CleaverBrooks 4WI - 4.3.4 - Burner Shutdown: Post-Purge; 4.4 - Flame Loss Sequence

750-211 (revised 2009)
Promethean Boilers, Model 4WI Manual
4-5
4.4 — Flame Loss Sequence
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.
The burner starting cycle is now complete. the LDL and FVL lights on the panel remain lit. Demand firing contin-
ues as required by load conditions.
4.3.4 — Burner Shutdown: 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 con-
trol (OLC) opens. When the limit control circuit is opened, the following sequence occurs:
The main fuel valve circuit is de-energized, causing the main fuel valve (MGV or OV) to close. The flame is extin-
guished. 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.
On a No. 6 oil burner, the air purge valve (APV) is powered from the blower motor start circuit via the contacts of
the air purge relay (APR) to provide an air purge of the oil nozzle. The damper motor returns to the low-fire posi-
tion if it is not already in that position.
The blower motor start circuit is de-energized at the end of the post-purge cycle and the shutdown cycle is com-
plete.
The program relay is not 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.
4.4 — 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 pre-purge period.
NOTE: Normal operation of the burner should be with the switch in the manual-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 effectiveness of nozzle purging is lost on a No.
6 oil burner.