Boiler safety component for low-water protection A ________ in a boiler is used to extinguish the fire in the furnace automatically when the water level in the boiler falls to an unsafe limit.

Introduction / Context:
Low-water conditions in boilers are dangerous because exposed heating surfaces can overheat and fail. A simple but effective protective device used in fire-tube and some shell boilers is the fusible plug, positioned to melt when uncovered by water, thereby giving an unmistakable warning and quenching the fire.

Given Data / Assumptions:

• Conventional shell/fire-tube boiler furnace or crown sheet area.
• Correctly installed fusible plug with low-melting alloy core.
• Adequate draft such that releasing steam/water into the furnace extinguishes the fire.

Concept / Approach:
The fusible plug contains an alloy insert that melts at a predetermined temperature slightly above normal water-wetted metal temperatures. If the water level drops and the metal becomes exposed to furnace heat, the temperature rises, the alloy melts, and steam/water discharges through the plug into the furnace, dousing the fire and alerting operators.

Step-by-Step Solution:
Normal operation: plug tip is submerged; metal temperature remains below fusible alloy melting point.Low-water event: crown sheet uncovered, temperature rises rapidly.Alloy melts; opening allows steam/water to blow into the furnace.Combustion is disrupted/extinguished, preventing further overheating and alerting operators.

Verification / Alternative check:
Inspection codes and boiler texts identify the fusible plug as a mandatory or recommended protection for certain boiler classes, distinct from safety valves (which limit pressure) and blow-off cocks (which drain sludge/water).

Why Other Options Are Wrong:
Blow off cock is for draining sludge/boiler water; stop valve isolates steam line; safety valve prevents overpressure; economizer bypass is unrelated to low-water hazards.

Common Pitfalls:
Assuming the safety valve protects against low water—pressure relief is not a substitute for level protection.

Final Answer:
fusible plug