The Fusible Plug and How it Works

1 / 4
2 / 4
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).
3 / 4
Figure #2: Illustration of a 3/4-inch fire-side fusible (or soft) plug shown actual size.
4 / 4
Figure #4: Examples of 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.”

His concern, clearly, is that an engineer knowing there is a fusible plug in his boiler will allow the water to go lower, assuming the plug will protect him and his boiler.

James Stephenson, in Farm Engines and How to Run Them (1903), doesn’t go so far as to condemn the use of the plug, but he clearly didn’t trust them, stating that, “In any case the fusible plug is not to be depended on.”

The book Maxims and Instructions for the Boiler Room (1903), by N. Hawkins (first name unknown), seems to indicate the fusible plug was not universally applied at the time his book was published. “All marine boilers in the U.S.,” Hawkins states, “are required to have fusible plugs, 1/2-inch in diameter, made of pure tin, and nearly all first-class boiler makers put them in each boiler they build.”

Concerns About Fusible Plugs

The old texts identified at least two factors that appeared to contribute to the fusible plug’s poor reputation. These factors involved the properties of the metal in the plug and the condition of the plug when in use.

“There is considerable diversity in the make-up of the material used for filling the plug,” Hawkins states, “which must not have its melting point at anything less than the temperature of the steam lest it should ‘go off at the wrong time.'” However, Hawkins certainly seems to contribute to the problem when he suggests that, “If the accident of low water occurs at a time where it is important to continue operations with the least possible delay, a pine plug may be driven in the opening left by the melting of the fusible metal.”

In 1928, G.F. Gebhardt in his Steam Power Plant Engineering, states:

“The melting point of fusible metals being sometimes uncertain, plugs occasionally blow out without apparent cause and at other times fail to act when the shell is overheated.”

Many of the authors showed little concern for the composition of the alloy in the plug. Maggard, for example, says that ” … if you have no extra plug you must remove the first one and refill it with babbitt. You can do this by filling one end of the plug with wet clay and pouring the metal into the other end, and then pounding it down smooth to prevent any leakage.” Charles Mason, in Arithmetic for the Steam Boiler (1913), says, “The U.S. Government rules call for pure Banca tin for boiler plugs, but this is not essential, and any good tin will serve the purpose.”

Many texts referred to Banca tin, but I was not able to find any definition of what is meant by the term “Banca.” Dr. Robert Rhode solved the mystery, however, when he informed me that Banca is an island in the Far East where the Dutch began mining tin in 1816.

Melting Points

William Dingee and Wallace MacGregor, writing in The Science of Successful Threshing (1911), say:

“If a plug ‘melts out’ in the field it can be temporarily filled with either lead or babbitt. The melting point of lead is 610 degrees F. A proper material for fusible plugs is commercial tin, the melting point of which is about 450 degrees F, or an alloy of two parts of lead and one of tin, having a melting point of 440 degrees F. In some places the law requires the use of Banca tin in fusible plugs. This metal has a melting point of 450 degrees F.”

I found the following information on the melting points of babbitts in the current catalog of the McMaster-Carr Supply Company (2001).

Babbitt Melting Points

Lead-base                         650 degrees F

Lead-base coppered          700 degrees F

Tin-base                            725 degrees F

I combined the above information with data from a table showing the effect of temperature on structural steel that I found in Machinery’s Handbook (1996). The result is seen in Figure #1. This shows that steel loses 18 to 30 percent of its strength as its temperature rises from the melting point of tin to the melting points of babbitt. The chart also shows that steel reaches it maximum strength near the temperature of 150 psi steam.

Later books reported much more strict requirements. Graham Lucas, Frank Graham and N. Hawkins, writing in Marine Engineers Guide (1937), cite a regulation that states:

“Fusible plugs for use in boilers of steam vessels under the jurisdiction of the Steamboat-Inspection Service shall be made of bronze casting with the bore tapering continuously and evenly from end to end, and filled from end to end with tin not less than 99.7 percent pure and to contain not more than 0.1 percent of lead and not more than 0.1 percent of zinc.”

