Boiler Controls, Instruments, and Safety Devices
Layers of control
Separate operating, limit and safety functions. Operating controls (aquastat, pressuretrol, feedwater regulator) hold the setpoint and cycle the burner. Limit controls (high steam pressure, high temperature, low-water fuel cutoff) are backups, with manual reset where required. Safety devices - safety valves, flame safeguard, low-water cutoff - protect against the hazards themselves. Learn what each device does not do: a low-water cutoff is not a level control.
Flame safeguard and startup
The burner management system proves airflow, pre-purges, proves the pilot, opens the main safety shutoff valves, proves main flame, then releases to modulation. Flame scanners (UV, IR, flame rod) must see the flame they are proving; UV scanners use shutters and self-checking circuits because hot refractory radiates UV. A loss of flame signal must shut the fuel off within seconds.
Instruments
Gauge glass is the only true level indication; the pressure gauge has a siphon so steam does not destroy the tube; try-cocks give an independent check. Know the difference between a gauge that reads and a transmitter that signals a controller.
Traps
- Treating the low-water cutoff as an operating level control.
- Jumping or bypassing manual-reset limits.
- Trusting a gauge glass that has not been blown down.
- Forgetting to prove airflow before purge.
Rule of thumb
Test the low-water cutoff and the high limit at the start of every shift and record it: blow the float chamber down, confirm the burner trips, then reset.