Safety Devices, Controls, and Interlocks
Safety Devices, Controls, and Interlocks
This domain tests the devices that prevent overpressure, low water, flame failure, and unsafe airflow. You must know what each device does, what it does not do, and how to test it.
Primary devices
The safety valve protects against overpressure. The low-water fuel cutoff (LWCO) stops firing when water falls below the safe level; it may be float, probe, or electrode type. The flame safeguard supervises pilot and main flame and closes fuel valves on failure. High-limit pressure or temperature controls stop firing at a set point. Airflow switches prove combustion air and purge; gas trains include pressure switches and safety shutoff valves.
Interlocks and testing
Interlocks enforce sequence: no purge, no light-off; no pilot proof, no main fuel; no airflow, no firing. Test LWCO by lowering water or using the test connection, not by guessing. Test safety valves by lifting with the try lever under controlled conditions, and record the result. Never plug, jumper, or bypass a safety device.
Worked example
A steam boiler trips on low water. The gauge glass looks normal, but the LWCO has tripped. The operator should not add water immediately to a hot, empty boiler; that can cause a violent collapse. Instead, trip the burner, let the boiler cool, investigate the LWCO and gauge glass, and inspect before refilling. The reasoning: the LWCO may be correct, and the glass may be falsely high because its lower connection is clogged.
Traps
A safety valve is not a pressure regulator. An LWCO is not a water-level controller. A high-limit control is not a substitute for a safety valve. Jumping a flame scanner or wedging a safety valve is a serious violation.