|
The air and fuel controllers get their setpoints from
the boiler master controller. In addition, each controller monitors the other
controller's flow
input to ensure that, on a load increase, air flow leads fuel flow, and on a decrease the fuel backs off before the
air does. This is the "cross limiting"
part. Oxygen control is implemented by passing the signal from the stack gas measuring
device to
the air flow controller. This directs the air flow controller to adjust its output very slightly to reach the ideal
excess oxygen level. All of the "cams"
for air / fuel ratio shaping and oxygen setpoint are in software curves in the controllers.
In both of the strategies, drum water level is measured, often along with steam flow and
feedwater flow, and another controller calculates the required feedwater delivery rate. It regulates
the feedwater valve to maintain drum water level.
CONTROL SYSTEM ARCHITECTURES
So your boiler plant has either a Single-Point Positioning strategy or a Metered, Cross Limited
strategy, combined with the separate requirements of the flame safeguard system for
complete
control. These strategies can be implemented in several control system types.
For the single point positioning strategy, it is
common with newer boilers to find a single control system "in a box" which
handles both combustion
controls and flame safeguard functions. These systems are now microprocessor based with
their
operator interface functions established by the manufacturer. They often use their own field measurement
devices, but usually come with
certifications of compliance with NFPA, IRI and/or FM. They also give a lot of diagnostic
information as a standard product.
Further, they can communicate digitally with most building environmental control systems (such as
Johnson Controls "Metasys") for data logging, etc. If, however, your existing
system has relatively
current flame safeguard equipment, and just the combustion controls need upgrades, it can be
accomplished by using Single Loop Controllers. These are controllers combined with hand-auto
|
|
stations, and can be used for the plant master,
boiler master and oxygen trim functions. These can also communicate with your in-house
environmental control system if properly chosen.
For Metered, Cross Limited applications used by larger boilers, more capability is needed in the
control system because the strategy requires a lot more computation. Usually, the flame safeguard
and combustion control functions are separated.
The flame safeguard system may be one of the packaged systems, similar to that used on the
smaller boilers, or it may be Programmable Logic Controller (PLC) based. The determining factor
in this decision is the relative complexity of the application. Single-burner, single-fuel boilers
making saturated steam are suited to the packaged approach, while multiple-burner, multiple-fuel
applications are custom enough to require a unique PLC solution.
There are three basic system types for combustion controls in the Metered, Cross Limited application:
Single Loop Controllers, Distributed Control Systems and Foundation Fieldbus.
For combustion controls, networked, micro-processor-based, single loop controllers are often
employed, as they provide both the ability to solve all of the control equations as well as provide a
good operator interface. In fact, some even have built in trending of the measured variable available
on the controller front. They can be interfaced with a PLC based Flame Safeguard system, as well
as communicating data to in-house environmental control and industrial process control
applications.
For larger boilers, especially those in petrochemical or Pulp and Paper plants who use
Distributed Control Systems ( DCS ), the DCS is usually used for boiler control. This control type,
or architecture, can look different from those described above. The only "controller faceplates"
might be on a PC screen or dedicated controller "look alike" modules. The main point is that all
control is conducted in the DCS cabinets. Further, the Flame Safeguard system may be incorporated
into the DCS logic, and may not be identifiable as
|