Fig. 1. Figure 1a (left) shows the outside of a fragmented furnace control system. Notice the single-loop temperature controllers and paper chart recorder. Figure 1b (right) pictures an integrated furnace control system with graphical user interface terminal.
Heat treating furnaces have historically been controlled by what can appear to be complex collections of flame safety relays, single-loop temperature controllers, high-temperature limit relays, and paper-chart recorders.
You may have to deal with systems like this every day: "cabinets of shame" that over time have had components replaced and have been cobbled back together, wiring that seems to go to nowhere, or systems composed of unsupported equipment that can't be replaced. These are the furnaces that experience the occasional random trip, always seeming to occur at the least opportune time. You may have to re-run or scrap entire batches because your furnace controls are no longer able to produce the same results twice, affecting quality and adding to your operating costs.
If any of these symptoms sound familiar, the recent development and release of 32 bit programmable automation controllers, may be of interest to you. Rockwell Automation's Logix family of automation controllers integrates the programming and control functionality of the traditional programmable logic controller (PLC) with the process control functionality of distributed control systems (DCS). The Logix programmable automation controller has the ability to handle all levels of plant automation including discrete-drives, motion, and process control in one integrated control architecture. Figures 1 and 2 are comparative representations that illustrate the external and internal differences between used, old-style equipment and newer, state-of-the-art systems.
Fig. 2. Figure 2a (left) shows the haphazard wiring of a typical "cabinet of shame." Figure 2b (right) shows a modern replacement system.
Process Control
Pressure to reduce processing costs has forced many maintenance engineers to replace these fragmented systems with automation controller-based furnace controls. Automation controller systems reduce operating costs by improving system control and reliability, and by increasing furnace automation and system availability. Automation controller systems can be integrated with PC-based supervisory Human Machine Interfaces (HMI), allowing for centralized data collection and storage. Everything from flame safety to furnace material transfer to recipe management can be programmed into one controller, which actually makes these systems simpler to develop and maintain than the older, cobbled-together systems.Programmable automation controllers (PAC) can be programmed specifically to meet the needs of a particular thermal process. If a line stops, the PAC is able to respond to a gap in material flow by reducing temperature set points or automatically removing burners from service, thereby saving fuel. When the line starts up again, the PAC can anticipate this action and begin ramping up temperatures immediately.
If a batching furnace has completed its heating cycle, a PAC can be programmed to ramp down and hold a lower temperature while awaiting the next batch. Multi-burner systems can be programmed with automation enhancements such as temperature profiling. Temperature-profiling logic uses furnace load data to determine whether specific burners may be removed from service to reduce fuel usage and improve heat distribution. Certain 32 bit programmable automation controllers provide auto-tuning PID loops, ensuring the tightest possible temperature control. Their ability to program separate subroutines with individual scan times allows programmers to run fast-acting control loops at higher frequencies than slower-acting loops. These methods improve system response, reduce fuel usage, and free heat treating furnace operators to focus on other tasks and responsibilities.
Programmable automation controllers can also increase heat treating system availability. Forced outages caused by intermittent furnace trips can be dramatically reduced. Advanced alarming and troubleshooting algorithms help maintenance departments locate process disruptions and take corrective actions. Historical alarming data and process trending can be used to help maintenance personnel predict failures.
Fig. 3. Electrical schematic depicts one method of implementing hardwired master fuel trip circuit. Notice that normally open watchdog relay contacts are wired in series with the emergency stop button. When completed, the hardwired master fuel trip circuit energizes the Main Trip Relay coil. Final control devices are powered in series with Main Trip Relay normally open contacts.
Programming Languages
One big advantage of the new systems is that the entire line of controllers is programmed using a single software package. Maintenance personnel can interface with several different processor types using the same development software. This integrated software package facilitates relay ladder, structured text, function block, and sequential function chart editors.Relay ladder and function block are the most common programming languages used for heat treating controls. Relay ladder allows maintenance personnel to easily understand the programming environment, as this language strongly resembles electrical schematics. The function block editor simplifies programming by offering programmers a catalog of packaged instructions to execute complex process algorithms. The function block editor also enables programmers to create custom user-defined function block instructions such as carbon potential, fuel ratio stations, and oxygen trim controllers. The dual nature of the automation controller, the discrete relay ladder environment, and the process function block environment, allow the user to apply a single technology to control all facets of heat treating furnaces -- a big advantage to using this type of system.
