"Screw Cone" fusible plug, in half longitudinal section

"Screw Cone" fusible plug renewable from the water side screwed, 2 in. gas thread, in half longitudinal section

A fusible plug is a safety device used in steam boilers. The plug has a fusible metal core which melts at a relatively low temperature, and is positioned inside the boiler between the furnace and water-space above it. If the water level falls too low, potentially causing an explosion, the plug is exposed. The fusible metal melts and steam is vented into the furnace beneath, alerting the operator to the potential danger.

In this construction, patented in 1888 by Mr. H. Hiller, the fusible alloy is used to secure a core into a readily removable plug screwed into a gunmetal seating, which is screwed into the furnace crown. The plug is provided with a bevelled edge similar to that of a valve, so as to ensure a tight joint, and the core is formed with a flange, which protects the fusible metal from the direct action of the fire, and prevents its gradual escape.