Fusible Plug Analysis

Why your old fusible plug may fail when you need it the most


| March/April 2002


The spectrographic analysis of the fusible metal in two fusible plugs indicated a combination of lead and tin (possibly solder?) and gave no indication that they had become oxidized. Instead of melting at 360-460° F they could not be melted in a metallurgical laboratory even when heated to more than 1000° F Would the same thing have happened if the fusible plugs had been properly filled with only tin ? The tin used in fusible plugs melts at 449.5° F, but an oxide of tin, SnO2, doesn't melt until 2966° F. As the metallurgical report from one plug concluded: "Considering that the purpose of the fusible plug is to prevent excessive temperatures inside the steam engine (boiler), the analyzed plug would have most likely failed to perform the required safety task."

As we have discovered in the previous article, fusible plugs have had a reputation of not always melting at a predictable temperature, and the unpredictable fusible plugs seem to be those that have seen extended service. In 1924 the American Society of Mechanical Engineers (A.S.M.E.) Code reacted to this observation, dictating that fusible plugs should be replaced every year. And while it seems evident that old fusible plugs carry with them a higher potential of failure, it should be stressed that not all fusible plugs are unpredictable. I have heard of four instances where fusible plugs in traction engines have melted out reliably, with no damage to the engine and no injury to the operator.

My Investigation

In an attempt to understand why some fusible plugs that have been in use for many years don't melt at the predicted temperature, I removed the plugs from three of my boilers so that I could analyze them.

The first plug came from my 1911 45 HP J.I. Case traction engine. After removing it I carefully split it lengthwise with a hacksaw and smoothed the surfaces with a belt sander. After splitting the plug, the first thing I noticed was that the appearance of the fusible metal in the plug was very different at the opposite ends. The half of the plug toward the water end had the metallic luster that I would expect to see. However, the other half of the plug lacked that luster except in some isolated spots. At this point I am not referring to the contents of the plug as tin, for I had no way of knowing if it was still filled with tin or if it had been refilled "in the field" (as often occurred) with some other metal, such as solder, babbitt or lead. The square head of the plug had wrench marks on it, so this is a distinct possibility. It's also important to keep in mind that one of my limitations in evaluating this plug is that I know nothing of its history.

The second plug came from my Brownell vertical boiler (horse power rating unknown), which was built some time around 1916. As with my Case boiler, I don't know anything about the Brownell's boiler history. The plug in the Brownell is a water-side plug screwed directly into a tapped hole in one of the tubes. To remove the plug I simply had to remove a hand hole cover that is there specifically for access to the plug. I was surprised at the small size of the plug. It had a 3/8-inch NPT pipe thread and was only 15/16-inch long. The boiler is four feet in diameter and eight feet tall. When I split this plug, I found the metal in the plug had a bright metallic luster from one end to the other. The bore of the plug tapered from 5/16-inch at the fire end up to 7/16-inch at the water end.

The third plug that I attempted to remove was from the boiler of my portable engine, a circa 1899 Peerless Model G. This plug was the biggest surprise of them all – it was simply a 3/4-inch cast iron pipe plug! I don't want to over generalize from just three fusible plugs, but so far, every fusible plug holds a surprise.