Fig 8 "National" Waterside Fusible Plug
Fig 8 Fusible Plug assemblies (Fig.9 sold seperately) are used to protect internally fired steam boilers. If overheating occurs due to low water conditions, the plugs are designed to operate and allow pressure to reduce, thereby preventing collapse of the boiler.
The plugs are of gunmetal construction with a central portion held in position by a low melting point alloy.
When the alloy fuses, this component is ejected by steam pressure. The Fig 8 has it contained in a separate, additional (Fig 9) replaceable cone.