I have heard reports of fusible plugs that “melted out” when the operator claimed that there was adequate water in the boiler. I now believe that there might be a plausible explanation for at least a few of these incidents

Early fusible plug manufacturers were not aware of the importance of tinning the bores of fusible plugs until the U. S. Steamboat Inspection Service and the U. S. Bureau of Standards recognized that leakage between the tin and the casing could eventually make a plug ineffective. When the filling in the plug was not tinned to the casing, it was common practice to thread the bore of the plug (first photo) or to provide a small counterbore (second photo) on each end to prevent the filling from falling out in shipment. In addition to requiring that the bores be tinned, the Steamboat Inspection Service also specified that the bores must be tapered. Eventually, ASME followed suit. There is no indication that the casing of either plug in the photos had been tinned. It is not difficult for me to imagine that the filling in the plug with the counterbores might eventually be blown out of the casing by the pressure of the steam.




Starting in 1940, the ASME Code for Power Boilers specified that “The casing shall be tapered continuously from the water end of the casing for a distance of at least 1 in. to a diameter of not less than 3/8 in. at a point not less than ½ in. from the fire end.” The 1940 code also specified that “The fusible metal shall extend from the water end of the plug to the point of least diameter of the hole and shall be carefully alloyed to the casing. A test shall be made to assure that the fusible metal is not lose in the plug.” The sketch below is from the 1940 edition of the ASME code.



ASME does not specify the nature of the test they state should be made. In 1930, the Steamboat Inspection Service designed a testing device that they found to be satisfactory in routine testing of plugs. In their test, a 2-pound weight is dropped 6-inches against the small end of the filling. “Three such blows will generally indicate whether the filling is loose by forcing it to slide partially out of the casing.” The photo shows my home-made version of this device.



The third photo shows the configuration of the tin in an ASME-stamped, 1/2-inch, water-side plug, and a similarly stamped ¾-inch fire-side plug.



Not all manufacturers produce plugs that comply with the ASME specifications: http://www.smokstak.com/forum/showthread.php?t=28191

The ASME code does not include any mandatory requirements for the design or use of fusible plugs.

Bruce E. Babcock