Instrumentation, Process Control and Automation
What the exam tests
Measurement of pressure, temperature, flow, level and composition; P&ID symbology; control valve sizing and fail action; PID tuning and loop stability; cascade, feedforward and DCS/PLC/SIS architecture.
Core rules
- Proportional band and gain are reciprocal: PB% = 100/K_c; proportional output change is K_c × error.
- PID form: u = K_c[e + (1/τ_I)∫e dt + τ_D de/dt]; apply derivative to measurement to avoid setpoint kick; use anti-reset windup against output limits.
- Tuning by Ziegler–Nichols ultimate gain and period, or by the process reaction curve; dead time caps achievable gain.
- Control valve sizing uses C_v; gas flow chokes near ΔP ≥ 0.5 P₁; fail-closed versus fail-open follows the safety case.
- Cascade loops need the slave much faster than the master; feedforward requires a process model.
- P&ID reading: line types, instrument bubbles, interlock versus SIS boundaries.
Worked example
A reverse-acting flow controller has PB = 20% and an error of +4% of span (measurement above setpoint). K_c = 100/20 = 5. Proportional output change = −5 × 4 = −20%, so the output falls from 50% to 30%. With τ_I = 1 min and the error persisting, the integral term adds (K_c/τ_I) × e = (5/1) × 4 = 20% per minute, driving the valve further closed until the error is removed.
Traps
- Confusing direct and reverse action, which produces positive feedback.
- Leaving derivative on error and getting setpoint kick on every operator change.
- Ignoring actuator air-failure direction during hazard review.
- Aggressive gain on a loop with large dead time, causing oscillation.
- Misreading a 4–20 mA signal converted across a 250 Ω resistor.