combustion or total air flow are used to trim the
parallel demands. Because actual fuel flow cannot
be measured, it is derived from pressure-
compensated steam flow that is a steady-state infer-
ential measurement of heat released from the fuel.
The result is a fast, responsive system that is the
characteristic of a parallel system but with the accu-
racy of a metered fuel system.
Overfire air damper, FD fan and induced
draft (ID) fan demands all use two element con-
trol strategies. With overfire air damper, boiler
demand positions the damper directly and is
trimmed by he duct pressure controller. The op-
erator can bias demand to account for changes in
coal quality and sizing. Air flow, as measured by
an orifice, venturi or anubar in the air flow duct,
is used by the feedback controller to adjust the
FD damper demand set by boiler demand.
The operator reduces clinker formation by
applying biases provided to adjust air flow. By
using FD fan demand as a feedforward to ini-
tially set ID fan demand, continuous operation of
the ID and FD fans during process upsets and
load changes is possible. It is then trimmed by
the furnace draft controller to position ID fan
motor or inlet dampers. Ideally, the controls
should be operated as close to atmospheric pres-
sure as possible to reduce air infiltration and im-
prove efficiency.
Oxygen or opacity measurements can be im-
plemented in the control strategies to further im-
prove boiler efficiency. Using oxygen
measurement lets the fuel-to-air ratio to be main-
tained within practical high and low oxygen lim-
its. As noted earlier, the operator-adjustable
oxygen set point helps reduce clinkers.
Opacity monitoring can warn of impending
operational problems; a high reading could indi-
cate inadequate air for combustion, too much air,
rapid air increases or sootblowing. As discussed
earlier, the advanced control strategies will
gracefully degrade to basic automatic control lev-
els when the gas analyzers are out of service.
Target Steam Flow Plant Master
The Plant Master generates a steam flow demand
for each boiler that feeds a common steam header.
The total demand to the boilers must match the
plant demand for steam in order to maintain steam
header pressure. The predominate plant master al-
gorithm used in DCS is the target steam flow plant
master. It gives a faster response to load changes
than either a single or two element plant master
and is best suited for use where loads move rapidly
and/or tight pressure control is critical.
Disadvantages of Single and Double
Element Plant Masters
A single element Plant Master controls demand
to the boilers based on the error between steam
header pressure and set point. As a result, this
type of Plant Master:
Overcompensates to changes in process
steam demand
May require different tuning parameters
over the operating range of the system
Could create operational inefficiencies and
thermal stress to the boiler due to steam
pressure oscillations.
The two element Plant Master uses steam
flow as a feedforward signal to the header pres-
sure error. The biggest problem associated with
this type of Plant Master is the tendency for over-
correction in changes in both steam flow and
header pressure, giving conflicting responses to
changes in header pressure and steam flow. For
example, header pressure will drop due to a de-
crease in BTU content of the fuel. The pressure
controller reacts to increase boiler demand. How-
ever, the decreased header pressure results in de-
creased steam flow. The feedforward index acts
to decrease boiler demand. Oscillations in boiler
demand then occur until the system can stabilize.
By contrast, the target steam flow Plant Mas-
ter gives stable steam header pressure by adjust-
ing the firing rate of all operating boilers and
adapts to varying boiler availability without op-
erator adjustment. The control action is adapted,
depending on which of the boilers are available
to respond to changes in steam demand and
automatically compensates for transient energy
effects. This combination of functions provides
stable and precise control of steam header pres-
sure with minimal operator intervention.
Furnace Pressure
(Induced Draft Damper) Control
Effective furnace pressure control improves
boiler efficiency and inhibits boiler deterioration.
Boiler load changes and combustion stability
both affect furnace pressure. The furnace pres-
sure control uses a feed-forward index and non-
linear control response, giving fast and smooth
furnace pressure control.
In a DCS, the furnace pressure control is part
of the air flow control. The FD fan demand is
characterized to program the ID inlet damper to
a position approximating the desired furnace
28 Council of Industrial Boiler Owners