In 1953, when I was 12 years old, my father took me to work with him one day at the H.B. Sherman Manufacturing Co. in Battle Creek, Mich. While there, I observed a man pouring tin into fusible plugs in the foundry. I can remember the superintendent explaining to me that this was a critical operation and that, if they spilled any tin, or did not fill one of the plugs completely, they were not  allowed to re-melt that tin to use in fusible plugs. They could use only virgin tin from ingots.

In 1978 Graham noted: ‘Each plug is stamped ‘A.S.M.E. Std.’ on the bronze casting to signify compliance with A.S.M.E. code specifications. A.P.I. Field Boiler Code requirements are the same as A.S.M.E. Code. A heat number stamped on the fusible core of each plug is the mark of approval of the Bureau of Marine Inspection and Navigation, Department of Commerce, and signifies that the samples of each ‘heat’ have been submitted to the Department for test and have been approved. Fusible plugs are made of bronze and filled with pure Banca tin.’

Function Conditions

Two factors are identified in the old texts as conditions that could affect the functioning of fusible plugs – the condition of the alloy and the deposits on the ends of the plug. Fredrick Hutton, in Mechanical Engineering of Power Plants (1903), identified both of these concerns, writing:

“The objection to such fusible plugs is, first, that the melting-point of most of these alloys change with time and is not always certain. Secondly, when covered with a crust of boiler-scale they may not be properly cooled by the water, and fuse when everything in the boiler is normal. On the other hand, they sometimes fail to act either from the first difficulty or from some unknown cause, and in any event, when blown out, it is annoying to replace them.”

William Kent, in the 1910 edition of Mechanical Engineers’ Pocket Book, agrees, but is more specific, stating:

“Alloys of bismuth have been used for making fusible plugs for boilers, but it is found that they are altered by the continued action of heat. Pure Banca tin is used by the U.S. Government for fusible plugs.”

Robert Kuss, in Steam Boiler, Care and Operation (1926), recommends:

“Fusible plugs are frequently unreliable. Sometimes they will blow out when there is no apparent cause, and sometimes remain intact when the plates have become overheated. If a coating of hard scale is allowed to accumulate over the plug, it may stand a considerable pressure even after the core has become melted. To provide against this, the plug should be replaced frequently. If allowed to remain in the boiler for any length of time, the composition of the alloy is likely to change, the plug thus becoming more or less unreliable.”

For these reasons many authors agree with Kuss and recommend cleaning and changing fusible plugs on a regular basis. Dingee and MacGregor in Science of Successful Threshing say that fusible plugs should be taken out two or threes times a season to be cleaned, and further suggest that they be replaced or re-poured at the beginning of each threshing season. Stephenson, in Farm Engines and How to Run Them, suggests:

“The careful engineer will never have occasion to do anything except to clean the scale off from the top of it on the inside of the boiler once a week, and put in a fresh plug once a month. It is put in merely as a precaution to provide for carelessness. The engineer who allows the fusible plug to melt out is by that very fact marked as a careless man, and ought to find it so much harder to get a job.”

How the Fusible Plug Works

Not all of the authors agreed upon just how the fusible plug protects the boiler. Some suggest that, when the plug melts, it protects the boiler by relieving the pressure and/or alerting the engineer by the sound of the escaping steam.

Kuss’ explanation seems to be the more plausible: “It does not necessarily follow that a hole 1/2-inch or 3/4-inch in diameter will liberate steam fast enough to prevent excess pressure. If, however, the quantity of escaping steam and water is considerable, combustion will be retarded and the fire will be partly extinguished.”

Several authors noted that fusible plugs are not used when pressures exceed 250 psi. Above this pressure the temperature of the steam approaches the melting point of tin.

