750-96 (revised 2010)
Model CB Packaged Boiler Manual
3-5
3.4 — 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 the cam controlled metering valve are actuated by the motor through a linkage and cam assembly to provide
modulated firing rates.
This is the end of the burner starting cycle. The LDL and FVL on the panel remain lit. Demand firing continues as
required by load conditions.
3.3.4 — 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 control
(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. 5 or 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 now in readiness 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 operat-
ing cycle.
3.4 — 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 a 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 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 lo fire, the damper will not be in a closed
position thus allowing more air than desired to flow through the boiler. This subjects the pressure vessel metal and
refractory to undesirable condition. The effectiveness of nozzle purging is lost on a No. 5 or No. 6 oil burner.
Courtesy of: C3 Surplus LLC
Surplus Industrial Superstore
305-428-2777
https://www.c3surplus.com
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