What is the function of fusible plugs in solid fuel fire-tube boilers?

Fusible plugs serve a critical safety function in solid fuel fire-tube boilers by preventing overheating. These plugs are typically made of a low-melting-point material that is designed to melt at a specific temperature, which is indicative of a dangerous condition, such as a drop in water levels within the boiler. When the temperature rises to the point where the fusible plug melts, it creates a pathway for steam to escape, thereby reducing the pressure inside the boiler and preventing the risk of an explosion due to overheating. This safety feature is vital in maintaining the integrity of the boiler and ensuring safe operation, particularly in systems that utilize solid fuels which can lead to rapid temperature changes if not properly monitored.

The other options do not reflect the function of fusible plugs; they are not used for regulating steam pressure, monitoring water levels, or ensuring proper fuel flow. Instead, fusible plugs are specifically a fail-safe mechanism aimed at controlling overheating, thus highlighting their essential role in boiler safety.