Boiler protection on dangerously low water level — identify the device Name the device used to automatically extinguish the furnace fire when water level in the boiler falls to an unsafe limit.

Introduction / Context:
Low-water conditions are among the most serious boiler hazards. A dedicated device exists to protect the furnace and heating surfaces when water level drops below a safe value, preventing metal overheating and potential failure.

Given Data / Assumptions:

• Standard shell or fire-tube boiler with a furnace crown exposed to flame.
• Water level monitoring and safety devices installed.

Concept / Approach:
The correct device is the fusible plug, which melts at a predetermined temperature when it is no longer covered by water, releasing steam/water into the furnace to quench the fire and alert operators. Since “fusible plug” is not listed among the options provided, the correct selection here is “none of these.”

Step-by-Step Solution:

Verification / Alternative check:
Boiler mountings list includes: safety valve, stop valve, water-level gauge, pressure gauge, blow-off cock, and fusible plug—the last of which handles low-water hazards.

Why Other Options Are Wrong:

• Blow-off cock: drains sludge/water, not automatic fire extinction.
• Stop valve: isolates steam line.
• Superheater/economiser: heat exchangers; no protective role for low water.

Common Pitfalls:
Confusing blow-off cock with an emergency dump; believing the safety valve responds to low water—it does not.

Final Answer: