California Portland Cement Co. serves the California, Arizona, and Nevada markets with plants in Colton and Mojave, Calif., and in Rillito, Ariz. The company also operates several terminals in these markets, and can import product through its Los Angeles Harbor import facility. The Colton plant has been in continuous operation since 1982 and is geographically the closest cement plant to the metropolitan Los Angeles area. To remain economically viable over the years, the plant has been upgraded and improved a number of times.

The Colton plant is rated at about 800,000 tpy, based on clinker production of 720,000 tpy. The primary fuel is coal, and the original quarry adjacent to the plant site continues in operation. The facility has two long dry kilns and is staffed by 85 hourly and 35 salaried workers.

The facility's design provided greater raw and finish grind capacity than clinker capacity. This has proven fortuitous since it enables the plant to carefully manage its electrical energy consumption. Southern California is a high-cost energy market, and the plant operates on a real-time pricing contract for electricity. This contract is vital to lowering energy costs, but the plant must shed electrical load when required to do so. The plant has an average electrical energy cost of about $0.04 per kWh. Plant personnel estimate that if they were not able to manage energy use, the average cost could be as much as $0.07 per kWh.

Colton has maintained its economic edge through several means. Both kilns are equipped for tire burning with mid-kiln injection facilities. In addition to Type II cement, the plant produces a number of specialty products, including Fast Set, High Early, Block, and Plastic cements. As part of its raw material mix, the plant has learned to use waste materials from other industries, such as oil refinery catalyst fines, filter cake from breweries, and waste from sand blast operations. Major emphasis also is placed on finding solutions to high equipment maintenance cost and improving system reliability.

One of the high-cost areas that plant personnel were determined to improved was the pneumatic conveying system used to transport raw meal to the kiln feed blend silos. The objective was to reduce the high maintenance cost associated with the existing screw pump systems and improve system reliability. Significant effort was expended to reduce cost with existing equipment by improving maintenance techniques and incorporating equipment modifications, with little actual improvement. Not only were the existing screw pump systems high in cost, but the unpredictable failure pattern of the screw pumps also made it difficult to include the raw mill operation as part of the energy management program. This was in spite of the fact that the raw mills need only work 70% of the time to supply the kiln. Plant workers decided to use a different approach.

Dual-pod conveying In 1996, the Colton plant selected a dual-pod conveyor system to replace the screw pumps. The first system came on line in August 1996 and the second followed in July 1998. Both systems were supplied by MODCO Conveying Corp.

The key to this company's type of pneumatic conveying, whether it be with pod conveyors or screw pumps, is its gas-solids injector (GSI) technology. This method results in heavily loaded, or high-density, dilute-phase conveying. In dilute-phase conveying, material and air move through the convey line at about the same speed (above minimum convey velocity) so that the material never drops out of the convey steam. The GSI pod conveyor systems are not dense phase and bear no operational resemblance to the old-style "pressure tank or pressure pot" equipment.

To achieve high capacity and efficiency, the GSI is force fed and operates in a chamber pressurized by the convey air. With pod conveyors, the pod vessels serve to force feed the GSI device. Less than one-third of the system-convey air passes through the pod vessels. The bulk of the compressed air enters the system through the pressurized GSI chamber. The system is closed, so blow back is not possible.

By the discharging action of the pod vessels, the material is forced into the inner GSI chamber where it is mixed with some convey air to lubricate the inner walls of the chamber. This high momentum mix of air and material then leaves the inner GSI chamber and enters the outer GSI chamber in the central core. This core of material is surrounded by a circumferencial layer of air that reaches velocities to 35,000 fpm. This velocity is proportional to the pressure in the GSI chamber. To achieve this velocity, as much as 40% of the total system pressure is used to pressurize the GSI chamber. The material air stream is then injected into the convey line in a near laminar flow pattern.

The dual-pod conveyor operates on a programmable logic controller-based timing program. In this application, there is no need for level sensors inside the pod vessels. Maximum conveying performance is achieved by extending system cycle time so the pod vessels are being nearly filled in every cycle. Depending on the system design, the total cycle time can vary between one minute to nearly three minutes. The objective is to achieve the longest cycle time in a given system. Extending the cycle time so that full fills are achieved results in maximum convey rate and favors valve maintenance.

The force-fed GSI technology precludes the requirement for high operating pressure systems. For a reasonably well-engineered, fully loaded, dilute-phase convey line, total system pressure will rarely exceed 35 psi. In short distances (to 500 ft), system pressures will rarely exceed 20 psi.

Pod conveying takes place in a cyclical pattern. System pressure is greatest during the discharge cycle. As the pod empties and the material is well down the convey line, system pressure drops off momentarily before the discharge of the second pod vessel starts. If the system is properly tuned, the valves operate at the momentary point of lowest system pressure, thus extending valve life.

In many pod conveyor applications, high-quality, split-bodied, resilient-seated butterfly valves provide adequate service. The seats of these valves are easy to change, and if regular periods of down time are available, this is a relatively minor preventative maintenance procedure. For higher-pressure, longer-distance systems, high-temperature (kiln dust), or if very abrasive material (clinker dust) is to be handled, then consideration is given to the use of metal-seated rotating sliding disc-type valves.

