Open-Loop Testing of Process Response ◾  115
can be with a vertical reflux drum with the level transmitter at
50% at steady-state conditions. Say the controller output is at
70% with the flow rate into the tank equal to the flow rate out
of the tank at 0.05 tank volumes per minute. A tank volume
can be defined as the volume in the tank between the levels
of 0% and 100%. This would give a residence time of 20 min
if the tank were at 100% level and 10 min if the tank were at
50% level. Increasing the flow rate out of the vessel from 0.05
to 0.06 tank volumes per minute would cause the tank level to
drop 1% per minute. With a constant liquid flow rate into the
tank, a step change to increase the outlet valve position would
give a liquid level response that would be a straight line sloping down with time.
Some guidelines for level control are to use a proportional
gain of 1.25 (proportional band of 80%) in the controller when
using the accumulator for level averaging to smooth the outlet
flow rate and allowing the level to deviate from setpoint to
accomplish this. The integral time constant can be set between
1 and 2 times the residence time of the material in the accumulator. A higher proportional gain can be used in the controller for tighter level control.
All of the tuning rules in Tables 10.2, 10.4, and 10.5 predict
that the controller gain, K c , can be set to a high value if the
dead time, DT, is short. As a result of this, the proportional
gain for a level controller can often be adjusted to very high
values without creating a sustained cycle if the integral time
constant is kept larger than the closed-loop response time for
the process variable to move 95% of a step change in setpoint
as shown in Chapter 9, Section 9.9. At some point, an increase
in proportional gain in the controller, K c , will simply amplify
the noise in the process variable signal into chatter (rapid
movement) of the controller output and the control valve position. The noise in the process variable signal can be caused
from splashing in a reflux drum or boiling in a reboiler. In
that case, the controller gain can be reduced until the rapid
movement in the control valve is reduced to 2% of the average