CHAPTER3
the modulating control (MC) depending upon the
position
of
the manual-automatic switch (MAS).
This allows operation 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 the
fuel valve(s) 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 switch in the
manual-automatic position and
under
the
direction
of
the
modulating control. The manual
position
is
provided for initial ad-
justment
of
the burner over the
entire fll'ing range. When a
shut-
down occurs while operating
in
the manual position at other than
low fll'e, 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
condi-
tions. The effectiveness
of
nozzle
purging
is
lost
on
a
No.6
oil fired
burner.
This
is
the end
of
the burner starting cycle. The
(LDL) and (FVL) lights
on
the panel remain lit.
Demand firing continues as required by load
conditions.
Burner
Shut,;.Down Post
Purge
- The burner
will fire until steam pressure
or
water tempera-
ture
in
excess
of
demand
is
generated. With
modulated
fll'ing, 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 caus-
ing the main fuel valve (MGV)
or
(OV)
to
close.
The flame
is
extinguished. The control panel
750·150
SEQUENCE
OF
OPERATION
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 fired 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 position
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
complete.
The program relay now
is
in readiness for sub-
sequent recycling and, when steam pressure or
water temperature drops sufficiently
to close the
contacts
of
the operating control, the burner
again goes through
its normal starting and operat·
ing 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
a flame
or
a flame simulating con-
dition occurs during the
pre-pu~ge
period.
The control will prevent start-up
or
ignition if the
limit circuit controls
or
the fuel valve interlocks
are open. The control will lockout upon any
abnormal condition affecting air supervisory
eontrols wired
in
the running interlock circuit.
CHAPTER 3·9
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