If safety was the feature that the San Leandro, CA, Georgia Pacific Gypsum Paper mill was most focused on through an electrical retrofit of its traditional steam-turbine-driven paper machine, increased throughput, reliability, control, and reduced downtime are added benefits the mill has realized.
"Yes, we needed more capacity in the mid-to-late 90s, when this conversion was planned and installed," said Fred Curcio, plant manager, "but the company, as a matter of policy and objective, is committed to ensuring that all its mills would be operated as safely as possible." San Leandro, he noted, has operated the past decade without any lost-time injuries, a record he wants to extend.
Built in 1952 and acquired by G-P in 1996 from Domtar Gypsum, the San Leandro mill is one of four G-P Gypsum paper mills integrated into the company's vertical gypsum operation -- from mining rock to paper and final wallboard production. Based just south of Oakland, the 400 ft long mill sources all of its own corrugated and flyleaf wastepaper from brokers for mill processing, and supplies 8-ply face- and back-side paper to G-P Gypsum board plants in Tacoma, Long Beach and Las Vegas, in addition to mills in Texas and western Canada.
The mill's steam turbine -- part of the original construction -- powered the paper machine through a traditional single line shaft, giant cone pulleys, belt drives, and gear boxes. The machine's maximum capacity was 750 fpm. "Reliability and safety become the issues," according to Curcio. Turbine failures, brought on by years of heat and wear, create downtime, and turbine failures are catastrophic. Line shafts can pretzel if the governing mechanism on the turbine-driven speed control fails or malfunctions. Record demand for paper throughout the 90s exposed these vulnerabilities and their costs, Curcio noted, not only at his plant, but also at any mill trying to maximize output.
G-P Gypsum's acquisition of the mill and completion of replacing the 50s technology with motor AC Variable Speed Drives, AC motors, and new gearboxes not only turned these concerns into mill assets, but also pioneered a first for the company and the paper industry: encoder-less operation of all the new sectionalized drive systems.
Electrical Retrofit Completed in Stages
Transformation to electrical control of the paper machine began in the mid-90s, when systems integrator Intec Solutions added ABB 100 and 150 hp drives and motors, respectively, to the second and third presses. A second phase included the addition of a 350 hp drive and a helper drive to the main press.
"The drives, which were synchronized to the existing line shaft via encoders that provided feedback of line speed, freed up horsepower to the steam turbine so the whole paper machine could speed up," said Chris Hall, Intec president. This addition of the motor controllers also provided, for the first time, much more precise draw control, giving the mill the ability to run tighter draws with fewer paper breaks -- improving run-ability and product quality.
Multi-drive Solution Uses Dryers' Regenerated Energy
With G-P Gypsum's purchase in 1996, Curcio and Intec began to discuss a complete retrofit of the mill's finishing end, including the dryer section, calendar stack, and reel. "With the capacity already installed in the front end, we designed completion of the project for 1,000 fpm," said Curcio.
Intec provided an innovative electrical solution: build and install a multi-drive, featuring seven ABB AC motor drives, ranging from 40-125 horsepower -- and all connected to a common 700-volt DC bus supply. "This configuration made sense, because the high-inertia dryers close to the calendar stack often generate, not motor," said Chris Hall. "And inherent to a true common DC bus multi-drive is the ability for all drives to share energy within the system. So regenerated energy from the end dryers being pulled by the calendar stack can be used by those near the presses."
These high-inertia processes within paper mills also provide the benefit of a manufacturer being able to maintain power through a power dip from the utility -- "ride-through" capability.
When a true common DC bus multi-drive is powered up, no pre-charge arrangements are required; all the drives come up at the same time. The common bus is the power supply to each drive and eliminates the 3-phase supply used for each drive in a shared-bus configuration. Input power semiconductors and pre-charge hardware are eliminated from all drives, reducing hardware count and increasing reliability. And a dynamic brake chopper and resistor are built in to handle any emergency stops.
"Additionally, the true common bus eliminates the need for additional reactors in front of each drive, to provide filtration," said Hall. He noted that use of ABB's ACS 600 series' open-loop Direct Torque Control technology also eliminated the need for installing encoders on G-P's finishing end equipment. The DTC feature, according to ABB, enables each machine's motor drive to calculate the state (torque and flux) of the motor 40,000 times per second, making the drive virtually tripless.
Elimination of so much hardware in the construction of this type of Multi-drive means "a lot less engineering and manufacturing are required," according to Hall. "That makes the common DC bus system cost effective and attractive to paper producers. Installation costs via a multi-drive's shared power feed are on par with the equipment costs of standalone drives, each of which needs its own power feed," he noted. "Plus you get the long-term regenerative benefit."
Four-Day Retrofit
Installation of the multi-drive and standalone ACS 600 drives on the mill's dryers, reel, stack, and winder were planned out for three months, as part of a week-long shutdown. Intec took full responsibility for all mechanicals and electricals related to the turnkey installation, including the building of a motor control room that tapped into the 115 kV substation the mill owns and manages. Removal of existing line shaft components, gearboxes, pouring new motor bases, installing motors, and connecting all drive controls were mapped out in detail.
"It is not unusual to be in full production 24/7 for six to seven months with just a couple of hours scheduled for shutdown, so this was the most ambitious shutdown we'd ever had," said Curcio. As part of the downtime, he planned extensive maintenance and repair work round the clock for crews made up from the 78 employees at the mill.