Case study Raw Grinding System The basic machinery-Two identical raw grinding circuits:

* Ball mill, 13-ft diam Yen 17-ft long; connected power is 1,600 hp.

* Parallel air separator system with two, 16-ft Sturtevant separators.

* dual-pod conveyor, Model DP-10-200 incorporating two, 100-cu-ft vessels. These units operate with split-bodied, resilient-seated butterfly valves throughout. No connected hp.

* A C175 and a C300 sliding vane compressor operate in parallel, providing 2,450 intake acfm, and a pressure capability of 35 psi. Total connected power is 400 hp.

* Rated capacity of each mill is 100 tph.

The product:

* The raw meal is 75% minus 200 mesh, and is relatively abrasive due to free silica content.

* Active bulk density is about 500 lb per cu ft.

The pneumatic conveying application:

* Effective convey distance is about 1,000 ft (the sum of 870-ft actual convey distance, 10 convey line bends, three diverter valves, and an inclined section of convey line).

* Existing 10-in.-diam convey lines.

* Average system operating pressure is 18 to 23 psi using about 310 of the 400 connected compressor hp.

Project development Prior to 1996, each raw mill was supported by a Model 250-M screw pump. Persistent maintenance problems were characterized by the requirement to rebuild each pump about eight times per year. Each screw pump initially operated with a single C300 compressor, and in 1995 each system was upgraded by the addition of a C175 compressor.

This did improve performance but had little impact on the basic screw pump maintenance problem. Also, shutdowns could not be anticipated and repairs often were required under crisis conditions. The screw pump systems operated at an average system pressure of 23 to 28 psi.

The inability to predict repair events to the raw mill pneumatic conveying equipment greatly reduced the plant's ability to effectively include the raw mills in the energy management program. Also, efforts to fine tune other aspects of the milling system were hindered by the unreliability of the system.

The existing compressors were used in the new dual-pod conveyor installation, and the energy savings per dual-pod conveyor were achieved by the elimination of the 125-hp screw pump motor. The installed cost for the project was budgeted at $100,000, and the predicted pay back was 1.4 years.

Results The first dual-pod conveyor was installed in August 1996. Initially, it was hoped that one system could handle the production of both raw mills through a single 10-in. line. However, the DP-10-200 achieved a capacity in the range of 170 to 180 tph through the existing 10-in. convey line.

After a few months, the decision was made to dedicate the dual-pod conveyor to just one of the raw mills. The conveyor was operated with the same combination of a C175 and a C300 providing the convey air, and it was able to handle the 100-tph production rate.

Problems encountered included handling the oily compressed air from the sliding vane compressors, which tended to plug up the dual-pod cone aeration jet system. This was resolved by increasing the size of the jets and by paying closer attention to the oil-removal system, which essentially consists of a drop leg early in the air delivery system and a second drop leg just before the air enters the dual-pod conveyor.

The plant experimented with various butterfly valve types and seats and concluded that high-quality, split-bodied butterfly valves with EPDM seats provided the best results.

The plant is now able to predict maintenance, so system shutdowns due to the failure of the pneumatic conveying system no longer occur. When a leaking valve is discovered, the repair usually can be delayed to a scheduled down time. In the worst case, it can be changed immediately with little lost time.

A second DP-10-200 was installed in July 1998. The plant continued to fine tune the systems, made mechanical changes to ease the task of changing valve seats, constructed walkways to provide access to the various valves, and installed a dedicated high-pressure air compressor for the pneumatic valve operators.

Currently, the plant has settled into a 21-day preventative cycle for replacement of the valve seats for the fill, vent, and discharge valves (six total per dual-pod conveyor). The time required to change these six valve seats on each dual-pod conveyor is about four to six hours. On an annual basis, all valves and actuators are replaced. The air supply butterfly valves on the clean air side of the system rarely require maintenance, but they are nonetheless replaced annually to ensure system reliability. The entire system is fully integrated into the central control facility, and the control room operator can determine the status of the system.

In addition to the direct cost savings of the dual-pod conveyor system, a number of intangible benefits were realized:

* The unscheduled and unpredicted shutdowns of the raw mill pneumatic systems were eliminated;

* The dual-pod conveyors operate on average in the 18- to 23-psi range, or about 5 psi lower than the previous screw pump systems, with a corresponding reduction in brake horse power for the compressors;

* With the stability in the pneumatic conveying side of the raw mill operation, the plant has been able to fine tune the raw milling system, achieving incremental increases in production rate;

* The plant can now effectively incorporate the raw grinding department in the energy management program; and

* The screw pump systems required vibrating screens in the feed system to each pump. These vibrating screens, which were difficult to maintain, were eliminated since the installation of the dual-pod conveyors.

No status quo The installation of the dual-pod conveyors to support the raw grinding mills at California Portland Cement's Colton plant has met the performance and economic pay back expectations. Plant personnel refused to accept the status quo of high maintenance and energy costs and took the risk of employing new technology.