Instrumentation, Controls, and Safety Devices
What this domain tests
This domain covers the sensors, transmitters, controllers, and safety interlocks that keep a power plant within safe limits. Questions test measurement principles, control loop behaviour, and the logic of trips and interlocks.
Core rules to have at your fingertips
- Sensors: thermocouples (Seebeck effect), RTDs (resistance changes with temperature), pressure transmitters (capacitive/strain gauge), flow (orifice, venturi, ultrasonic, Coriolis).
- Control loops: open loop (no feedback) vs closed loop (feedback). PID control: proportional responds to error, integral removes offset, derivative anticipates change. Tuning order: P, then I, then D.
- Signal standards: 4–20 mA current loop (live zero allows detection of broken wire), 0–10 V, HART, Foundation Fieldbus.
- Safety devices: low-water fuel cutoff, high-pressure limit, flame scanner (UV/IR), high-temperature limit, and emergency stop. Trips are fail-safe - de-energise to trip.
- Interlocks: purge before ignition, prove flame before opening fuel valve, trip on loss of flame.
Worked example
A pressure transmitter has a range of 0–2 000 kPa and outputs 4–20 mA. It reads 12 mA.
Span = 20 − 4 = 16 mA. Fraction of span = (12 − 4)/16 = 0.5. Pressure = 0 + 0.5 × 2 000 = 1 000 kPa.
If the controller setpoint is 900 kPa with a proportional band of 10 % (200 kPa), the output starts to change when error exceeds 100 kPa - at 1 000 kPa the error is 100 kPa, so the controller is at the edge of its proportional band. Integral action would then remove the remaining offset.
Traps that cost marks
- Assuming a 4–20 mA signal starts at 0 mA - 4 mA is the live zero.
- Confusing proportional band with gain - gain = 100 / PB %.
- Forgetting that trips must be fail-safe (loss of power = trip).
- Ignoring the purge requirement before burner light-off.
- Mixing up RTD and thermocouple temperature ranges and linearity.
Exam strategy
For signal questions, convert mA to % of span, then to engineering units. For trip questions, state the sensor, the setpoint, and the action taken.