26-07-2012, 01:08 PM
Utility Boiler Attemperator Controls
Effective control of main steam temperature on a coal
fired utility boiler provides a difficult challenge. Feedback
only control is not adequate due to significant dead time
between the interstage attemperator (control element)
and the superheater outlet temperature (process
variable). Several variables affect the superheater outlet
temperature such as boiler load, boiler pressure, excess
air, gas recirculation, pulveriser biasing or burner tilt and
sootblowing. Additionally the attemperator spray causes
variances in the boiler main steam flow, which can have
unwanted affects on boiler master and unit master
control. Another problem is that attemperator spray
nozzles and control valves are normally significantly
oversized to handle a wide specification of fuels and
extreme upset conditions.
The design approach for attemperator control is based
on the understanding that for a given fuel energy input to
the boiler there is a fixed steam energy output.
Attemperation can then be considered a function of the
saturated steam generated at that boiler load.
Main Steam Flow = Saturated Steam Generated +
Attemperator Spray Flow
Knowing the affects that variables such as boiler load,
boiler pressure, excess air, gas recirculation, and
pulveriser biasing have on saturated steam generation;
feedforward signals can be developed that will predict
the required attemperation. The feedforward signals are
then tuned with lag or dead time to match the dynamic
response of the system. If the control system is not
capable of supporting the coordination of the
attemperator and boiler master controls, then the
feedforward signals can be tuned to reduce the affect of
attemperator spray on the boiler master during load
swings.
To ensure successful implementation, all aspects of the
attemperation system are considered including the
physical condition of the attemperator, nozzle and
control valve sizing and current and historical operating
data.
• Review of current and historical boiler operating data to
determine actual required attemperator spray flows.
• Internal (videoprobe) inspection of attemperator to assess
physical condition.
• Review of attemperator nozzle and spray valve sizing.
• Development of control strategy using feedforward signals.
• Complete DCS graphic and control configuration
• Commissioning and startup services
• Control system tuning
The implementation of the modified attemperator control
strategy in conjunction with resizing of attemperator nozzle
and spray control valve has proven to be successful in
improving main steam temperature control during load swings
on a coal fired utility boiler. Additionally, the attemperator
controls have been tuned to reduce unwanted effects of the
attemperator spray on the boiler master. The overall result is
a significantly improved response of the boiler to unit load
demand changes.