Design and Role of Fusible Plugs in Fire tube Boilers

What is a Fusible Plug?

A fusible plug is equipment that boosts the safety of the steam boiler and acts as a warning device against overheating. Fusible plugs are found in fire tube boilers and are usually considered the last resort or warning to avert any hazardous failure such as a boiler explosion. Just like safety valves help to protect the boiler system from overpressure, fusible plugs help to safeguard steam boilers from over temperature. Fusible plug is widely acknowledged as the preferred safety equipment for fire tube boilers by many industries due to its simple design, effectiveness, and improvement in safety.

Design and Material of Fusible Plug:

Fusible plugs include a fusible alloy made of tin material. The fusible plug itself is made of brass, bronze, or gun material with a threaded plug body that makes it easy to install. It is a hollow gun metal plug screwed at the apex of the firebox. The main metal plug and hollow gun metal plug are separated through an annulus fusible material. A flange protects the material from the fireside. The fusible plug has a high melting point, while the fusible alloy has a low melting point of 232°C (450°F).

The Role of the Fusible Plug:

The fusible remains immersed in water with the water in the fire tube boiler at a normal level. As the temperature does not reach the melting point of the fusible alloy, the heat is transferred to the water. As the temperature in the boiler increases, the water level starts decreasing and generates steam. On the other hand, the fusible plug above the firebox gets overheated by the exceeding temperature, and the fusible alloy starts melting. It results in steam and water immediately rushes through the fusible plug to extinguish the fire in the firebox, thus preventing boiler explosion. The fusible plug body is shaped in a way to alert the operating personnel of system pressure vents by a whistling sound. Fusible plugs reduce the possibility of boiler accidents taking place due to low water firing. The low water level in the boiler system leads to overheating of the heat transfer surfaces that can cause catastrophic accidents. It is advisable to replace the fusible plug after two years as they probably get defective after a long period of use.

Where is the Fusible Plug Installed?

Fusible plugs in fire tube boilers are located around 1 to 2 inches above the top row of tubes. Usual locations for the fusible plugs include the crown sheet i.e., the apex of the combustion chamber, rear tube sheet, and front tube sheet. The fusible plug location depends on the boiler design.

Faults and Inspection of Fusible Plugs:

• The fusible plug body is impacted by corrosion in fire tube boilers. As a result, the lifecycle of the fusible plugs starts declining. Therefore, it is recommended to record the washout before refilling it. Usually, the lifespan of the fusible alloy is 2 to 3 washouts periods, and that of the fusible plug is 4 to 6 washout periods.
• The fusible alloy may get affected by corrosion, depending on the water level in the boiler. The lead and body interface ensures corrosion, particularly if the tinning has been used in plug manufacturing. 
• In a fire tube boiler, the soot from the combustion may accumulate on the fusible plug that forms an insulating barrier. During such occurrences, the operations of fusible plugs are hindered. Therefore, it is important to remove the soot from the heating surface during maintenance.
• Fusible plugs need to be inspected regularly for leakages. The plug must not be tightened while the boiler is in operation if leakage is detected. If required, plugs can be checked when the boiler is cool and can be removed, inspected, cleaned, and refitted after partially draining the water in the boiler.

Conclusion:

Rakhoh Boilers are one of the globally leading boiler manufacturers for more than 38 years. We have provided efficient and reliable water tube boilers, fire tube boilers, waste heat recovery systems, thermic fluid heaters, and boiler accessories for over 20 process industries worldwide. With proficiency in thermal solutions, we offer the best boiler services like energy audit, steam trap assessment, boiler automation, annual boiler maintenance, etc., that enhances the productivity and lifespan of the boiler.

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