The retrofit was completed in early March, 1999, in four days (Monday-Thursday), and the mill was making paper within two hours of startup. "We had tuning to do, but we had scheduled 48 hours for getting paper on the reel," said Curcio.
Encoder-less Operation
Encoders, those fragile glass disks wired to motors and providing critical feedback to traditional PWM drives, are the "weakest link" in the electrical chain, according to the experience of both Curcio and Hall. Encoders are required to outperform in hostile, harsh environments. If they fail or short out, the processing line stops until the specific encoder is identified and replaced.
G-P's retrofit of the finishing end of the line at San Leandro included no encoders. The use of ABB's DTC technology (incorporated into the company's large-horsepower drives series) eliminated this consideration. "Encoder-less operation of a paper mill is a quantum leap in simplification," said Mike Giraudo, vice president of Intec. "Mechanically, the encoders and wiring to them cease to be necessary; they're gone in one fell swoop." Two full years of round-the-clock production at the mill illustrates the dramatic possibilities of this technology, he noted.
Seamless, encoder-less operation of the finishing end led to the subsequent removal of encoders from the forming end, as well. Intec replaced the PWM drive on the third press with a DTC drive and then created a mini multi-drive between the second and third press. "The second press is operated in torque -- not speed -- mode at a set point," said Hall. "That sometimes generates energy, which the third press drive then can use."
The DTC drives -- a total of 13 on the paper machine -- have operated without a hitch, according to Curcio. "I don't think we've lost a drive motor in two years," he said. "We have a total of 35 VF drives operating anything in the mill that has a variable output."
Touch Screen Control
Such a retrofit can provide operators exceptional operational control of the machine. All drives and motors at San Leandro are wired to a master SAF controller platform, for which Intec has created proprietary software.
This programmable linear controller utilizes a processor with input cards and, via fiber optic connection, talks directly to the drives. The fiber optic cable is the I/O to the drives. SAF, in turn, reports to a SCADA program that provides historical trends of data (for troubleshooting faults or identifying operating problems).
Via SAF, all controls for the paper machine are created for easy understanding and use on touch screens -- installed as part of the retrofit. "This kind of linear controller makes it easy to create a control system to provide exactly what a mill wants," said Hall. "That could be touch screen or PC control; and the controller has the flexibility to integrate a wide platform of equipment vendors."
The precise control now makes it easy to adjust the paper machine on the fly. Push a button on the touch screen and the whole machine comes back to the tailing speed. Or use the fast take-up control to take out the slack ahead of the calendar stack. Also, it is easy to reverse the stacks to clear a jam.
Startups are far more efficient, too, saving enormous downtime. "It used to be when the steam drove the turbine and the steam went through the turbine into the dryer section, you couldn't have people working on dryers, which is the major focus of any shutdown," said Curcio. The startup process could not begin until the dryer work was completed. "Now we'll start an 8-hour shutdown, and six hours into it, we'll be starting the lead/wet end if it's done. We'll get that all lined up, get felts lined out, press sections -- we'll get all that running, broke in, we'll start the calendar stack: all these things can be running and set up for startup."
Speed variations are history, too, he noted. With the turbine, variations in steam pressure would result in speed changes and product variability. With the new drives, that problem is history. The process is much more stable.
Increased Tonnage
Since the retrofit, the mill has begun producing heavier paper, for use in G-P Gypsum's premium ToughRock™ wallboard. The retrofit, notes G-P, has provided throughput capacity that has let the mill increase tonnage of this heavier paper from 175 to 225 tons per day.
Production is, in large part, a function of uptime, notes Curcio. And with the retrofit, the machine runs with very little delay. "We're down to 2% many months," he noted -- a full 5% below the industry average.
* * *Beyond more efficient energy usage, the compelling reason for an electrical retrofit is safe, reliable operation of equipment, says plant manager
A total energy-use reduction was not planned, anticipated, nor the goal of retrofitting the traditional steam-powered line shaft paper machine at Georgia Pacific's gypsum paper mill in San Leandro, California.
"What we got," notes Fred Curcio, "is improved safety and more precise operability of the paper machine. We got rid of a lot of 50's technology, including the steam turbine."
But removal of the turbine did not reduce dramatically the quantity of steam needed for the dryers. In fact, he notes the cost of gas affects the mill's margins more than electricity, at least to date. The cost of gas has been going straight north, he said, and he predicts electricity costs, too, will go out of sight.
The mill has advantages: its own 115 kV substation from the utility line, through which the plant buys and manages large quantities of power; and access to landfill gas, which provides 20-50% of the total needed to generate steam.
"You probably will become more efficient in your use of energy as a result of retrofitting steam power with electronics," says Curcio. "But the more compelling reason here was to create a much safer operation in which the equipment could be operated with more reliability."
G-P's quest to increase capacity at the mill continues. Under review now is the addition of a new press section, which Curcio said would be more energy efficient. "We have the drives and motors in place to run it," said Curcio.
The San Leandro mill's retrofit quickly became the model for fellow G-P paper plants. G-P's Pryor, OK, location followed suit almost immediately, with Intec Solutions providing a turnkey installation that includes running the entire paper machine from a large single multi-drive configuration.
Managers and operators in Pryor are pleased, too, said Curcio and Hall of Intec. "It makes you appreciate how much easier it can be to operate equipment," Curcio said.
