Process Control Systems and Loops
The Process Control Systems and Loops domain tests whether you can read a loop's dynamics and documentation, select the right control strategy, and tune it without destabilising the plant. Level III questions mix PID behaviour, cascade and feedforward structures, valve characteristics, and P&ID and loop-sheet interpretation.
Core concepts
- Proportional action gives speed but leaves offset; integral removes offset but adds phase lag and can wind up; derivative anticipates but amplifies noise, so it is rarely used on flow or level.
- Dead time (θ) is the tuning constraint. Lambda tuning sets the closed-loop time constant λ relative to θ: for a first-order-plus-dead-time model, Kc = τ / (Kp (λ + θ)) and Ti = τ for a PI controller.
- Cascade control puts a fast secondary loop (flow, valve position) inside a slow primary loop (temperature, level); the primary output becomes the secondary setpoint.
- Feedforward measures a disturbance and acts before the error appears, then feedback trims the result.
- Valve characteristics matter: equal-percentage trim on a mostly low-flow application gives better resolution than linear.
Worked example
A self-regulating process has Kp = 2, τ = 60 s and θ = 10 s. Choosing λ = θ = 10 s for a robust, non-oscillatory response gives Kc = 60 / (2 × (10 + 10)) = 1.5 and Ti = 60 s. A larger λ produces a slower, gentler response; a λ close to or below the dead time produces tight control but risks cycling.
Traps
- Tuning with the controller in manual and wondering why nothing changed.
- Leaving derivative on a noisy flow signal and blaming the valve for the chatter.
- Ignoring integral windup in cascade or split-range loops during start-up.
- Reading a P&ID symbol as fail-open when the loop sheet specifies fail-closed.