FUSIBLE PLUG.
Lunkenheimer
Fusible Plug.
A Fusible Plug is a short brass bolt which has a hole running through its center, filled with a metal that melts at a low temperature. This plug is screwed into the crown sheet directly over the fire, and as long as it is covered with water the metal will not melt and run out; but should the water become low, exposing the crown sheet to the intense heat of the fire, the metal will run out, and the steam rushing through the hole puts out the fire and many times saves the crown sheet from injury.
Q. In what condition would the plug become useless and of no value?
A. By allowing it to become covered on the inner end with scale and sediment. It should be unscrewed and occasionally examined, at least two or three times during the season, and all scale and dirt removed from the end of the plug before replacing. Examine the crown sheet to see that no scale has formed over the hole to prevent the water from reaching the plug.
Q. How can a plug that has melted out be refilled?
A. Unscrew the plug from crown sheet and cap one end with clay, then melt the lead or babbitt metal in a shovel, spoon, or even a piece of bent sheet iron, and refill the plug. Now, with a light hammer, close the ends tight, and screw the plug into the crown sheet.