Instrumentation, Controls, and Safety Devices
Core concepts
Understand the four process variables (pressure, temperature, level, flow) and the common sensors for each. Know transmitter output (typically 4–20 mA) and how it maps to a range. Study control modes: on/off, proportional, PI, PID, and the meaning of setpoint, error, offset, and reset. Understand safety devices: safety valves, high-limit switches, low-water cutoffs, flame scanners, and pressure switches. Familiarize yourself with burner management systems (BMS) and their function in purge, light-off, and trip sequences.
Traps to watch
- Confusing proportional offset (droop) with integral reset - only integral eliminates offset.
- Forgetting that low-water cutoffs must be tested regularly.
- Mixing safety valve (pressure relief) with safety switch (electrical trip).
- Assuming a 4–20 mA signal represents 0–100% - check the calibrated range.
- Overlooking the required pre-purge before light-off.
Worked example / rule of thumb
A 4–20 mA transmitter spans 0–1,000 kPa. At 12 mA: (12 − 4) / 16 = 0.5, so pressure = 500 kPa. Rule of thumb: 4 mA = 0%, 20 mA = 100%, and 12 mA is always mid-range.