Safety Devices, Controls, and Interlocks
Core concepts
Safety devices protect the boiler when the operator cannot respond fast enough. The safety valve is the final overpressure protection; it must be set at or below MAWP and must be tested and sealed. The low-water fuel cutoff shuts off fuel when the water level drops below the safe limit; it must be tested regularly and must never be bypassed while firing. The high-pressure limit switch and high-temperature limit trip the burner; the flame safeguard (flame scanner and programmer) proves flame on startup and shuts fuel off on flame failure. The burner management system sequences prepurge, pilot, main flame, and shutdown.
Know interlocks: low-water, high-pressure, flame failure, low fuel pressure, low atomizing steam or air, and combustion air proving. Know that prepurge (typically several air changes of the furnace) must complete before ignition to clear combustible gases. Know the difference between a trip (automatic shutdown) and an alarm (notification only).
Traps
Questions test whether the low-water cutoff can be bypassed (no), the purpose of prepurge (purge combustibles, not cool the furnace), the difference between a limit switch and an operating control, and what the flame scanner proves (flame presence, not fuel quality). Another trap: assuming a safety valve set at MAWP allows operating pressure above MAWP.
Worked example
Rule of thumb: if the low-water cutoff trips repeatedly, find the cause (level control, feed pump, or fouled probe) rather than adjusting the cutoff to a lower level; lowering the trip point defeats the protection.