What is a Fusible Plug?
A fusible plug is a small but critical boiler mounting that protects the boiler from damage or explosion due to low water level and overheating of heating surfaces.
Basic purpose
The primary function of a fusible plug is to act as a temperature‑sensing safety device that responds when water level falls below a safe limit and heating surfaces get overheated. Unlike a safety valve (which responds to pressure), the fusible plug protects specifically against overheating caused by low water level.
Construction and materials
A typical boiler fusible plug consists of a gunmetal/brass/bronze body with a central tapered hole filled with a low‑melting alloy such as tin or a tin‑based fusible metal. The body is threaded to screw into the firebox crown or tube plate, and the fusible metal is protected on the fire side by a cap or flange so it sees metal temperature rather than direct flame erosion.
Location in boiler
In fire‑tube/smoke‑tube boilers, the fusible plug is usually fitted on the crown sheet or front tube plate just above the tube bank but below the minimum safe water level inside the shell. This ensures that under normal conditions it remains submerged in water, but becomes exposed to steam and flame as soon as the water level drops dangerously low.
Working principle
When water level is normal, the surrounding metal is cooled by water and the fusible alloy temperature stays below its melting point. If the level falls below the plug, heat transfer to water stops, metal temperature rises rapidly, the alloy melts, and an opening is created through which water/steam rushes into the furnace, quenching the fire and relieving pressure.
Typical features and maintenance
- Melting temperature of the fusible alloy is selected slightly above normal saturation temperature but below the safe metal temperature (for example around 232 °C for tin‑based alloys).
- Once a fusible plug has operated (melted), it must be replaced; it is not reusable and should be inspected periodically for choking, corrosion, or incorrect alloy.
