Buy Watertube Boilers Now because of the reduced quantity of water in the Steam Boiler shell, the steam volume in the shell rises and because of continuous combustion in the furnace the temp of this steam goes up and it gets super heated. When the water level goes below the fusible plug, the lead present inside the Plug gets melted because of super-heated steam and now the water from the boiler shell comes out and wets the flue gases and even disrupts the combustion thereby protecting the Steam Boiler from any mishap that may have happened otherwise. FAQs What will happen if a fusible plug is not used in the boiler? When the steam temperature reaches the melting point of the fusible plug ,plug melts and sends the steam out through the plug hole. This escaped steam avoided over pressure in the steam boiler vessel which eliminated the risk of burst in steam boilers.
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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 mentions a fusible plug, even though some texts give extensive coverage of the problem of overheating the crown sheet. Little confidence is now placed in this contrivance; for it has been known to fail completely in various cases of boiler explosions.
More than years later, in , Frank D. 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. Hawkins first name unknown , seems to indicate the fusible plug was not universally applied at the time his book was published. These factors involved the properties of the metal in the plug and the condition of the plug when in use. 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.
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. Government rules call for pure Banca tin for boiler plugs, but this is not essential, and any good tin will serve the purpose.
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 The melting point of lead is degrees F. A proper material for fusible plugs is commercial tin, the melting point of which is about degrees F, or an alloy of two parts of lead and one of tin, having a melting point of degrees F.
In some places the law requires the use of Banca tin in fusible plugs. This metal has a melting point of degrees F. 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 psi steam. Later books reported much more strict requirements. Graham Lucas, Frank Graham and N. Hawkins, writing in Marine Engineers Guide , 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 Sherman Manufacturing Co.
While there, I observed a man pouring tin into fusible plugs in the foundry. They could use only virgin tin from ingots. Field Boiler Code requirements are the same as A. Fusible plugs are made of bronze and filled with pure Banca tin. Fredrick Hutton, in Mechanical Engineering of Power Plants , 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. Pure Banca tin is used by the U. Government for fusible plugs. 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.
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.
If, however, the quantity of escaping steam and water is considerable, combustion will be retarded and the fire will be partly extinguished. 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. Case book, The Science of Successful Threshing , quite clearly shows a fusible plug with a square head and an extension on the water side. However, an illustration in the Case Threshing Machinery Catalog shows a much shorter fusible plug.
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: , Stout Rd.
Case Threshing Machinery Catalog Dingee, William W. Science of Successful Threshing, Dow, Carl S. Practical Mechanical Engineering, Ernst Gage Company Catalog , Gebhardt, G. Steam Power Plant Engineering, Graham, Frank D. Power Plant Engineers Guide, Hawkins, N.
Maxims and Instructions for the Boiler Room, Hutton, Fredrick. Mechanical Engineering of Power Plants, Kent, William. Kuss, Robert. Steam Boilers, Care and Operation, Lucas, Graham and Hawkins, N. Maggard, James H. Mason, Charles. Arithmetic of the Steam Boiler, Medina County Sheriff, Report on the explosion of the Case, Rankine, William John MacQuorn.
Severns and Degler. Stephenson, James. Farm Engines and How to Run Them, Tulley, Henry. Handbook on Engineering, Winton, John F. Woodruff, E.