Foundation
Proportional action, and the offset it leaves
Before you start
The feed steps up. Proportional-only control answers, but never all the way.
Proportional action moves the valve in proportion to the error right now. Twice the error, twice the correction. It is fast and it is honest, and it is what you feel first when you turn up a controller.
It also cannot finish the job. A proportional controller only produces a correction while an error exists, so the moment the level returns to setpoint the extra valve opening disappears. If the feed is permanently higher than before, the valve must sit permanently further open, and the only way this controller holds it there is to keep a permanent error.
That standing error is called offset, or steady-state error. It is not a defect in your tuning. It is what proportional-only control is.
Turning Kp up shrinks the offset without removing it, and buys the reduction with a more violent valve. That trade is the whole reason the next lesson exists.
Your task
- Integral and derivative are locked off for this lesson; only Kp is yours.
- Run the feed step and watch where the total level settles after the disturbance.
- Try a higher Kp and run it again. Compare where the level settles, and what the valve did to get there.
Configure both controllers before starting. Gains are locked during the attempt.
SEP-101 horizontal separator with gas, oil, and water. True total level 0.0 percent; measured total level 0.0 percent. True interface 0.0 percent; measured interface 0.0 percent. Total inlet 0.0 cubic metres per hour; oil outlet 0.0 cubic metres per hour; water outlet 0.0 cubic metres per hour. Actual oil valve 0.0 percent; actual water valve 0.0 percent. Safety: Filling: low-level protection bypassed.
SEP-101 · engineering fallback
Separator process cutaway
Filling: low-level protection bypassed
- Total true
- 0.0%
- Total measured
- 0.0%
- Total SP
- 50.0%
- Interface true
- 0.0%
- Interface measured
- 0.0%
- Interface SP
- 20.0%
- Total inlet
- Stopped / 0.0 m³/h
- Oil outlet
- Stopped / 0.0 m³/h
- Water outlet
- Stopped / 0.0 m³/h
- Oil command
- 0.0%
- Oil actual
- 0.0%
- Water command
- 0.0%
- Water actual
- 0.0%
- Active safety
- Filling: low-level protection bypassed
2D engineering fallback · process values and safety logic remain fully active.
LIC-101
Ready to tuneTotal level / oil valve
More valve opening → more oil out → lower total level
- Total level process value
- 0.0%
- Total level measured value
- 0.0%
- Total level setpoint
- 50.0%
- Oil valve command
- 0.0%
- Oil valve actual position
- 0.0%
- P / I / D
- 0.0 / 0.0 / 0.0
Advanced settings
IIC-101
Ready to tuneInterface / water valve
More valve opening → more water out → lower interface
- Interface process value
- 0.0%
- Interface measured value
- 0.0%
- Interface setpoint
- 20.0%
- Water valve command
- 0.0%
- Water valve actual position
- 0.0%
- P / I / D
- 0.0 / 0.0 / 0.0
Advanced settings
Synchronized historian
Live control trends
Oil/water interface
Truth, LT-102, SP and protection limits · %Phase flows
Feed and phase flows · m³/h; water cut · %Valve response
Commands and actual positions · %Four synchronized trends compare both controlled levels, phase flows, water cut, and both valve responses.
Debrief
Run the exercise to see how you did.