Information Management
Data sharing is another advantage of using PACs. The open architecture of programmable controllers provides a gateway to data sharing. Product information can be entered in the system manually or by barcode scanner. This barcode information can be used to recall and check a product queue waiting to be loaded. Once confirmed for loading, this data can be used to trigger a specific ramp soak cycle already stored in the system database. Another data sharing capability is that furnace controller process data can be collected, displayed, and trended on a plant-wide HMI system. The historical data can be logged and reported by lots, allowing heat treating vendors to easily retrieve heat treat reports and logs. Process data can be stored on corporate-wide network servers, providing ready access to all users.
Fig. 4. Diagram depicts one method of implementing input test scheme. Output from the PAC generates interrogation power for field inputs. De-energizing the interrogation power allows the PAC to test for input "burn-in."
The Safety Factor
Another concern often voiced by plant engineers is safety. Can you configure a PAC to meet safety standards?The answer is that PAC-based systems can be designed to meet National Fire Protection Association (NFPA), Factory Mutual (FM), and IEC 61508 SIL standards. The system design needs to meet the requirement (NFPA 86, 2003 Edition, Section 7.3.2.2 (1)) that, "The programmable logic controller shall not interfere with or prevent the operation of the safety interlocks." In other words, in the event of an unsafe condition, the controller must not prevent the system from reverting to a safe condition.
NFPA 86, section 7.3.3.2 requires that a watchdog timer, external to the PAC, must be used to allow the system to revert to a safe default condition if a controller failure is detected. The external watchdog timer is wired in series with the electro-mechanical master fuel trip relay and the emergency stop circuit. This creates a hardwired master fuel trip circuit that provides power to outputs that drive critical devices (i.e. fuel valves). If tripped, the master fuel trip circuit will open, de-energizing the final control elements (see Figure 3).
The external system watchdog timer requires timed output pulses from the controller. To keep the enable signal active, the PAC changes the state of these bits every program scan. If these bits are not continually serviced, the enable signal is de-activated, which will halt the oscillation of the outputs and trip the watchdog relay, preventing a potential accident.
The watchdog timer relay monitors the processor's timing function, its ability to cycle through an algorithm, and the ability of the processor to turn outputs "on" and "off". This feature prevents:
- Failure of the internal processor clock.
- Memory failure in the processor.
- Loss of processor communication with I/O modules.
- Cessation of processor execution of the logic program.
Furnace safety systems operate on a "de-energize to trip" philosophy. This means that in the event of a loss of power for any reason, the fuel gas safety shutoff valves are designed to spring closed. Likewise, all safety inputs must remain powered for the furnace to operate. For instance, the combustion air fan running interlock input must be proven on for the furnace to operate. If this signal opens for any reason, the furnace will trip.
To protect against input "burn-in," safety interlock inputs to the system should be repeatedly tested by the system to prove their ability to turn "off," or initiate a trip. The input test periodically buffers input data and then briefly removes interrogation power from the field inputs. The logic then tests all inputs to verify that they have turned "off." Any input remaining "on" will be considered failed. If a field device connected to the failed input were to open, and that input was configured as a safety interlock, then a trip would not occur as a result of the field device. Safety interlock inputs proven to have failed will cause a system trip (see Figure 4).
Outputs powering safety shut-off valves, ignition transformers, or other critical devices can be monitored by an input to verify the execution of the command. If a discrepancy exists between the commanded state of the output and the state reported by the output monitor (input), an output failure has occurred. If the failure detected is such that the output is in a failed "on" state (i.e., valve energized state), the system will alarm the failure. If an output failure, alone or in combination with another similar failure, is determined to allow fuel to enter the unit (i.e. open fuel path), then a fuel trip must occur.
The PAC verifies the health of the inputs and outputs and reacts accordingly if a critical failure is detected. The external system watchdog monitors the health of the PAC. Both design features ensure that the furnace control system will operate safely.
Conclusion
As you have seen from this discussion, PACs provide an integrated heat treating furnace control solution that can simplify your operations. The open architecture of these advanced platforms can be configured to meet the hardware requirements of any furnace design. The ability of PACs to execute relay ladder and function block logic allows them to be used to control all facets of furnace operation. Advanced networking capabilities can transfer information from the controllers to be stored on network drives and made available to multiple users, improving process visibility.Techniques such as automatically reducing temperature set points between heating batches or material flow gaps reduce fuel usage. Improved alarming and diagnostics increase system availability. Auto tuning features and individually configurable subroutine scan time adjustments allow for improved system response and tighter process control. Built-in safety features can reduce accidental shut downs while preventing accidents.
The importance of finding an experienced and qualified team of industrial combustion controls experts cannot be overstated when considering a replacement furnace control system. By using all these advanced programming techniques, PAC-based furnace control systems increase reliability, drive down operating costs, and make everyone's job easier.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: programmable automation controller, programmable logic controller