Charles Fialkowski| Aug 05, 2011
Implementing combustion safeguards (i.e. flame scanners) in modern BMS designs
In the past, NFPA 86 (Standards for Ovens and Furnaces) stated that the combustion safeguard (i.e. flame scanner) de-energize the fuel safety valve in the event of flame failure.
This statement and it's interpertation drove some to design their system using their flame scanner as a hardwired connection to the fuel valves (when the flame signal fell, its contact switch would open, thus removing power from the solenoid operated main fuel valve). Pretty simple design, with no logic required.
In some cases, where users were trying to claim SIL (safety integrity level) credit for their designs this cause many challenges with implementing voting schemes, lack of SIL certification for scanners, increasing nuisance trip rate, etc.
The 2011 edition has re-written the definition, and removed the statement of directly controlling the fuel valve.
The benefit I see here, is that it now opens the oppurtunity to utilize the safety, diagnostics and fault tolerance of a SIL rated PLC where it can:
1. Directly and safely monitor flame presence
2. Accurately recieve and interpert scanner diagnostics (if available)
3. Improve system availablity by voting multiple devices in a 1oo2, or 2003
4. Simply overall designs by directly using the flame scanner as input signal (much like you'd use a temperature or pressure transmitter) and control the operation of the fuel valve directly.
This will ultimately provide a safer and more reliable burner managment system.
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