Safety Devices, Controls, and Interlocks
This domain tests the protective devices that trip fuel or limit operation before a boiler is damaged. Expect safety valves, low-water fuel cutoffs, pressure and temperature limits, flame safeguards, fuel-train interlocks, and testing requirements.
Protective devices
- The safety valve is the final overpressure protection and must be set at or below MAWP with adequate capacity.
- A low-water fuel cutoff trips the burner when water falls below a safe level; it may require manual reset after a trip.
- High-pressure and high-temperature limits trip the burner before the safety valve lifts.
- Flame safeguards prove flame and close fuel valves on flame loss.
- Fuel-train interlocks include proof-of-closure, low fuel pressure, combustion-air proving, and purge timing.
Testing and response
Test low-water cutoffs by blowing down or evaporating the level as the manufacturer directs, and confirm the burner actually trips. Test high limits and flame scanners through their normal trip paths. A safety valve is tested by qualified personnel; it must never be gagged or plugged.
Worked example: A 150-psig boiler has a high-pressure limit switch set at 140 psig. The burner trips at 140 psig, below MAWP. That is acceptable protection. If the switch were set at 160 psig, it would not protect the boiler because the safety valve could lift first or the limit would allow pressure above MAWP. Correct the setting and verify the trip.
Traps: jumpering a low-water cutoff because of nuisance trips; resetting a manual-reset device without finding the cause; testing only the alarm and not the fuel trip; plugged sensing lines; adjusting safety valves in the field.