Closed-Loop Tuning of Controllers ◾  99
The typical response times for variables in distillation column
control are as follows:
◾ Feed, reflux, or steam flow rate controller, 0.1 to 0.4 min
◾ Column pressure controller, 5 to 15 min
◾ Reflux drum or column base level controller, 10 to 30 min
◾ MRT (most responsive temperature) controller on a distillation column, 10 to 120 min
9.8 Pattern Recognition Tuning of
Integrating Control Loops
The level controllers for the reflux drum and the column
base level for a distillation column are examples of integrating process response. If a level controller is put in manual
output mode and the valve position for the liquid flow out of
a reflux drum is increased, the level in the drum will integrate the difference, that is, sum the difference over time,
and eventually empty the drum. Similarly, decreasing the
valve position will allow the drum to fill and eventually overflow. Consequently, the Robbins method of pattern recognition for tuning of level controllers uses a different set of rules
as follows:
1. Record the existing settings in the controller.
2. Turn off the reset, or set the integral time constant to 30
min or longer.
3. Turn off the derivative action, that is, D = 0.
4. Set the proportional gain to 1.25.
5. Introduce a step change in the controller setpoint.
6. Record the response time for the control loop to reach
about 95% of the step change in setpoint after introducing
the change.