Flame Safeguard Instrumentation; Application Of Boiler Controls - Honeywell AUTOMATIC CONTROL Engineering Manual

Figure 46 shows how flame safeguard controls are integrated
with combustion controls on a small oil fired steam boiler. The
ultraviolet (UV) flame detector is located where it can see the
flame and will shutdown the burner when no flame is present.
OPERATING
CONTROL
FLAME SAFEGUARD
CONTROL
HI LIMIT
UV FLAME
DETECTOR
LOW WATER
CUT-OFF
Fig. 46. Combustion Controls with
Flame Safeguard Circuit.

FLAME SAFEGUARD INSTRUMENTATION

In addition to the combustion, safety, and flame safeguard
controls shown in Figure 46, additional instrumentation often
provided on larger burners measures:
– Percent of O 2 or CO 2 in flue gas (to monitor combustion
efficiency).
– Flue gas temperature.
– Furnace draft in inches of water column.
– Steam flow with totalizer or hot water Btu with totalizer.
– Oil and/or gas flow with totalizer.
– Stack smoke density.
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
MAIN BURNER
THERMO-
COUPLE
PILOT
BURNER
Fig. 45. Simple Flame Safeguard for a Gas Furnace.
RELAY
BURNER
WITH
ELECTRIC
IGNITION
C2930
OPERATING
CONTROLLER
HI LIMIT
POWER
PRESSURE
REGULATOR
VALVE
PILOTSTAT
AUTOMATIC
VALVE
PILOT
GAS LINE

APPLICATION OF BOILER CONTROLS

Boilers have to provide steam or hot water whenever heat is
needed. A conventional BMCS is often set to provide a
continuous hot water or steam supply between October and
May at anytime the OA temperature drops to 60F for more
than 30 minutes and an AHU is calling for heat. The BCMS
should include a software ON-OFF-AUTO function. Unlike
chillers, boilers can be left enabled at no-load conditions, during
which time the water temperature will be held at the design
temperature. Frequent warm-up and shutdown of boilers causes
stress buildup. Boiler manufacturers recommendations provide
specific guidelines.
Unless a water temperature low limit is used, hot water boiler
burners are not controlled to provide water temperatures based
on outdoor temperatures because the reset schedules require
water temperatures to be supplied below the dew point
temperature of the flue gas. Some boilers require entering water
temperatures to be above 140F before going to high fire. In this
case, if a building is using hot water and the boiler is locked
into low-fire because the entering water is too cold, the system
may never recover.
333 ENGINEERING MANUAL OF AUTOMATIC CONTROL
GAS
FLOW
SHUTOFF
VALVE
M15050