Boiler Safety, Protective Devices and Emergencies
Protective devices
Safety valves are the ultimate overpressure protection: they must be set to lift at or below the design pressure, be tested regularly, and never be gagged. Low-water cut-out cuts off fuel when the water level falls dangerously; it must be tested regularly, often by blowing down the float chamber. High-pressure and high-temperature cut-outs, flame-failure devices, fusible plugs and alarm systems complete the set. A fusible plug melts at a set temperature to warn of low water and overheating.
Low water and explosions
If low water is indicated, stop firing and do not feed water into a boiler that may have overheated - sudden feed can flash and cause a violent explosion. Shut down and let it cool. Boiler explosions arise from overpressure, low water, scale overheating, or explosion of unburnt gas in the furnace.
Emergencies and first aid
For a fire, isolate fuel and use the correct extinguisher. For burns, cool with clean running water, cover, and seek medical help. For electric shock, isolate before touching. Keep escape routes clear and know the emergency procedure.
Traps
Questions test the rule "never feed a boiler with suspected low water" and the difference between a safety valve and a relief valve. Also remember that a low-water cut-out is a protective device, not a level control.
Worked rule of thumb
If the gauge glass shows no water and the low-water cut-out has tripped: stop firing, do not feed, evacuate and cool. Feeding a hot, empty boiler is the classic fatal error.