A soft plug, or fusible plug, is a safety device for boilers. Installed in the crown sheet, the low temperature alloy melts if the sheet becomes overheated due to low water. It blows out and extinguishes the fire.
Ben Nixon style
Ben Nixon wrote the following in the "The Smoke Stack", Niagara Frontier Live Steamers, 1 December 1996:
- A Crown Sheet Soft Plug to protect live steam boilers. Build up an area to 1/4" or 3/8" and drill and tap for a 1/8" brass pipe plug. Modify plug as shown and soft solder copper rivet in place. Install plug and test for leaks.
- If crown sheet becomes dry immediate temperature increase will melt solder and the remaining water in boiler will gently steam fire out with no damage to boiler.