Redundancy is Key to Glass Plant's Future

Combining a very intense and hot continuous process with long periods between shutdowns calls for a robust and resiliant control system. This application uses a redundant control system and reused prior control cabinents and wiring.

07/01/2009


While many continuous process plants operate for long periods without shutting down, glass plants are certainly among the most demanding. Typically, float glass plants shoot for around 15 years before they stop, allowing their massive 3,000 °F furnaces to cool completely. Cleaning, repairing and upgrading the furnaces and production lines during this repair period take on a feverish pace since every minute spent on the outage affects the bottom line.

However, system failure can cause months of downtime and ruin a multi-million dollar furnace. As a result, producers depend on successful cold repairs to allow the non-stop ribbon of glass to keep flowing.

When planning cold repairs, operators want everything staged so they have all the possible items they need. For

Cardinal was able to reuse cabinets and wiring from the earlier system, which saved on installation costs and time.

Cardinal was able to reuse cabinets and wiring from the earlier system, which saved on installation costs and time.

one such outage, the engineers at Cardinal Glass Industries in Menomonie, WI, had much to consider. High on the priority list was the control system responsible for every aspect of the “hot end”—from ensuring the right mixtures of raw materials to the quality of the finished product to meet or exceed customer specifications.

Recently, the Menomonie float glass plant completed a cold repair cycle within a 90-day timeframe. Mark Gehrke, the company's electrical engineer, played a key role determining what control system modifications would be needed to take the plant through the next 15 years of supplying glass to residential window manufacturers.

While the existing system worked adequately, looking out another 15 years Gehrke knew an upgrade was in order. He needed a fully redundant control system with open communication interfaces and flexibility. He also wanted a system that would provide comprehensive and easy-to-use trending of process data for analysis. But, above all, reliability topped his selection criteria.

Redundancy wins

After evaluating options from a number of vendors, he chose an integrated automation and control system from Siemens Energy & Automation.

“The redundancy of the Siemens PCS 7 system was much better than the other systems we looked at,” Gehrke says. “That was one of the main driving forces. There were fewer steps involved in implementing a Siemens redundant system. If we wanted to add I/O on the other systems we would have had to flip ones and zeros. The Siemens system was also very easy to implement.”

The upgrade solution chosen for the Menomonie plant was a fully redundant Siemens Simatic PCS 7 control system, operator station, engineering station, Masterdrive VFDs (variable frequency drives), Profibus I/O, and more than 30 Sitrans pressure and temperature transmitters. To take advantage of a digital fieldbus and to bring it all seamlessly together, Cardinal Glass replaced the electronics in its existing analog 4-20 type instruments with Profibus PA communications capable modules.

“Profibus helped us eliminate a lot of manual tweaking at the engineering station,” says Gehrke. “Also, we chose to place a controller in each of our production areas even though we could have used just one controller for the entire operation. We chose to keep it broken up because of redundancy and the ability to isolate problems.”

Gerhke believes a big reason why the Menomonie plant met its 90-day schedule for the cold repair was he was able to retrofit the existing control cabinets with the new hardware. “We basically gutted the cabinets and started over,” he says. “We saved the wire connections and put terminal blocks in the bottom. It was the only way we could have done it on time and on budget.”

From batch house to cutting

The float glass manufacturing process begins when a precise formulation of eight raw materials are mixed in the plant's batch house. The PCS 7 system communicates to two S7 417 controllers, one each for manual operations and unloading, as well as one for batching. At the same time, cullet (recycled glass) is mixed with the other raw materials.

With up to three days worth of glass melting at a time, even a slight error in batching, heating, or cooling, can ruin many tons of product. “Think of the production process as a river,” Gehrke says. “The flow has eddies that will bleed off into the ribbon for days or weeks or months. The control system minimizes the risk of these problems that can go on for days, weeks or months.”

As the glass enters the tin bath, which is the forming section of the line, the temperature is approximately 2,200 °F. Once inside the tin bath, the glass floats atop a pool of molten tin where the top roll machines precisely adjust the width and thickness of the molten glass. The tweel, or refractory gate, controls the flow of the molten glass into the tin bath. Resistive heating elements hanging above the molten tin, heat different areas of the ribbon to ensure correct thickness and width.

