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Realizable gains from an upgrade can impact plant operations in many areas, including lower operating expenses (labor, fuel, material, etc.), improved product quality (reduced breakage and increased product consistency), increased production and reduced maintenance costs.
Understanding Your Current OperationWhen deciding whether a new kiln control system is the right solution for your plant, you should spend time critically examining and understanding how the kiln is currently controlled. How much control is manual? Of those processes that are manual, how many can be performed automatically? Is there a difference in product between shifts? What about during the weekend? Can a given style and color of product be accurately produced—even if that particular style and color was first fired years ago? Automation can often help all of these factors by providing repeatability and thus quality.
The next key question is whether adequate, basic operational data is currently obtained relative to kiln performance. For instance, how many pieces per day come out of the kiln? What is the breakage rate? What is the fuel consumption per piece? How many times per year is unplanned maintenance performed on the kiln? What is the floor labor cost, and how much of it is tied to the kiln?
After developing these perspectives, the operations, engineering and management teams should be brought together to decide whether to merely automate processes that have previously been performed manually, or whether there is a potential to achieve substantial gains through automation in overall plant operations. Everyone should agree with the systems changes that will take place. Operations will be using the new control system daily, engineering will need to specify the system, and management must approve it (benefits versus cost).
Think big—you’ll get the biggest “bang for your buck” by automating as much as possible. You may want to increase the number of temperature control zones in the kiln, switch to a different firing technique, automate the kiln blowers and fans to work together, change the method of car pushing, or make other changes specific to your plant’s operation.
Choosing a Control SystemNow for the most difficult decisions of this entire process: What type of control system equipment should you purchase? The choices are numerous:
- Control using discrete loop controllers (i.e., cascade control systems based on loop control);
- Programmable logic controllers (PLC);
- Distributed control systems (DCS);
- PC-based control systems (also known as “open-architecture” control systems); and,
- Variations and hybrids of the above.
Because of the differences between the continuous nature of analog equipment and the discrete function of digital designs, we must now consider new factors when applying digital controls. For example, digital controllers calculate new output values at intervals that range from many times per second to once in several seconds. Slow variables such as temperature in a large furnace can be updated infrequently with no impact on control quality, while flow should be updated every second or faster. Ratio systems demand even faster response—dedicated ratio controllers operate at 10 updates per second (see Figure 1).
Sometimes, because of their universal nature, PLCs are used for tasks that would be better accomplished by other equipment. Proper application of a PLC to industrial combustion systems requires an understanding of the limitations of the control equipment, experience with the process, and a working knowledge of industry and national standards. It is not possible to convert a relay-based ladder diagram directly to a PLC system without understanding the inherent differences between the technologies. Easy configuration (the major feature of a PLC) is a double-edged sword when security is important, such as in systems that control hazardous equipment.
Distributed Control Systems (DCSs). The trend to lower the cost of computing power (which encouraged the growth of PLCs) also allowed designers to distribute computing power throughout the system. With a DCS, the controller is placed close to the process and transmits information through a network to other controllers and operator interface equipment.
This allows a building-block concept, where each element is installed and debugged without affecting the remaining process elements. Tasks are concentrated at the lowest possible system level, minimizing network communication and reducing the impact of catastrophic failure. Local devices can operate in a “fall-back” mode that keeps the process running while repairs are made. The elements located at the process provide stand-alone basic control, while communication with a computer provides advanced strategies. This isolates the minimum day-to-day control operation from the complex. Computer-intensive activities such as process graphics and data storage are focused at the top of this hierarchy.
Distributed control is moving even closer to the process with the advent of “intelligent” transmitters. Intelligent field devices can improve sensor performance by correcting for temperature variations and linearizing transducer data. Corrected data can then be transmitted to the control equipment. Industry standard bus systems allow many field devices to transfer information over a single cable (wire or optical), eliminating separate wiring between the control equipment and each field device.
