luokai0 Process Control Expert
Before Starting
- Single loop or multivariable control?
- New design or troubleshooting existing loop?
- Basic regulatory or advanced process control?
Core Expertise Areas
Feedback Control
PID controller: most widely used in chemical process control. Proportional: reduces error but leaves offset for load disturbances. Integral: eliminates offset, adds phase lag, can cause windup. Derivative: improves speed of response, amplifies noise. Direct vs reverse acting: configure based on process gain sign.
PID Tuning
Open loop test: step test gives process gain, dead time, time constant. IMC tuning: lambda tuning, set closed loop time constant to lambda. Ziegler-Nichols: ultimate gain and period from sustained oscillation test. Cohen-Coon: open loop tuning rules from FOPDT model. Autotuning: relay feedback test, automated parameter estimation.
Advanced Process Control
Cascade control: secondary loop controls manipulated variable faster. Feedforward: measure disturbance and compensate before it affects output. Ratio control: maintain fixed ratio between two streams. Override control: select between multiple controllers based on constraint. Model predictive control: MPC optimizes over prediction horizon with constraints.
Safety Instrumentation
SIS: safety instrumented system, independent from basic process control. SIL: safety integrity level determines required probability of failure on demand. High high and low low: independent alarm and trip levels beyond normal control range. Proof testing: periodic tests verify SIS functions correctly.
Best Practices
- Tune controllers conservatively, tighten only if performance insufficient
- Implement anti-windup for all integrating controllers
- Segregate safety systems from regulatory control systems
- Document control philosophy and loop descriptions in control narrative
Common Pitfalls
| Pitfall | Fix |
|---|---|
| Controller windup on output limits | Implement anti-windup in all PID controllers |
| Wrong controller action direction | Verify direct or reverse action before commissioning |
| Cascade with detuned primary | Tune secondary first then primary with secondary in auto |
| SIS on same DCS as regulatory control | Use separate independent SIS hardware |
Related Skills
- process-design-expert
- reaction-engineering-expert
- safety-engineering-expert