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CONTROL SYSTEM FUNCTIONS AND
STRATEGIES
Figure 2. The control strategy is defined using
graphics.
There are two systems to consider: Combustion
Controls and Flame Safeguard Systems.
Combustion controls are the measurements, control calculations, and valve and damper drive
commands which regulate the delivery of fuel and air to the combustion process, as well as the
feedwater flow to the steam drum. These processes are all analog, or exhibit continuous
smooth changes. The objective of the combustion controls is to hold steam pressure (or water
temperature) at the setpoint, the pressure or temperature needed in the plant. The Flame
Safeguard System, also referred to as the Burner Management System, checks for safety related
switched signals to ensure safe operation of the boiler. It checks the status of safety issues such as
high or low fuel pressure, low (or high) drum level, correct boiler preignition air purge duration,
and a host of other on / off switches. It also watches the boiler flame through the use of a
"flame scanner", a photocell, to shut the unit down if the flame is lost. The two functions, flame
safeguard and combustion control, are packaged differently in the control system, depending on the
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strategies as described below.
Single Point Positioning Control
For this discussion, there are two fundamental methods of combustion control employed today.
A method called "single point positioning" is used on firetube boilers (the round ones) and on the
smaller watertube designs up to about 30,000,000
BTU's. First, a "plant master controller" continually checks the steam pressure or water
temperature and sends the individual boiler master a load signal. The boiler master then sends its
signal, in response, to a single actuator which positions a jackshaft, a mechanical link connected
to both the air damper and the fuel valve. Fuel-to-air ratio characteristics, which vary
over the firing range, are adjusted by cams on the fuel valve,
shaped for the individual application. But the main idea is that everything moves at once.
Everything is hard linked, mechanically.
For better efficiency, oxygen trim can be added to this control strategy. Excess oxygen is measured
by a device in the stack, compared to an ideal reading established when the controls are first
installed and adjusted, and the resulting correctional signal sent to a fuel pressure regulator
to minutely adjust fuel pressure to correct for deviations from the optimum.
Metered, Cross Limited Control
The drawback of single point positioning control comes in boiler size. The bigger they are, the
harder it is to get a jackshaft to reach all of the components. The natural "play" in the jackshaft as
the components wear over time is also a potential efficiency problem. For larger boilers, up to 300
million BTU per hour or so, this leads to the second combustion control strategy: Metered,
Cross Limited Control.
In Metered, Cross Limited Control, both air and fuel flows are measured continuously with the
flow signals fed to controllers which manipulate the final control elements to control the flows to a
setpoint. This is the "metered" part.
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