How to tune a PID loop on a running plant
A plant-floor procedure: read first, refuse mechanical problems, change one constant at a time, and never cycle the loop to find a number.
Tuning guides by loop type
What each class of loop wants, what usually goes wrong with it, and what to check before you change any constants.
How to tune a cascade PID
Tune the inner loop first, in automatic, then the master. Why cascade hunting happens when both are moved at once, and what the structure is for.
Averaging level control
Most level loops exist to absorb variation. How averaging level control uses the volume, why offset is intended, and why tight level control dumps the problem downstream.
Boiler drum level control
Why single-element drum level fails on shrink and swell, and how three-element control uses steam and feedwater flow to hold the drum.
Tuning flow control loops
Flow loops are the fastest and noisiest loops on a plant. Why they want low gain and fast reset, why derivative belongs at zero, and what usually goes wrong.
Tuning level control loops
Level is integrating, not self-regulating. Why most level loops should be tuned loose, and why tight level control causes problems downstream.
Tuning pressure control loops
Gas pressure loops behave like slow flow loops; liquid pressure is faster still. How volume, compressibility and valve sizing shape the tuning.
Tuning pump and fan speed loops
On a VFD loop a significant part of the dynamics lives in the drive. What to check in the drive before touching the controller.
Tuning temperature control loops
Temperature loops are slow and lag-dominated, and the one common case where derivative action genuinely earns its place.
Fired heater and heat exchanger temperature
Coil outlet and process temperature on fired heaters and exchangers: cascade onto fuel or coolant flow, and why flame or fouling problems are not a PID.
Tuning analyzer and composition loops
Analyzer, conductivity and composition loops are dead-time dominated. Why pH is not a substitute, and why the sample system is usually the constraint.
Tuning pH control loops
pH loops combine extreme process nonlinearity with long sample dead time. Why tuning is rarely the answer and what usually is.
Compressor anti-surge control
Anti-surge is a protection loop, not a PID you retune from a trend. Why detuning a recycle controller is dangerous, and what actually belongs to the surge line.