surprise: Installing the belt conveyor was his first big project since hejoined Gullfiber 3 years ago. What would he do if he had the chance to lead the project again? "I wouldn't do anything different," he said. "We're really satisfied with this one."