Practice
A noisy interface, and what derivative does to it
Before you start
Derivative predicts. On a noisy measurement it mostly invents.
The oil/water interface is the hardest measurement on the vessel. The boundary is not a clean line, and the instrument sees a band rather than a level.
Derivative action responds to how fast the measurement is changing, which lets it start correcting before the error grows. On a clean measurement that is genuinely useful.
On a noisy one it is close to poison. Noise is, by definition, fast change. Derivative amplifies exactly that and turns sensor noise into valve movement, so the valve works continuously while the actual interface goes nowhere. The wear is real; the control is not.
This is why the derivative term here acts on a filtered measurement rather than the raw one, and why the honest answer on a noisy loop is usually a small derivative gain, or none at all.
Your task
- The interface transmitter has ±1.5% noise and holds each sample for half a second.
- Tune the interface loop to track its setpoint without letting the water valve chatter (move in rapid, repeated small steps).
- Try raising Kd and watch the valve travel figure, then decide whether it was worth it.
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.