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

Hi Bruce:
Thank you so much for posting this information!
After reading the document, we cut a couple of our used fusible plugs in half and heated them to see if they would have functioned as designed in service and both did.
Both had been removed after 4 or 5 seasons use, one because it had a drip around the filling under hydro and the other because the top of the filling was no longer smooth/ the ASME stamp was no longer legible. When cut, the filling on the latter was found to be loose.
The filling on the first one melted at about 400F, the latter at a bit over 300F, perhaps it was some sort of a tin-lead alloy.
Be well,
J.David

Nathan manufactured fusible drop plugs. See http://www.martynbane.co.uk/modernsteam/tech/drop.htm for more info.

Fusible drop plugs were invented by the Southern Pacific Railroad. This is discussed in a paper published in a Master Boilermaker's annual convention proceedings, sometime in the 1930s (IIRC). I believe that S.P. held the patent, but let others use it free of charge...? The drop plug diameter was initially smaller than shown in the Nathan catalog. I have early S.P.-made examples of the smaller-diameter design.

Drop plug diameter, and later the number of drop plugs installed, were increased when it was found that engine crews didn't necessarily notice if the plugs had dropped because of low water while working the engine hard. Southern Pacific had one "Mechanical Circular" that showed the required drop plug locations in the crown sheet, for each class of steam power.

S.P. made their own fusible drop plugs. The Mechanical Circulars include dimensions/drawings, assembly procedure and fusible material specs (pure tin, IIRC), and required quality control tests and sampling plan.

I believe that Strasburg RR made copies of Nathan drop plugs at one time. Maybe they still do? Also I have foundry patterns for the plug bodies - somewhere - if anyone needs to borrow them.

- Doug Debs

Here as a comparison is one type of fusible plug we use in the UK. I think it's called a Narvick. It has like a rivet from the fire side which is then held in by the tin mix.

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I am pleased to hear a reason why fusible plugs have a specific life. I wasn't really willing to accept that they spontaneously went bad. Could pure block tin not have functioned as well as the alloy used? That would alleviate the issue of alloy migration to the interstitial space and also wouldn't have the issue of phase change and solid solutions to deal with. Was the specified alloy intended to be a eutectic ratio?

In 1914, the Steamboat Inspection Service mandated that zinc and lead contamination be limited to 0.1 percent each, and that the filling must be at least 99.7 percent tin.


The non-mandatory appendix of the 1971 edition of Section I of the ASME Boiler and Pressure Vessel Code specifies:

Pure tin, percent ................ 99.3
Copper, max. percent ........... 0.5
Lead, max, percent ............... 0.1
Total impurities, percent ....... 0.7

ASME also specifies that the melting point be 445 to 450 F.


Bruce E. Babcock

Pardon my ignorance, but I have very little experience with steam and none with full scale. I have always been curious about these fusible plugs, there design and application. I appreciate seeing this posting here. I would like to know if they are generally all the same basic design. Looks like the top picture is NPT? What size? How many are usually in a firebox, and is it based on the area of heating surface? Does the type of boiler change the requirements for plug design, quantity or mounting location?

Thanks
Eric

According to the non-mandatory appendix to Section I of the 1971 edition of the ASME Boiler and Pressure Code, there are two basic types of plugs;

“Water side plugs are fusible plugs which are inserted from the water side of the plate, flue, or tube to which they are attached. Fire side plugs are fusible plugs inserted from the fire side of the plate, flue or tube to which they are attached.”

The code provides that “The fusible metal in either of these may be partly replaced by a bronze plug loosely fitted in the hole … .” This plug must be properly alloyed to the casing.

They also state, “The bore of the casing shall be tapered continuously from the water end of the casing for a distance of at least 1 in. to a diameter of not less than 3/8 in. at a point not less than ½ in. from the fire end.”

I have seen ASME plugs ranging from 3/8 inch to 1 ½ inch NPT. The internal dimensions do not change with the size of the plug.

I do not know of any criteria for determining the number of plugs to be installed in a boiler.

Bruce E. Babcock

Unless things have changed in the past few years since I last had occasion to investigate, there was no requirement to have fusible plugs, just requirements for them if you did use them. I've never liked them since the only time I've had one drop there was plenty of water in the boiler and all was well, except for the faulty fusible plug. A bad one can ruin your day....and nobody incapable of maintaining water in the boiler should be placed in control of one in any case so it sort of makes them redundant and troublesome from my perspective. Technology can't idiot-proof machines, only removing idiots from the picture can. Good personnel policies and standards and practices are much better solutions.

dave

In an earlier post, I mentioned fire side, and water side fusible plugs. The plug on the right in the photo is a ¾ inch NPT fire side plug. The other is a ½ inch NPT water side plug. As they are positioned in the photo, the bottom of the plugs would be exposed to the fire, and the tops would be submerged in the water. If the plug on the left were cross-sectioned, the profile of the tin would be exactly the same as the profile of the tin in the fire side plug.

Bruce E. Babcock