Careful attention to variable frequency drive considerations pays off in major redesign of cement plant.
Lone Star Industries redesigned its cement plant with one mission in mind—to increase production (Photo), without compromising safety, product consistency, on-time deliveries, or product quality. Variable frequency drives (VFDs) played a large part in the conversion that enabled the Greencastle, Ind., cement plant to nearly double its production levels from 750,000 tons per year to 1.3 million tons.
Lone Star boosted throughput by converting the nation’s largest wet-process plant to its first-ever semi-dry process plant. This involved shortening the kiln from 580 feet to 255 feet, and adding a 322-foot one-stage preheater tower with an in-line calciner, a dual de-dusting cyclone, and a hammermill dryer.
George Glassburn, the plant’s electrical superintendent, says Lone Star upgraded many of its motors to higher efficiency motors and installed 40 variable frequency drives on key motors throughout the entire plant. “These are key pieces of equipment that we can’t stand to fail,” he says. “Any one piece in a sequence of electrical equipment can take us down.”
In this plant, AC variable frequency drives power motors that range from 3 hp (feeder weights) to 800 hp (the ID fan drawing air through the kiln into the stack). Six 150 hp drives control the slurry pumping stations and feed pumps into the kilns, while two 600 hp drives power the primary air fan and baghouse cooler exhaust. VFDs also control the primary OSEPA exhaust fan (Photo 2) and a 200 hp drive controls the OSEPA separator in the finish mill. The plant uses smaller motor controllers on fuel-feeding applications, such as the alternate fuel pump and agitator, as well as the feeder from the primary rock crusher to the conveyor at the quarry site.
Drive strategies that work. With all of these drives going in, how can Greencastle’s people keep up? Two things really work in their favor. First, Ralph Tennis, the plant’s chief electrician, says the drives are easy to install, set up, and start up. Secondly, the plant relies on a local distributor to handle drive start-ups for the facility. “If I put anything in, I won’t energize it until the technician says it’s hooked up right,” he says.
Consider the electrical environment of a cement plant. Part of the process involves crushing rock. For an across-the-line motor, that means significant disturbance of the distribution system. You might think adding an electronic drive would worsen the situation by adding harmonics, but with the drives Lone Star selected, this is not a problem. The drives are at unity power factor and are harmonics-corrected. But what about the static electricity this kind of process generates? The Lone Star engineers protected the company’s investment in people and equipment through diligent application of the bonding and grounding required by Art. 250.
Correct sizing of loads helped the engineers reduce the necessary number of motor and drive models. “We size motors to the largest horsepower an application requires, and then we slightly oversize the motor,” Tennis says. That strategy allows Lone Star to specify the same motor to handle the variety of loads within a process, such as feeder weights or slurry pumps.
The plant extends this specification strategy to the motor drives as well. “We use the same type of drives to control these motors across different functions,” Tennis says. Such standardization provides multiple benefits. For example, the electrical technicians need training once to operate drives throughout the plant—reducing training costs and mistakes. Another important benefit is that the reduction in spare parts count reduces administration and storage costs while improving the availability of spare parts, should something fail.
One key to the plant’s success is extensive planning for maintenance and service throughout the year. “We know a little preventive maintenance goes a long way in providing us that uptime,” Glassburn says. “We schedule our downtimes, pick and choose them sparingly throughout the year—and plan our work. That includes getting in a maximum of maintenance and service routines for the electrical equipment in the shortest possible time.”
This kind of smart management within a cement processing plant focuses both on trouble-free uptime and using energy as efficiently as possible to contain costs. “We have more than 19,000 hp on line,” Glassburn says. “That eats up kilowatts and costs you money, even here, where we enjoy competitively priced electricity. If you save 1% of your consumption a year, that is a significant savings. If you do that with a higher-efficiency motor, putting in a VFD that controls a motor right to the rpm you need, that saves energy.”
All on the upside. A high-performance drive controls the 900 hp motor that rotates this shortened kiln at 3rpm—three times faster than it rotated in its previous configuration. For the first time, the plant also has medium voltage technology—a 5,000 hp drive and motor to power the new ID fan in the one-stage preheater.
Lone Star views the timing to be right for increasing capacity. The Greencastle, Ind. facility will be a long-term producer, with the largest (and lowest cost) plant in the market—able to address the needs of a thriving economy.
Tuch is a Senior Engineer, ABB Automation Inc., Drives & Power Products Group, New Berlin, Wis.