Burner Management Systems (BMS) are traditionally defined as systems to monitor and control fuel burning equipment during all startup, shutdown, operating, and transient conditions. Their primary purposes are:
- To protect against startup when unsafe conditions exist.
- To protect against unsafe operating conditions and admission of improper quantities of fuel to the furnace.
- To provide the operator with status information.
- To initiate a safe operating condition or shutdown procedure if an unsafe condition exists.
“BMS are making heating environments significantly safer,” says Bruce Darling, president of Olive Branch, Miss.-based Thermal Economy, Inc., a leading HVAC supplier in the greater Memphis, Tennessee metropolitan area. “They protect personnel as well as heating equipment such as boilers, furnaces, and kilns from damage during operation. They also keep the buildings in which the equipment is employed, as well as their immediate surroundings, less subject to damage from unexpected events.”
In any fuel-fired heating system, the potential for steam or fuel explosions exists; but with proper BMS, fewer are occurring. According to Darling, BMS are used with any fuel-fired equipment, regardless of the type of fuel employed (e.g., coal, natural gas, biomass, fuel oil).
The History and Evolution of BMS
“Original burner management systems were completely manual,” says Darling. “They employed the ‘LOP approach’: light the burner, observe, and pray.” With these systems, explosions and injuries were not uncommon, but the advent of flame scanners proved the end of LOP by introducing automation into the BMS equation. The early systems used flame scanners and relays, which quickly evolved into electronic systems that combined timers, relays, and flame scanner/amplifiers.
These early systems used non-programmable hardware. A number of vendors developed simple flame safeguarding systems that delivered minimal information about burner shutdowns, employed very simple burner management logic, and had limited capability and flexibility.
“The next step was the development of microprocessor-based systems, based on the original hardware designs,” says Darling. These provided more information via display modules regarding burner shutdowns and the sequencing of events leading to failures. Darling says that these systems first became prominent in the fire tube boiler market, and many are still in use there today.
Programmable logic controller-based systems constituted the next phase of development. “These PLC-based BMS are more versatile and powerful than relay-based and microprocessor-based systems,” says Darling. The early PLC-based BMS used lights, pushbuttons, and selector switches for operator control, but current ones include human-machine interfaces (HMI) and greater communications capabilities that provide the operator with more control.
A comparison of microprocessor and PLC BMS shows the advances made in this evolutionary stage:
- Provide limited flexibility.
- Are not appropriate for multi-burner systems.
- Have a single flame scanner.
- Yield limited information regarding status and shutdowns, with often-cumbersome communications.
- Provide great flexibility.
- Extend communications capabilities dramatically.
- Have both single- and multiple-burner capability.
- Provide diagnostics, if programmed into the PLC.
- Are much easier to troubleshoot.
- Allow use of various flame scanners.
As BMS systems have evolved, so too have the standards that govern them and establish the best practices for their implementation and use. The National Fire Protection Association (NFPA) and UL have written most of the regulations. Other guidelines are provided by factory documentation.
Specifically, NFPA 85 (Boiler and Combustible Systems Hazard Code) covers boilers delivering less than 12.5 million BTU/hr.; NFPA 86 (Standard for Ovens and Furnaces) covers virtually all applications other than boilers. UL 795 (Commercial Industrial Gas Heating Equipment) applies to systems delivering less than 400,000 BTU/hr., while UL 508 (Industrial Control Equipment) governs construction standards. Generally accepted design standards can vary based on the manufacturer.
For PLC-based BMS, implementation is strictly governed by the NFPA. Implementation of the PLC logic and other components, such as timers and relays, is crucial for safe systemic operation. “Importantly, safety integrity levels (SIL) can be affected by improper implementation of the hardwired portion of BMS systems,” adds Darling. “So the BMS system design is critical.”
Key Design Features
Safety design features can be implemented via software, hardwiring, PLC architecture and, in the case of Safety PLCs, internal component software.
Safe system design demands include:
- Compliance with NPFA requirements
- Appropriate shutdown processes
- Input checking logic
- Redundancy for fuel valve circuits
- Systems failing to a safe state
“Generally, valves are other devices that are de-energized when systems fail to a safe state,” says Darling.
Other design features should include easy operation, intuitive operation, and provision of sufficient alarm and shutdown information to the operator. Systems should be adequately designed for electrical loads, and systems designed for special or classified environments will have further design requirements (e.g., NEMA 4/12/4X enclosures). The best systems will also provide detailed diagnostic information to facilitate troubleshooting, communications to other systems, remote monitoring and alarms, and historical information.
The Advantages of Safety PLC-Based BMS
“The advent of safety PLCs has taken BMS design to a new and better level,” says Darling. Safety PLCs are specifically designed to be reliable through integral redundancy; they are typically applied for safety functionality or to minimize the commercial impact of serious system failures.
“When applied to BMS, safety PLCs provide significant advantages to the design of systems,” says Darling. “By providing reliable and proven safety logic for PLC inputs and outputs, they assure that the critical input checking logic is designed properly and operates correctly. Further, they assure that the safety outputs to safety valves operate correctly. Outputs are checked continuously by the safety PLC to verify that no failure has occurred. With proper implementation, safety PLC-based BMS exceed NFPA requirements.” Other benefits Darling attributes to safety PLC-based BMS include enhanced program security and integrity, password protected safety code, and the logging and recording of program changes.
With the increasing profile of functional safety and growing awareness of functional safety standards, the importance of meeting safety integrity levels is rising. “SIL integrity can be achieved with safety PLC-based BMS,” says Darling. “SIL 2 is standard and SIL 3 is achievable.” Moreover, he adds, these systems are highly flexible. “They are applicable to any boiler, furnace, or other fired burner equipment,” he says. “They integrate with other PLC systems for high-level communications, and are compatible with standard operating systems.”
Darling notes that the cost of this technology is not a barrier, and that safety PLC hardware is available at pricing equivalent to that of standard PLCs. “Safety PLC-based BMS provide users with maximum and cost-effective protection,” he concludes. “In these types of applications, that should be the bottom line.”Have an Inquiry for Siemens about this article? Click Here >>