We have written many articles about flame safe guard controls on Control Trends but we have not really given an explanation on the basic principles of these controls.
The programmer, (also called the blue box, the FSG, the fire eye and the brain) is the mastermind that controls the starting sequence and the firing cycle of a burner. The programmer controls the operating sequence of the blower, burner motor, ignition system, fuel valve, and all other components also called interlocks in the control system.
The programmer also provides, if necessary, a suitable purge period before ignition and after burner shut down allowing for the removal of explosive combustibles. The programmer is designed to de-energize all fuel valves within 4 seconds after loss of flame signal. In addition, the programmer automatically restarts a new cycle each time a temperature controller or pressure controller closes or after a power failure, lock outs must be reset manually after any flame failure.
Example of basic operation of programmer on steam boiler:
When the steam pressure within the boiler drops the pressure controller, (like Honeywell L404F1102), completes an electric circuit which starts a timing sequence in the programmer. The first timing sequence closes and starts the burner motor that rotates the primary air fan. The primary air fan blows air into the furnace to purge any unburned fuel present in a gaseous condition in the furnace.
This process is called pre-purging the furnace. By pre-purging the furnace before pilot ignition, the danger of a furnace explosion is reduced. The purge cycle is controlled by the time card (ST7800) in most programmers and can last varying lengths of time depending on the furnace size.
The programmer is still operating and when the second timing sequence contact closes, the circuit to the ignition transformer (similar to Donagan AO6-SA) is completed. The ignition transformer creates a high voltage spark in front of the gas pilot tube. At the same time, a solenoid valve is opened in the gas pilot line, allowing the gas to flow through the gas pilot tube and be ignited by the transformer spark. The scanner ( like the C7027A1023 or similar) is located on the front of the boiler and positioned in such a manner to sight the flame from ignition.
Sighting the pilot through the will verify that the pilot is lit. This process is called proving the pilot. The programmer timing sequence is still operating and the next contact closes to complete the circuit to the main fuel valve, which opens only when the scanner proved pilot. The main fuel valve is then opened and the fuel enters the furnace and is ignited by the pilot.
The programmer continues to operate for a few more seconds, securing circuits to the ignition transformer and the gas pilot. After the circuits are secure the programmer stops. The burner is now regulated by the pressure control and modulating pressure control. If the scanner senses a flame failure, the system is purged and secured. The programmer is then manually reset to restart the cycle.
The programmer needs very little maintenance. Most common failures are due to heat and moisture causing corrosion to the contacts of the programmer. Care must be taken to eliminate these factors. Check all wiring to the programmer sub-base for loose damaged connections or cracked wiring.
Please be aware this is a GENERIC ARTICLE on the workings of a burner programmer. There are MANY variations of these controls and many applications using these controls. Extreme care must be exercised when working with FSG CONTROLLERS and refer to factory and manufacture’s guides when working with these controls.
When your needs turn to the flame safe guard control please give Stromquist and Company at call at 1-800-241-9471.