The Fusible Plug and How it Works

Cursed and praised for more than 100 years, fusible plugs offer some protection but tend to be unreliable

| March/April 2002

  • fusible plugs

  • Installation of a fire-side plug
    Figure #3: Illustration of typical installation of a fire-side plug in a locomotive-style boiler (left) and a water-side plug in a vertical boiler (right).
  • Inch fire-side fusible (or soft) plug
    Figure #2: Illustration of a 3/4-inch fire-side fusible (or soft) plug shown actual size.
  • Early fusible plugs
    Figure #4: Examples of early fusible plugs.

  • fusible plugs
  • Installation of a fire-side plug
  • Inch fire-side fusible (or soft) plug
  • Early fusible plugs
In some important ways, a fusible plug is a lot like our type of government:
It may not be perfect,
Sometimes it promises more than it can deliver,
If we want it to work well we have to give it a lot of attention,
Yet it is better than anything else that is available.

My concerns about the reliability of soft plugs (now known more commonly as a fusible plug or safety plug) led me to see what I could learn about them from the old textbooks. What follows is a collection of quotes that I found to be interesting and that I thought might be of some value to others who have an interest in steam engines. Please note this is not intended to be presented as a comprehensive research project, as I limited my search to just those texts that I have in my personal library.

Looking at the Books

Of those books in my collection, only one text dated prior to 1890 mentions a fusible plug, even though some texts give extensive coverage of the problem of overheating the crown sheet. The earliest description of a fusible plug in my collection is from the 1870 edition of professor William Rankine's The Steam Engine and other Prime Movers, first published in 1859. In this early description (I don't know if this description appeared in earlier editions), Rankine describes the purpose of the plug and then addresses an issue that exists to this day:

"The fusible plug is a piece of metal or alloy stopping an aperture in some part of the boiler which is directly exposed to the fire, and of such a composition as to melt at a temperature lower than that at which the pressure of the steam would become dangerous. Little confidence is now placed in this contrivance; for it has been known to fail completely in various cases of boiler explosions."

From this description, it sounds as if they were trying to use a fusible plug as a pressure relief device rather than a safety device. More than 100 years later, in 1978, Frank D. Graham, in Power Plant Engineers' Guide, echoes a similar concern when he concludes that fusible plugs are overrated. "They are unreliable," Graham states, "sometimes blowing out when there is no apparent cause and sometimes remaining intact when the plates become overheated."

And yet, even though these plugs have been cursed and praised for more than 100 years, they seem to offer a degree of protection that would not otherwise be available. James H. Maggard, author of The Traction Engine, Its Use and Abuse (date of publication unknown, but clearly early 20th century), takes something of a psychological approach in his criticism of the use of the plugs:

"I would have no objection to the safety plug if the engineer did not know it was there. I am aware that some states require that all engines be fitted with a fusible plug. I do not question their good intentions, but I do question their good judgment. It seems that they are granting a license to carelessness."


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