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A modern fusible plug showing the core of low melting-point metal.

A fusible plug is a threaded metal plug, usually of bronze, brass or gunmetal, which has a tapered hole drilled completely through its length. This hole is sealed with a metal of low melting point, usually lead or tin. It is screwed into the crown sheet (the top plate) of a steam engine firebox, typically extending about an inch into the water space above. Its purpose is to act as a last-resort safety device in the event of the water level falling dangerously low: when the top of the plug is out of the water it overheats, the lead core melts away and the resulting noisy release of steam into the firebox serves to warn the operators of the danger before the top of the firebox itself runs completely dry. Water must be added to the boiler immediately (but see “Operators’ response”, below) and the fire thrown out.^[1] The hole is too small to have any great effect in reducing the steam pressure and, as little or no water passes through, it is not expected to have any great impact in quenching the fire.^[2]

The temperature of the flue gases in a steam engine firebox can reach 1000 °F (550 °C), at which temperature copper, from which historically most fireboxes were made, softens to a state which can no longer sustain the boiler pressure.^[1] A severe explosion will result if water is not put into the boiler quickly. However, experiments conducted by the Franklin Institute, Boston, in the 1830s had cast doubt on this practice: they fitted a steam boiler with a small observation window of glass and heated it beyond its normal operating temperature with the water level below the top of the firebox. When water was added they found that the pressure rose suddenly and the observation glass shattered. They reported that the high temperature of the metal had vaporised the added water too quickly and that an explosion was the inevitable result.^[3]

It was not until 1907 that this assumption was challenged: a steam locomotive belonging to the Rhymney Railway, Wales, was inadvertently sent out with its safety valves wrongly assembled. The pressure in the boiler built up to the extent that the injectors failed; the crown sheet became uncovered and violently blew apart. The investigation led by Colonel Druitt of the Railway Inspectorate dismissed the theory that the enginemen had succeeded in starting the injectors and that the sudden flood of cold water had caused such a generation of steam that the boiler burst. He quoted the results of experiments by the Manchester Steam Users Association, a national boiler certification and insurance body, that proved that the weight of copper present (considered with its specific heat) was insufficient to generate enough steam to raise the boiler pressure at all. Indeed, the addition of cold water had caused the pressure to fall.^[4] From then on it was accepted that the correct action in the event of the operation of the fusible plug was to add water.

The device was invented in 1803 by Richard Trevithick, the proponent of high-pressure (as opposed to atmospheric) engines, in consequence of an explosion in one of his new boilers. His detractors were eager to denounce the whole concept of high-pressure steam, but Trevithick proved that the accident happened because his fireman had neglected to keep the boiler full. He publicised his invention widely, without patent, to counter these criticisms.^[5]

Investigations by the U.S. Bureau of Standards have found that, in use, encrustation and oxidation of the fusible core can increase its melting point and prevent it from working when needed: melting points in excess of 2000 °F (1100 °C) in used examples have been found. The current U.S. standard specifies replacement after every 500 hours of use.^[6] The British Office of the Rail Regulator allows for between thirty and sixty days of use, according to locomotive type and operating pressure.^[7]

Fusible plugs are sometimes fitted to the receivers of air compressors as a precaution against the ignition of any lubricating oil vapour that might be present. Should the action of the compressor heat the air above a safe temperature the core will melt and release the pressure.

• Boiler explosion

1. ^ ^{a b} Anonymous (1957):Handbook for railway steam locomotive enginemen. British Transport Commission, London.
2. ^ Snell, John B (1971): Mechanical Engineering: Railways. Longman, London.
3. ^ Anonymous – Benjamin Franklin Institute of Technology (undated ca 1830): Steam-boiler explosions. Reprinted 2005 as Explosions of steam boilers. Scholarly Publishing Office, University of Michigan Library. ISBN 1-4255-0590-2.
4. ^ Hewison, C H (1983): Locomotive Boiler Explosions. David and Charles, Newton Abbot, UK. ISBN 0-7153-8305-1.
5. ^ Payton, Philip (2004): Oxford Dictionary of National Biography, Oxford University Press.
6. ^ Bruce E. Babcock, "A Short History of the Fusible Plug, With References to Regulations and Standards", Website of Steam Traction magazine.
7. ^ Unknown authors (2005):The management of steam locomotive boilers. Health and Safety Executive, Sudbury, Suffolk, UK.