Page 125 - Boiler plant and distribution system optimization manual

110 Boiler Plant and Distribution System Optimization Manual

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Tuning Boiler Controls Tuning Boiler Controls
Measurement and Control Response Time Superheat and Reheat Thermocouples

1. Fast measurements improves quality and sta-
bility of control loops • Check for good thermocouple contact with the
bottom of the thermowell
2. Fast response times improves quality and sta-
bility of control loops • At 80% load increase spray valve position 5%
as a step function. Observe steam temperature
3. Valve stroking rates of 2 to 3 seconds Is re- time constant.
quired in many cases.
• Time constant should be less than 30 seconds.
4. If one loop Is upsetting other loops, a compro-
mise must be reached ————————————————————————
Figure 9.4—Tuning boiler controls, superheat and
5. In the final analysis, good engineering judg- reheat thermocouples
ment Is required for tuning loops

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Figure 9.39—Tuning boiler controls, measurement
and control, response time
Tuning Boiler Controls
Steam Temperature Control

• Fast accurate temperature measurements are
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desirable

Tuning Boiler Controls • Slow temperature constants can be compen-
PID sated for with derivative action at the tem-
Proportional Gain-Reset-Derivative perature controller

• It is desirable to use as little PID as possible ————————————————————————
Figure 9.42—Tuning boiler controls, steam tempera-
• Stability Is more Important than quality of ture control
control

• Excessive reset action is deadly for most con-
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trol loops
Tuning Boiler Controls
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Figure 9.40—Tuning boiler controls, PID proportion-
Adaptive Tuning
al gain -Reset-Derivative
• Final steam temperature controller must be
tuned at 5 points.

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Figure 9.43—Tuning boiler controls, adaptive tuning