Process Control and Instrumentation
This domain tests feedback control, PID tuning, loop stability, measurement selection, control valves, and advanced strategies such as cascade and feedforward. The exam expects you to diagnose oscillations, offset, and valve problems from process behavior.
Core rules
- A feedback loop measures the controlled variable, compares it with setpoint, and adjusts the manipulated variable. Proportional action leaves offset; integral action removes offset but can wind up; derivative action anticipates error but amplifies noise.
- Loop stability depends on gain and dead time. Excessive controller gain or dead time causes oscillation; derivative action on a noisy flow signal is usually harmful.
- Cascade control uses a secondary fast loop inside a slower primary loop. Feedforward acts before the disturbance affects the output but cannot correct model error alone.
- Control valves need a defined fail-safe position. A valve that fails open or closed must be chosen from the process hazard, not convenience.
Worked example
A proportional-only controller has Kc = 4 and the process steady-state gain is Kp = 2. For a setpoint change of 1 unit, the steady offset is ΔSP/(1 + KcKp) = 1/(1 + 8) = 0.111 units. The reasoning: proportional action increases output as error appears, but a nonzero error is still needed to hold the new output; the loop gain determines how much error remains. Adding integral action would drive this offset to zero, subject to stability.
Traps
Adding derivative action to a noisy flow loop, ignoring integral windup during startup, reversing controller action, and assuming a valve's fail position is safe without a hazard review.