The heat of the tin bath is maintained by a Thyro-P power controller that is integrated into the PCS 7 control system. Each top roll machine is controlled by a Masterdrive VFD and synchronous motors connected to the control system via Profibus. Coordination between the drives, motors, and control system delivers the precise adjustments to form the glass to within 0.001 in. while maintaining an internal atmosphere of nitrogen and hydrogen to prevent oxidation of the molten tin.

“We maintain the atmosphere in the tin bath with actuators and Sitrans pressure transmitters that are connected by Profibus PA to the hazardous area controller,” Gehrke says. “This patented process keeps defects out and provides a clear surface on top of the glass.“

After exiting the tin bath, lift-out rolls pull the ribbon of glass into the lehr for annealing. The lehr, controlled by Masterdrive VFDs, gradually cools the glass. This controlled cooling process, automated by the PCS 7 system and hazardous area controller, ensures the proper strength and the ability to cut the finished products, ranging from 1.6 to 7.0 mm thick residential windows.

Because the variable frequency drives are programmed and communicate using Profibus, engineers can quickly replace drives and configurations as needed. Gerhke says this ability to download the configuration parameters and changes quickly online reduces downtime and does not affect operation.

Additionally, because operators can now monitor the temperature and health of all its instrumentation devices, downtime is further reduced.

“We get much better feedback over Profibus,” Gehrke says. “Operators know right away when a device is about to go bad or is overheating. Those diagnostics allow us to make adjustments before an instrument goes down.”

As the glass exits the lehr, it is tested for proper thickness across the entire width. If any portion of the glass falls outside of specifications, it is automatically marked and sent to be recycled in the batch house. Additionally, a laser inspection booth positioned over the line permits an even closer analysis of 100% of the glass to detect defects and distortion.

Once the furnace is running at 3,000 degrees Fahrenheit, operators want to keep it in production for a long time.

Computer-controlled cross cutters score the glass according to particular customer orders. The main line then snaps the glass at the scores and trims edges to specifications. Waste is fed back into the batch house to be used as cullet. The finished products are finally steered down individual lines where they are packed and shipped to residential window and door manufacturers.

“Throughout the entire process, operators can look at parameters that we never had access to before,” Gehrke says. “Now they can reset faults themselves and analyze process data trends. Before, they had to call me to handle the problem.”

Non-stop reliability

Chris Granley is engineering manager at the Menomonie plant, and his primary job is to ensure everything in the plant runs non-stop until the next cold repair in another 15 years. “I have been very impressed with the reliability of the Siemens PCS7 control system,” he says. “We have had no issues with the hot-end control system since the switchover.”

He likes the new system tools, particularly the trending, diagnostic, and alarm features to monitor system performance and diagnose any problems with the process equipment.

Mark Kehne, production manager at the plant, has also learned to rely on the system to help meet his production objectives. Referring to himself as an end user, Kehne works closely with Gehrke to improve quality and output.

“Our new process is much more streamlined,” Kehne says. “I use the system as a tool to help me problem solve and do root cause analyses. The PCS 7 system has removed much of the potential for error. If it sees something moving this way when it should be moving another way, it either automatically compensates for the problem or alerts the operator to the situation.”

Kehne says he now achieves results faster and can fine tune the closed loops more easily. In the past, he says production adjustments were often too fast or too slow. Operators also tended to over- or under-compensate parameter changes from time to time. “Now I can give Mark Gehrke feedback, and together we accomplish our goals with fewer iterations of revisiting the same thing as we typically experienced prior to cold repair,” Kehne says. “The new control system is far superior to what we had before.”

Gehrke adds, “Reliability is the most important part of the control system. The PCS 7 system has met all of our expectations for reliability—not only by performing flawlessly, but helping the plant to resolve operation problems quickly. It is nice not getting the 1 a.m. wakeup calls about production events that our operators can now correct on their own.”

Today, Gehrke says the PCS 7 control system continues to automate the entire hot end of the production process reliably and provides the trending, diagnostics, and communications needed to keep the plant running for the next 15 years.


Author Information

Moin Shaikh is process automation systems marketing manager for Siemens Energy & Automation, Inc. Reach him at moin.shaikh@siemens.com .




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