Safety Devices, Controls, Interlocks and Protective Equipment
The protective stack
Protective devices sit above the operating controls. The operating pressure or temperature control cycles the burner; the high limit is a separate device set above it that must never be used as the operating control. Low-water fuel cutoffs may be float, probe or conductance type, and many designs require manual reset after a trip - resetting without finding the cause is a classic wrong answer. Test the cutoff by lowering the water (or using the test connection) and verifying the burner actually shuts down, not merely by blowing the glass.
Flame safeguard and fuel train
A modern programmer provides prepurge, pilot proving, a limited trial-for-ignition period, main flame supervision, postpurge, and lockout on flame failure. Gas trains add valve-proving (leak test), high and low gas pressure switches, and combustion air proving switches. Oil systems add low oil temperature and low atomizing pressure interlocks.
Relief and mechanical protection
Safety valves must be free, piped to a safe discharge, and periodically tested with records kept. Hot water boilers use relief valves, often with a separate high-temperature limit.
Traps and rule of thumb
Never jumper an interlock to keep a boiler on line. The high limit setting is above the operating control setpoint, and the safety valve setting is above both. Personal protective equipment - arc-rated clothing, hearing and heat protection - belongs in the answer whenever the question involves opening a panel or working near a burner.