The Viability of Fusible Plugs,
According to the U. S. Steamboat Inspection Service,
1914 - 1930
In the era of the steamboat, the U. S. Steamboat Inspection Service, in conjunction with the U. S. Bureau of Standards, identified two scenarios that were causing fusible plugs to fail to function.
In one of these scenarios, the tin fails to melt, and consequently does not allow the release of steam into the firebox.
In the second scenario, the tin melts but is retained in the casing of the plug, and as above, it does not allow the release of steam into the firebox.
According to the first scenario, when tin is heated above 350 degrees Fahrenheit, contaminants, (primarily lead and zinc) over time, migrate to the grain boundaries, where they oxidize. The fusible metal becomes a mass of minute ceramic capsules that are filled with pure tin. They found that after just 500 hours of service, signs of this deterioration were detectible in the laboratory. They did not go so far as to claim that the plugs were not viable at this time. After a fusible plug has been in service for an extended period of time, even the tin will oxidize, resulting in a fusible plug that might not function below the melting temperature of steel. The researchers were unable to identify oxidized plugs by visual inspection.
In the second scenario, they found that when the casing of a plug was not thoroughly tinned prior to being filled with the fusible metal, water from the boiler could seep through between the fusible metal and the casing. When the water reaches the firebox side of the plug, it immediately evaporates. Boiler water is seldom, if ever, free of minerals. As a result, these minerals are deposited in the cavity at the fireside end of the plug and can eventually fill it. When this happens, these minerals will prevent the melted tin from exiting the plug. An investigation that followed a multiple-fatality explosion on a steamboat led to this discovery. (I have an ASME-stamped fusible plug, that was purchased in the last 15 years, that was given to me because the owner found that it was leaking between the filling and the casing.)
Nowhere in the government reports did I find any speculation as to whether or not their findings relating to marine service might be relevant to fusible plugs used in locomotive or other types of service.
The attached photo shows one of two fusible plugs that I had tested at a metallurgical laboratory. The water side of the plug is filled with a combination of lead and tin that has completely oxidized. At the right is filling from this plug that did not melt when heated to 1,000 degrees Fahrenheit. The lower portion of the plug contains minerals that were deposited there by water that leaked through the plug. Note that the shape of the filling in the plug was designed to prevent the filling from falling out of the casing in shipment. Prior to the Steamboat Inspection Service’s investigations, there was no awareness of the importance of tinning the casings. The design of this plug is consistent with what was installed in steam traction engines in 1911, the year my boiler was manufactured. There are wrench marks on it that might indicate that it has been removed and refilled.
The non-mandatory appendix to the 1971 edition of Section I of the ASME Boiler and Pressure Vessel Code specifies that the fusible metal “… shall be carefully alloyed to the casing. A test shall me made to determine that the fusible metal is not loose.” The appendix also states, “The bore of the casing shall be tapered continuously, …” A representative of ASME told me that they have no record of who is stamping “ASME STD” on fusible plugs.
It was my intention to limit this post to verifiable facts. I do, however, have one observation. If a fusible plug is melted out as soon as it is removed from a boiler, one of two things will occur. If the filling melts, we can be assured that it was not oxidized, that it was still a viable device, and that it will not be reinstalled. If the fusible metal can’t be melted, or if it comes out as a viscous, semi-fluid mass, we will know that it was beyond its useful life.
Bruce E. Babcock
July 8, 2014
