Question

Which boiler mounting is used to extinguish the fire in the event of water level in the boiler shell falling below a certain limit?

• A. Blow off cock valve
• B. Safety valve
• C. Fusible plug
• D. Spring loaded safety valve

Hint:

Remember that safety valves protect boilers against overpressure, while the fusible plug specifically acts as a thermal safeguard against low water levels by physical melting.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Boiler mountings are essential safety devices and control instruments mounted directly on the boiler shell to ensure safe and efficient operation.
When the water level inside the boiler falls below a critical safety limit, the heat from the furce can overheat the dry boiler shell, leading to catastrophic structural failure or explosion.
A dedicated safety device must automatically trigger to put out the furce fire under these low-water conditions.

Step 2: Detailed Explation:
A fusible plug is a safety mounting installed at the lowest permissible water level, typically screwed into the crown plate of the furce or combustion chamber.
It consists of a hollow gunmetal plug containing a core of a low-melting-point metal alloy, typically composed of tin, lead, or bismuth.
Under normal operating conditions, the upper surface of the fusible plug remains completely submerged under water.
The water keeps the temperature of the plug below the melting point of its fusible alloy core.
If the water level falls dangerously low, the crown plate and the fusible plug become exposed to high-temperature combustion gases without the cooling effect of water.
As the temperature rises rapidly, the fusible alloy core melts and flows out of the plug assembly.
This leaves an open passage connecting the high-pressure steam space in the boiler shell directly to the furce.
High-pressure steam and hot water immediately escape through this opening, spraying directly onto the grate and extinguishing the furce fire.
This automatic mechanism prevents further heating of the dry boiler shell, thereby elimiting the risk of a major boiler explosion.
Other mountings like the blow-off cock are used for discharging sediments, and safety valves are used to relieve excess steam pressure.

Step 3: Fil Answer:
The boiler mounting designed to extinguish the furce fire during a low-water event is the fusible plug.
Hence, the correct option is (C).