It wouldn’t be wise to bet your life on a fusible plug. The following is a segment from a report about the traction engine that exploded in Medina, OH:
“The center area of the crown sheet had been cut away to allow metallurgical examination of the metal and evaluation of the fusible plug. The fusible plug did not melt and it was reported that it might have been installed with the use of brazing. A subsequent report from Lieutenant John Detchon confirmed this and that the plug metal was tin. The area of the crown sheet which was removed was reported to have a red colour tone, which is indicative of overheating.”
The following is from an article written by Bruce Babcock, an engineer who investigated fusible plugs on his own time after Medina because he is also a traction engine owner (copied with permission):
“If the above statements are true, I cannot understand how a good fusible plug could have been brazed into the bushing in the crown sheet without melting the tin out of the plug. The lowest melting point for a brazing alloy that I could find in an industrial supply catalog was 1125 degrees F. Also, if the red color of the crown sheet is “indicative of overheating,” it seems that it would also support the need to question the viability of the fusible plug.”
He found that the tin used in soft plugs melts at 449.5° F but after time spent at elevated temperatures, it can spontaneously change into an oxide of tin, SnO2, which doesn’t melt until 2,966° F (steel melts at about 2,700 degrees). He found that several older soft plugs failed to melt when subjected to temperatures of over 1,000 degrees. Initially the lab didn’t want to heat the soft plug he had brought in to test, because the spectrograph showed the material to be tin, and the book said that tin melted at 450 degrees. The lab was subsequently embarrassed to find that they were not able to melt the sample.
All this ties in with it being good practice to replace fusible plugs with new ones every 92 service days.
Even so, the book Three Barrels of Steam describes in detail the boiler of an SP engine which exploded and was found to have all three fusible plugs melted open. The crew of a passing train “rolled by” their train seconds before, and waved at the engine crew who waved back apparently in no distress, never even noticing the blowing fusible plugs over the noise of the working engine.
I have an old text around here that recommends against using fusible plugs because they are unreliable and are likely to give the crew a false sense of security.
“Eternal vigilance is the price, not only of liberty, but of peace of mind about a steam boiler.” Wise words by William McFee.
