Chapter 4 Sequence of Operation
4-4 750-177
With a proven pilot, the main fuel valve(s) (OV or MGV) is energized and the main fuel valve light (FVL) in the panel is
lighted. The main flame is ignited and the trial period for proving the main flame begins. It lasts 10 seconds for light oil
and natural gas. At the end of the proving period, if the flame detector still detects main flame, the ignition transformer
and pilot valve are deenergized and pilot flame is extinguished.
Note: If the main flame does not light, or stay lit, the fuel valve will close. The safety switch will trip to lock out
the control. Refer to flame loss sequence (section D) for description of action.
The cause for loss of flame or any other unusual condition should be investigated and corrected before
attempting to restart. Failure to follow these instructions could result in serious personal injury or death
Run Cycle - With main flame established, the program relay releases the modulating damper motor (MDM) from its low
fire position to control by either the manual flame control (MFC) or the modulating control (MC), depending upon the
position of the manual-automatic switch (MAS). This allows operation in ranges above low fire.
With the manual-automatic switch (MAS) set at automatic, subsequent modulated firing will be at the command of the
modulating control (MC), which governs the position of the modulating damper motor (MDM). The air damper and fuel
valves are actuated by the motor through a linkage and cam assembly to provide modulated firing rates.
Note: Normal operation of the burner should be with the manual-automatic switch in the automatic position and
under the direction of the modulating control. The manual position is provided for initial adjustment of the
burner over the entire firing range. When a shutdown occurs while operating in the manual position at other
than low fire, the damper will not be in a closed position, thus allowing more air than desired to flow through
the boiler. Excess air flow subjects the pressure vessel metal and refractory to undesirable conditions.
The burner starting cycle is now complete. The (LDL) and (FVL) lights on the panel remain lit. Demand firing continues as
required by load conditions.
Burner Shudown-Post Purge - The burner will fire until steam pressure or water temperature in excess of demand is
generated. With modulated firing, the modulating damper motor (MDM) should return to the low fire position before the
operating limit control (OLC) opens. When the limit control circuit is opened, the following sequence occurs:
The main fuel valve circuit is deenergized, causing the main fuel valve (MGV) or (OV) to close. The flame is extinguished.
The control panel lights (LDL) and (FVL) are turned off. The blower motor continues to run to force air through the boiler
for the post purge period.
The blower motor start circuit is deenergized at the end of the post purge cycle and the shutdown cycle is complete.
The program relay is now ready for subsequent recycling, and when steam pressure or water temperature drops to close
the contacts of the operating control, the burner again goes through its normal starting and operating cycle.
D. FLAME LOSS SEQUENCE
The program relay will recycle automatically each time the operating control closes, or after a power failure. It will lockout
following a safety shutdown caused by failure to ignite the pilot, or the main flame, or by loss of flame. Lockout will also
occur if flame or flame simulating condition occurs during the prepurge period.
The control will prevent start-up or ignition if limit circuit controls or fuel valve interlocks are open. The control will lock
out upon any abnormal condition affecting air supervisory controls wired in the running interlock circuit.
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