TM 5-810-15
are shown in figure 9-11. The oxygen setpoint is
operation. The system must be made to comply
calculated from boiler load by a characterizing
with the appropriate NFPA regulations and the
function generator applied to steam flow. Signal
recommendations of the boiler manufacturer.
(2) Purging and interlocks. The specific purg-
limiters are used to establish minimum and
maximum corrections to the fuel/air ratio since
ing and interlock requirements will depend on
major excursions are possible due to malfunctions
whether the boiler is gas/oil fired, stoker fired,
of the oxygen analyzer. Automatic oxygen trim
pulverized coal fired or ACFB fired. Regardless of
controls should not be used on stoker fired units.
the type of firing system, certain functions must be
Since the fuel bed on a stoker cannot be increased
included in the furnace safety system. These
or decreased quickly the firing rate on a stoker is
functions include a prefiring purge of the furnace,
varied primarily by changes in combustion air flow.
establishment of permissives for fuel firing, emer-
Changes in combustion air flow will cause similar
gency shutdown of the firing system when required,
changes in flue gas oxygen content. Automatic
and a post firing purge. Pulverized coal, ACFB and
oxygen trim would attempt to correct the air flow
gas/oil firing require additional functions such as
and would cause unstable operation during load
establishment of permissives for firing the ignition
changes. Stoker fired units should be provided with
system and continuous monitoring of firing
manual oxygen trim.
conditions. The prefiring purge is required to
ensure that all unburned fuel accumulated in the
furnace is completely removed and is accomplished
9-3. Boiler controls.
by passing a minimum of 25 to 30 percent air flow
a. Furnace safety system.
through the furnace for five minutes. The
(1) General requirements. The main function
conditions that would cause an emergency
of a furnace safety system is to prevent unsafe
shutdown (trip) for pulverized coal, gas/oil and
conditions to exist in the boiler including preven-
ACFB boilers are shown in table 9-2. The furnace
tion of the formation of explosive mixtures of fuel
safety system can be either deenergize-to-trip to
and air in any part of the boiler during all phases of
9-11
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