A while back I had an interesting conversation with a well respected burner management (BMS) expert, Mr. Mike Scott from AESolutions.  We were chatting about the requirements for PLC based BMS systems, and what the latest release of NFPA standards (NFPA 85 and NFPA 86) mandate.

Something that we found quite interesting was with NFPA 86 -2007 - Standard for Ovens and Furnaces.  This updated standard had several new updates, but the one we were particulary interested with was around the use of different type of approved logic solvers. 

Section 8.3 allows the use of five (5) different types of logic solvers, yet under this same section lists only 4 (four).  I found that particularly odd and confusing?  Mike pointed out that the fifth system is invoked though appendix A, and states:

Furnace controls that meet the performance-based requirements of standards such as ANSI/ISA 84.00.01 Application of Safety Instrumented Systems for the Process Industries, may be considered equivalent.  The determination of equivalency will involve complete conformance to the safety lifecycle including risk analysis, safety integrity level selection, and safety integrity level verification, which should be submitted to the authority having jurisdiction.

So what does all this mean?  Finally, NFPA standards are starting to recognize a difference between a certified, safety PLC to that of a generic, non-safety rated PLC.  Bottom line to a user, if you choose to use a system with the right integrity, you can remove the requirement for that external watchdog timer and master fuel trip relay. 

Congratulations!