30
5 Firing Rate Controller (optional)
I > faster approximation to the setpoint
danger of overshooting!
• D term
The derivative term is calculated from the variation of the actual values. It has acceleration
and respectively slowing effects.
In practice the adjustment of the PID-controller is specified to each controlled system. With
the characteristics of the controlled system you try to deduce acceptable data, i.e. the values
are defined experimentally
• P term, I term, D term are added together and act as adjustment on the firing rate default
for the ratio control. The value is added up, starting from the instantaneous internal firing
rate.
• As long as the actual value is below the setpoint, P term and I term are positive, that is to
say both these term will increase the firing rate default.
• Only the D term is negative in such a case (assuming that the boiler temperature is rising).
Use the D factor sparingly because it leads to a higher burden of the actuators.
• In order to avoid excessive overshoot during burner start-up, the parameters should be
adjusted to achieve a suitably large D-term.
• If, despite a large setpoint deviation, the burner is not running at full firing rate or base firing
rate, the P term should be increased.
• The longer the adjustment time, the quieter the group. However, this also increases the
actual value's deviation from the setpoint value and leads to slower adjustment.
Values must be adjusted according to the controlled system.
* Default value
Fig. 5-5 Controller operation with P-term too high
Hot Water Installations Steam Boiler Installations
P term 120* 600
I term 60* 300
D term 20* 25
Adjustment time 15* 20
Characteristic Control Process Control Mode Start-up Procedure
P term higher decrease of
attenuation
stronger reaction with
overshoot
faster start-up with
overshoot
P term smaller increase of
attenuation
less reaction, less ten-
dency to oscillate
slower startup
Characteristic Control Process Control Mode Start-up Procedure
I term higher decrease of atten-
uation
stronger reaction with
tendency to oscillate
faster start-up with ten-
dency to oscillate
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