TM 5-810-15
of boiler air flow demand. This feed-forward signal
may be fuel flow, boiler master, or other demand
index, but will not be a measured air flow signal.
(2) Furnace implosion protection. Boilers that
have a large capacity and large draft losses due to
air quality control equipment may require ID fans
with a head capacity large enough to exceed design
pressure limits of the furnace and ductwork. If this
possibility exists, the furnace pressure control
system must include furnace implosion protection.
The furnace implosion protection system will
comply with the guidelines established by NFPA
85G. These guidelines include redundant furnace
pressure transmitters and transmitter monitoring
system, fan limits or run-backs on large furnace
draft error, feed-forward action initiated by a main
fuel trip, operating speed requirements for final
control elements, and interlock systems.
d. Steam controls.
(1) Steam pressure control. Steam pressure is
controlled by boiler firing rate. As discussed in
combustion control, steam pressure is used to
establish the master demand signal that controls
fuel and combustion air flow.
(2) Steam flow control. Steam flow is a func-
tion of boiler load demand. Steam flow is also a
function of fuel Btu input and can be used to trim
combustion air flow as discussed in combustion
control. Steam flow is also used to calculate boiler
load for use in oxygen trim controls and as a feed-
forward signal in feedwater controls.
(3) Steam temperature control. Boilers that
produce saturated steam do not require steam
temperature controls. Boilers that produce super-
heated steam require a control loop to maintain
superheater outlet temperature. A single element
loop with feedback as shown in figure 9-15 is
normally adequate for control of steam tempera-
ture.
e. Blowdown controls.
(1) Continuous blowdown. Continuous blow-
of automatic blowdown will be dependent on
down is the continuous removal of concentrated
whether blowdown heat is to be recovered and a
water from the boiler. The rate of blowdown is
LCCA.
f. Sootblower control. Sootblower control
controlled by manually adjusting the setting of the
continuous blowdown control valve. Continuous
should be an operator-initiated automated sequence
blowdown can be used on boilers of any capacity
control. After the start command the system should
and permits heat recovery of the blowdown. The
step through the sequence for all sootblowers
use of continuous blowdown heat recovery is de-
including opening the valve for the sootblowing
pendent upon life cycle cost evaluation.
medium, timing the length of the blow and closing
(2) Automatic
blowdown.
Automatic
the valve. The system should automatically move to
blowdown systems as shown in figure 9-16
the next sootblower and continue the sequence
continuously monitor the boiler water and adjust
until all sootblowers have been completed.
the rate of blowdown to maintain the conductance
of the boiler water at the proper level. Control
9-4. Nonboiler controls.
action can be two-position or modulating. The use
a. Low pressure steam controls.
9-16
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