Closed-Loop Tuning of Controllers ◾  87
6. Set the integral (reset) time constant in the controller to
be equal to the response time.
This will give an excellent response to load changes. Generally,
no derivative action is desired for distillation controllers,
because any noise in the signal for the controlled process variable can be translated into controller output and valve movement. Smooth controller output to a control valve is usually
desired for distillation. When derivative action is used, the
recommended setting is 0.10 to 0.15 times the response time.
The recommended method for tuning level control loops is
given in Section 9.8. It is different from the method for tuning
controllers with a self-regulating process variable response.
During start-up of a new distillation column, one question
can be asked about which controllers should be tuned first
and which ones should be tuned last. First, the liquid level
control loops for the reflux drum and the column base liquid
level can be set with the starting tuning constants recommended in Section 9.8, that is, a proportional gain of 1.25 and
an integral time constant of 30 min. Next, the fast-responding liquid flow rate control loops should be tuned. When
a cascade (master-slave) temperature controller is used, the
slow-responding temperature control loop is the master that
manipulates the setpoint to a fast-responding flow rate slave
control loop. The master temperature controller can be placed
in manual output mode to introduce a step change in setpoint
to the flow rate slave controller. The slave control loop needs
to be tuned before the master control loop is tuned.
One of the guidelines for Step 4, when there is no peak in
the process variable after a step change up to a higher setpoint, is to double the proportional gain. Of course, if the step
change is down to a lower setpoint, the response time would
be the time to reach the first valley in the process variable
response. Once the height of the peak-to-valley response is
in the range of 5% to 50% of the size of the step change, then
the proportional gain can be changed by an increment of