Fusible Plug Safety Tips

How much do you know about the fusible plug on your steam traction engine?

| September/October 2003

  • Fusible pulgs 1

  • Fusible plugs 2
    Figure 2: A 3/4-inch fusible plug showing the matrix type of oxidation at the top and the infusible crust at the bottom. The filling in this plug did not melt when it was heated to over 1,000 degrees F at a metallurgical lab.
  • Fusible plugs 3
    Figure 3: A 3/4-inch fusible plug showing three modes of failure: The plug of scale could possibly withstand the pressure of steam if the tin melted; the filling did not melt when heated to over 1,000 degrees F and is completely converted to the matrix type of oxidation; the lower portion of the plug was filled with infusible crust, which crumbled when the plug was cut in two.

  • Fusible pulgs 1
  • Fusible plugs 2
  • Fusible plugs 3

As much as I enjoy auctions and good flea markets for antique steam traction engine parts, I no longer consider them acceptable venues for buying fusible boiler plugs, especially in the absence of sufficient information to assure me that what I am buying is right for the boiler I'm going to use it on. The following are some steam engine safety tips, specifically boiler safety tips, pertaining to fusible plugs.

If you purchase a fusible plug at an auction, a flea market, old-stock in a supply house or from anyone lacking knowledge of these critical devices, I suggest you know the answers to the following questions:

Is it a fireside plug or a waterside plug? Do you know the difference? On some plugs the only way to tell is by noting the location of the small end of the tin. The small end is always on the fireside.

Does the plug have an American Society of Mechanical Engineers (ASME) stamp on it? 



Is the ASME stamp on the fusible metal or on the brass? If the stamp is on the brass, try to figure out why it is not on the fusible metal.

Is there anything about the plug that might indicate the melting point may be something other than 450 degrees F? The 1946 and 1992 codes state that plugs may be filled with metals other than tin that melt at temperatures exceeding 450 degrees. However, such plugs shall not be marked as 'ASME Standard.'