Two Types of Fusible Plugs

There are two distinctly different types of fusible plugs available. One type is designed to be installed from the fire side of the crown sheet. This is the type that would be used on traction engines with locomotive style boilers. The other type is designed to be installed from the water side. This type would be used on vertical boilers. Figure #2 shows a full-size drawing of a fire-side fusible plug. Figure #3 shows how a water-side plug would be installed in a vertical boiler, and how a fire-side plug would be installed in a locomotive-style boiler.

The illustrations of fusible plugs in the early texts show very short plugs. It appears that the extended length was a later development that may have been intended to prevent premature melting of the tin by exposing more of the plug to the water. The illustration of a fusible plug in the J.I. Case book, The Science of Successful Threshing (1911), quite clearly shows a fusible plug with a square head and an extension on the water side. However, an illustration in the 1914 Case Threshing Machinery Catalog shows a much shorter fusible plug. The 1929 edition of Severns & Degler’s Steam Air and Gas Power showed the short plug, but their 1933 edition showed the long one.

Another development in the history of the fusible plug appears to have been the uniform taper of the hole for the tin (Marine Engineers Guide). Earlier designs showed an abrupt step or a barrel shape.

I would like to thank Dr. Robert T. Rhode, who contributed important information and valuable advice in the preparation of the article.

Steam enthusiast Bruce E. Babcock is a regular contributor to Iron-Men Album. Contact him at: (740) 969-2096, 11155 Stout Rd., Amanda, OH 43102.


J.I. Case Threshing Machinery Catalog 1914.
Colvin & Stanley. American Machinists’ Handbook, 1940.
Dingee, William W. and MacGregor, Wallace. Science of Successful Threshing, 1911.
Dow, Carl S., Ed. Practical Mechanical Engineering, 1907.
Ernst Gage Company Catalog 104, 1997.
Gebhardt, G. F. Steam Power Plant Engineering, 1928.
Graham, Frank D. Power Plant Engineers Guide, 1978.
Hawkins, N. Maxims and Instructions for the Boiler Room, 1903.
Hutton, Fredrick. Mechanical Engineering of Power Plants, 1903.
Kent, William. The Mechanical Engineers’ Pocket Book, 1910.
Kuss, Robert. Steam Boilers, Care and Operation, 1926.
Lucas, Graham and Hawkins, N. Marine Engineers’ Guide, 1937.
McMaster-Carr Supply Company Catalog Number 107, 2001.
Machinery’s Handbook, 25th Edition, 1996.
Maggard, James H. The Traction Engine, Its Use and Abuse, date unknown.
Mason, Charles. Arithmetic of the Steam Boiler, 1913.
Medina County Sheriff, Report on the explosion of the 110 Case, 2001.
Rankine, William John MacQuorn. The Steam Engine and Other Prime Movers, 1870.
Severns and Degler. Steam Air & Gas Power, 1933.
Stephenson, James. Farm Engines and How to Run Them, 1903.
Tulley, Henry. Handbook on Engineering, 1907.
Winton, John F. and Milar, W. J. The Engineer’s Encyclopaedia, 1891.
Woodruff, E.B. and Lammers, H.B. Steam Plant Operation, 1935.

Sources for Fusible Plugs

Presented below is a list of companies we presently know of where steam engine owners can purchase fusible plugs. This list is by no means complete, and we encourage readers to forward additional sources to our attention. Please write to us at: Iron-Men Album, 1503 SW 42nd St., Topeka, KS 66609-1265, or e-mail:

Conbraco Industries
701 Matthews Minthill Rd.
Matthews, NC 28105 (704) 841-6000

D & H Spring, Machine and Welding
3919 Montana Ave. Billings,
MT 59101 (800) 382-3917

Ernst Gage Company
250 S. Livingston Ave.
Livingston, NJ 07039-4089 (201) 992-1400

Oliver’s Boiler Repairs
R.R. #1 Auburn, Ontario
Canada NOM 1EO (519) 526-7640

Stellar Industrial Technologies
1918 Yorktown Court Lancaster,
OH 43130-1242 (740) 654-7052

Farm Collector Magazine
Farm Collector Magazine
Dedicated to the Preservation of Vintage Farm Equipment