Sequence Of Operation - Harsco Industrial SC-2000 Installation & Owner's Manual

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Once the boiler begins the ignition
sequence, the firing sequence will
continue until main flame is
reached regardless of heat
request. The sequence can be
interrupted by turning the power
switch off.
If any "Manual Reset" limit device
trips, DO NOT reset without
determining and correcting the
cause. Manual Reset Limits
include: Flame safeguard, high or
low gas pressure, high
temperature limit, stack
temperature, low water level.
Attempting to operate a boiler
without diagnosing the cause of
failure can result in unsafe
operation, increasing the risk of
injury. Never attempt to bypass a
safety limit device.
Figure 5.4
Harsco Industrial, Patterson-Kelley Technical Service 1.877.728.5351

5.4 Sequence of Operation

1. When the Boiler On/Off switch is turned on, power is provided through
over-current protection (fuse and/or circuit breaker) to the boiler control
and the combustion blower.
2. When the outlet water temperature drops below "CH SETPOINT"
minus "CH DIFFERENTIAL ON", a heat request is generated.
3. Provided all limits are made and the remote enable signal (if installed)
is active, the boiler will attempt to start.
4. The NURO control checks to make sure air pressure switch is open
indicating there is no airflow through the boiler. The combustion blower
is then driven towards the pre-start fan speed. When the air pressure
switch closes, the "PRE PURGE TIME" timer is started. Once the "PRE
PURGE TIME" timer expires, the blower is driven to the ignition speed.
5. A trial for ignition begins and the sequence of events is illustrated
graphically below in Figure 5.4. A valid flame signal must be detected
within 4 seconds after the gas valves open (elapsed time 7 seconds).
6. After a successful ignition, the fan is driven to low fire and the "CH
LOW FIRE HOLD TIME" timer is started. Once the "LOW FIRE HOLD
TIME" timer expires, the boiler is released to modulation.
7. The NURO control modulates the firing rate between low and high fire
to
8. The burner will continue firing until the outlet water temperature
exceeds "CH SETPOINT" plus "CH DIFFERENTIAL OFF".
temperature, the gas valves are closed combustion air fan continues to
run until the "POST PURGE TIME" timer has expired.
9. When the outlet water temperature is reduced by the load on the
system, a heat request is generated.
recycle to
maintain the desired
Step 3
.
41
Gas Fired Boiler
outlet water temperature.
The operating sequence will
At this