In today’s cost-conscious environment, metal producers, heat treaters and their suppliers are looking for every possible edge.
|Fig. 1. Typical cast refractory shapes|
In the case of a manufacturer of proprietary refractory shapes, a small investment in improved process control is paying big dividends. By simplifying the operation of their firing furnace, the producer now enjoys improved yields, reduced equipment supervision and lower operating costs. The system changes and the resulting improvements are detailed in this article.
The supplier offers refractory shapes (Fig. 1) for a variety of thermal-process industries, including metals production and heat treatment. The casting and curing operation includes a box-type, gas-fired furnace that uses profile control to execute cycles that can be over 80 hours long. The critical success factors in the economical production of this class of products are threefold: tightly controlled heating rates, correct process definition and prompt response to equipment issues. Driven by tightening margins and increased competition, the staff determined that replacing the older-technology furnace controls would pay back handsomely if those factors could be optimized.
Energy Services Group (ESG), a furnace builder and control-system integrator based in Channelview, Texas, was called upon to perform the controls upgrade. ESG controls engineer Mike Hunter analyzed the shortcomings of the original control system, surveyed current control technology and selected the West ProVU Advanced Process Controller as a powerful yet economical solution for the project. Specifically, Hunter recognized that a large number of outputs would be needed to improve process control and equipment monitoring. He also concluded that the profiling function of the ProVU would minimize the risk of mistakes in process definition and selection.
Heating Rate Control
By nature, cast refractory contains a high weight fraction of water. This makes precise control of heating rates at low temperatures absolutely critical, because overheating will damage the product if the retained water is vaporized too quickly. Unfortunately, the original controller could not adequately manage the excess-air burner system during the slow ramp-up from ambient temperature to the first soak at 300°F. Furthermore, problems at the high end occasionally resulted in overheating. Even worse, the occasional damage caused by poor control could not be detected until the end of the run several days later when the product was cooled, removed and inspected. The impact of suboptimum control was costly in terms of lost time, wasted energy and material, and missed deliveries.
The problem was solved by splitting the temperature range and using on-off control below 300°F and proportioning control up to the maximum process temperature of 1500°F. At low temperatures, as the profiling function ramps up the setpoint, a deviation band is used in combination with alarm contacts to cycle a burner control solenoid. Once the 300°F crossover point is reached, the solenoid is disabled, and a linear actuator takes over for proportioning control. The result is optimum process control over the entire curing cycle, which is made possible by the flexibility of outputs and alarms in the controller.
|Fig. 2. Display screen with plain text|
While the curing cycles themselves are not complicated, they do vary considerably in terms of heating rate, soak time and temperature. In the previous setup, these cycles were listed on an operator sheet at the furnace control panel and were numbered 1 through 8 as dictated by the controller/recorder/profiler. The procedure to add or change a cycle was cumbersome and, because of display limitations, programming errors were not easily detected. The results of simple mistakes were often dramatic: If a temperature or ramp rate was set incorrectly or if the wrong cycle was selected, a four-day run would result in a pile of ceramic shards instead of a set of precise shapes.
The profiler utility in the new controller provided an elegant solution. The controller display screen uses clear, plain text rather than mnemonic codes to display information, which drastically reduces the risk of programming errors. Navigation between segments and within features of a segment is simplified, and once completed, the profile is saved with a plain-text name. The operator sheet is eliminated because the list of cycles is now on board the controller (Fig. 2), and the risk of choosing an incorrect cycle is all but eliminated. The result: higher yields and improved deliveries.
An off-line engineering tool provides utilities for configuration, profile construction and visualization (Fig. 3), and simulation of controller behavior. Start-up risks are minimized because all aspects of the controller can be analyzed and tested virtually before committing equipment or product for real-world validation.
In addition, the new controller has a sealed, front-mounted USB port that allows the user to back up the configuration of the device (inputs, outputs, alarms and events) as well as all of the programmed cycles. Conversely, data can also be uploaded from a standard USB memory stick if changes are desired or if another controller requires an identical setup.
Once the work on the furnace was completed, the controller and profile data became part of the electronic file for the project.
|Fig. 3. Off-line engineering tool|
Prompt Response Capability
The older technology of the original control system did not allow for remote signaling of equipment conditions. Given that the furnace is located in a remote part of the manufacturing complex, stationing an operator nearby to constantly monitor it was not cost-effective. Because of the value of the product, however, it was often a necessary evil, especially on weekends when manpower was further reduced.
Many of the problems with the initial system stemmed from poor temperature control. The instability of the burner system at low temperatures and the tendency to overshoot in the upper range required frequent attention and intervention. If the furnace was unattended, however, slow response and delayed corrective action could produce off-spec results.
The new controller has sufficient capacity to dedicate individual outputs to specific equipment conditions in addition to all of the other tasks noted previously. By using these outputs in combination with a simple PLC and an integrated paging system, key personnel now receive a detailed text message whenever attention is required. Burner flame problems, temperature deviations and end-of-cycle conditions can be automatically reported, allowing immediate response. Even so, the improvements in burner control noted previously have all but eliminated the need for urgent corrective measures. Compared to the old system, therefore, direct operator monitoring is reduced as is the turn time between furnace cycles. The result: reduced costs.
By making a small investment in improved process control and monitoring, significant savings are being realized in refractory production. The new system is simple to operate because of a plain-text controller display, an easily understood profiler utility that minimizes mistakes and an innovative USB feature. Temperature control is improved over a wide range because of the creative use of multiple controller outputs, and operating costs are reduced because of the resulting higher yields and reduced manpower for monitoring. IH
For more information: Contact Steve Maus, heat treatment market manager, West Control Solutions, 1675 N. Delany Rd., Gurnee, IL 60031; tel: 317-626-6118; e-mail: Steve.Maus@West-CS.com or visit www.West-CS.com.