With the release of NFPA 86 2015, designers and providers of industrial heating systems could finally control all aspects of their heating control system—from startup to shutdown—using a single programmable logic controller (PLC). This development opened up new doors for Safety PLCs in burner management systems (BMS).
Now, in addition to directly controlling combustion safeguard systems, Safety PLCs can perform standard process control functions—eliminating the hard-wired interconnections between separate combustion safeguard systems, PID controllers and control devices. Combining safeguard and process functions in this way provides a number of benefits.
Let’s explore the role of the Safety PLC in BMS applications, including the ways it can lower your costs, enhance operator safety and improve overall productivity.
Traditional Safeguard System Designs
Combustion safeguard systems monitor fuel and air flow systems—ensuring both are available within the prescribed pressure and flow ranges. Once these “limits” are met, the flame safeguard system governs the start sequence of the equipment, including purging, pilot ignition and main burner valve positioning, ignition and monitoring.
Traditionally, thermal process control—temperature, fuel and air flows—wasn’t considered part of the combustion safeguard system. As a result, it didn’t require the use of safety-rated (SIL) equipment. Until 86 NFPA 2015, these two functions—safeguard and process—were separate tasks performed by different control devices.
Optimizing Your PLC for Process and Safety
Under the new NFPA standard, you can use a single PLC for both safeguard and process functions. In terms of their processing power, modern PLCs easily perform the process control tasks required by thermal process systems. They usually include PID control loop capabilities, enabling a range of control strategies—from simple temperature control, to complex cascade control.
You can also easily program lead/lag ratio control concepts. And, since PLCs easily communicate with higher-level computer systems via open networks, you can employ advanced mathematical modeling and predictive (AI) methods.
Putting the Safety PLC Into Practice
Combining the inherent processing power of modern PLCs with the safety functionality of SIL-rated Safety PLCs means you can implement your control strategy using a single, cost-effective and flexible device. This solution is ideal for a range of BMS applications, including:
- Ovens and furnaces
- Thermal oxidizers
- Process heaters
- Reactors—and more
Simplify your control design with flame switches. Since detecting a flame requires low microamp or millivolt signal levels, a PLC’s typical safety hardware can’t ascertain the strength of a sensed flame. Instead, it’s better to employ a flame sensor that acts as a flame switch, which can sense the presence of a flame, evaluate the signal strength and provide a YES or NO signal to the PLC as a discrete input.
In addition to simplifying system design, this approach allows equipment manufacturers to select the most appropriate flame detection system for the burner/chamber geometries independent of the PLC supplier.
Enable helpful diagnostic functions. The flame signals, along with other safety-relevant inputs—Low Gas Pressure, High Gas Pressure, High Air Pressure and more—should be individually wired to the safety-rated input channels. By wiring these devices individually, the Safety PLC logic can easily provide performance information to the operator.
By contrast, older systems, which employ devices wired in series to a single input channel, provide little diagnostic functions to troubleshoot a burner shutdown. Even modern standalone flame detection systems may only offer a “first out” fault indication.
Similarly, safety-relevant control devices—including fuel safety shutoff valves and purge control valves—should be individually wired to the PLC. Many safety-rated PLC input and output (I/O) modules have built-in diagnostics to detect issues like broken wires, shorted circuits, open circuits and crossed circuits. These extensive diagnostics are vital to ensure all safety components operate properly.
Tailor your control strategy. Going beyond the combustion safeguard functions, safety-rated PLCs can pass operational and performance data to their standard programs. This is where equipment manufacturers can take advantage of the safeguard information to create control strategies that maximize fuel efficiency, increase production and reduce emissions—all while operating within the safe parameters of the combustion system.
For example, regenerative burner systems, with their constant ON/OFF switching of burners, will benefit from a safety-integrated PLC system. Due to their inherent instability on cold startups, high-temperature recuperative systems will also function more reliably when the combustion safeguard and process control systems are directly connected.
Get More, Spend Less
Safety PLC controllers bring several cost benefits to today’s industrial heating equipment. By combining Safety PLC and traditional PLC functions in a single device, the cost of a safety-integrated CPU is less than the cost of two separate CPUs.
And, since modern control systems take advantage of networked devices, distributed I/O systems that can support both standard and safety-rated I/O in remote locations—PROFINET, for example—will lower your wiring and installation costs. You can achieve even more cost savings if the standard and safety I/O reside in the same rack.
In addition, adding modern HMI devices to your control system allows you to place operator interfaces at key locations throughout your facility. Since you can configure PLCs to send messages to phones, tablets and other mobile devices, downtime and production losses are kept to a minimum if a shutdown occurs.
Looking to the Future
While current NFPA standards call for SIL 2 safety-rated PLCs, bear in mind that future NFPA editions will require SIL 3 safety ratings. Fortunately, some Safety PLC systems in place today already achieve this higher performance level. Since a combustion system’s SIL rating depends on the performance of all its components—PLC, I/O, sensors, switches and more—it’s important to stay abreast of current and proposed new standards.
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