Personal Computers (PCs). Personal computers are common in today’s factory. High production volumes, standardized hardware and the availability of robust operating systems have moved the PC from the office to the factory floor. With a graphical interface package, they are ideal as supervisors in distributed control systems. Standard configurable software allows manufacturing managers to gather and analyze process data using factory floor PCs. Industrially hardened personal computers are replacing PLCs in many control applications.
Features of Advanced ControlAdvanced control systems are generally hierarchical in nature, including levels of equipment reaching from the process controller to a supervisory computer. Multiple levels are possible because these systems are software-based. This software is often hidden from the user and designer by a configuration language, which frees the designer from complex programming tasks.
In addition to basic loop control, advanced control systems acquire data to display process trends. This information can be stored for analysis and used to establish performance benchmarks, or to document batch history. Historical data can establish process capability and highlight degradation from standards caused by equipment deterioration, process variation or procedural change. Plant-wide information and product tracking is possible with systems equipped for network operation. The objective is consistent system performance.
Alarm logs, batch history, equipment performance reports and maintenance records help diagnose equipment, process or operator problems. Early warning systems can expedite maintenance and ensure uniform product quality. Computer-supplied messages can help service or operate the equipment. Step-by-step start-up and maintenance instructions are available, guiding operators through infrequently used procedures.
System-wide availability of information, coupled with distributed computing power, allows alternatives to conventional proportional/integral/derivative (PID) control. For example, in a multivariable process, the system could attempt to optimize desired conditions while operating within specified constraints. By monitoring the effect of small variations, the system could self-learn and continuously improve its operation.
Complex process systems are often described by a mathematical model to predict performance based on measured conditions. These predictions are compared to actual performance, or can run in “real-time,” adjusting operation to keep performance at the calculated level. Batch operations can benefit from “on-line” storage of recipes, allowing quick changeover from one product to another. The system can calculate required adjustments to recipes based on measured conditions.
Additionally, the method of communicating with a process operator can often be improved with computer technology. Visual displays using graphics and color compress large amounts of information into small, easy-to-understand formats. Computing power is used to simplify information and reduce language barriers.
A pulse-controlled firing system is a prime example of an advanced control system that regulates the furnace energy input according to the demand requirements dictated by the PLC, PC or temperature controller. It adjusts the thermal input by sequencing the cycling of multiple burners (within a specific zone) from high-to-low for controlled periods of time.
The burner control provided by the pulse-controlled system produces maximum heat transfer of the furnace gases for optimum temperature uniformity, resulting in better product quality and increased production. The on-ratio precision of a pulse-controlled system ensures efficient heat transfer with a minimum of fuel consumption, reducing one of any plant’s most costly operational expenses. As the demand for heat is reduced, the “low time” is increased and the burner rotation time is lengthened. The “high time” is constant for some control algorithms. Others constantly recalculate the high and low times, keeping the rotation times constant. The principal goal of these firing methods is to prevent stagnation of the furnace gases and improve thermal turndown. Additionally, at high fire, the pulse-controlled system produces a more thorough mixing of the products of combustion with the hot burner gases and low excess air, reducing the level of NOx emissions.
Meeting Current and Future NeedsYou should spend your available dollars in the areas that will meet your needs today and continue to meet those needs well into the foreseeable future. This involves building a system that is flexible, adaptable, extensive, and has a strong market presence that will continue to provide you with support. If you choose a system other than a top-tier name brand, be absolutely sure that there is a large advantage over the name brand—one that is not just the initial cost of the equipment. Chances are good that you will very likely spend more time in engineering, programming and installation than you will for the equipment. By the same token, beware of the cheapest cost for engineering, programming and installation. As with any product or service you purchase, you get the quality—and results—that you pay for.
Also note that no matter how good a control system is, it cannot fix mechanical problems. If a plant has frozen valves, inferior clay or poor forming equipment, a new (or even old) control system cannot make the valves work, create better clay, or reform the product.
Once you have determined that your plant’s mechanical systems are adequate, and have chosen a reputable supplier, upgrading your kiln’s control system may be the most viable solution. With the advancing state of control technology, a simple upgrade can often dramatically improve the performance of a kiln while reducing operational costs.