A Short History of the Fusible Plug

With References to Regulations and Standards

| September/October 2003

The November/December 2002 issue of Iron-Men Album included an excerpt from The Book of Modern Marvels published by Modern Publishing Co. in 1917. Submitted by Ed Gladkowski, the excerpted quotation mentioned an exhaustive investigation of fusible plugs undertaken by the U.S. Bureau of Standards in 1914. As soon as I read this, I embarked upon a search for a copy of the Bureau's report. Not only did I find a copy of the report, I also discovered that the Bureau of Standards completed two major, additional investigations of fusible plugs; one in 1920 and the other in 1930. Knowing that the investigations had been conducted was only the first step, however. The second, and more difficult, step was obtaining copies of the reports. Had steam historian and author Robert T. Rhode not been willing to use his influence to extract copies of these and other documents from the Library of Congress and from university libraries, this project could not have been completed.

My current compilation of information is too extensive for publication as an article in Steam Traction, and the following consists of highlights from the larger document. The details of the American Society of Mechanical Engineers (ASME) standards were taken from copies of the 1914, 1924, 1933, 1943, 1946, 1952, 1956, 1971, 1992 and 2001 ASME Boiler and Pressure Vessel Code. There were other editions of the code produced at roughly one- to three-year intervals, so the dates of some changes are only approximate. Additional information was taken from various editions of General Rules and Regulations of the Steamboat Inspection Service and the Canadian Interprovincial Regulations for the Construction and Inspection of Boilers, Tanks and Appurtenances.

Figure 2: A -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 degree F at a metallurgical lab.

In addition to the documents found, I had, thanks to the generosity of many people in the steam community, close to 40 variations of fusible plugs for study. I'm alarmed at how easy it was to find examples of every defect identified in the government investigations, and I even found one type of defect that was not listed in the studies (the slug of scale at the left in Figure 3).

Figure 3: A -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 oxidization; the lower portion of the plug was filled with the infusible crust, which crumbled when the plug was cut in two.


1803 - Richard Trevithick, Cornwall, England, invents the fusible plug.