Fusible plug
A fusible plug is a threaded cylinder of metal, usually
Purpose
A fusible plug operates as a safety valve when dangerous temperatures, rather than dangerous pressures, are reached in a closed vessel. In
History
The device was invented in 1803 by Richard Trevithick, the proponent of high-pressure (as opposed to atmospheric) steam 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 of water. He publicised his invention widely, without patent, to counter these criticisms.[3][4]
Experiments
Experiments conducted by the
Cored fusible plugs
The original design was a simple solid plug filled with a slug of low-melting-point alloy. When this melts, it first melts as a narrow channel through the plug. Steam and water immediately begin to escape through this. The cored fusible plug was developed in the 1860s to give a wide opening as soon as the alloy softens. This version has a solid brass or bronze centre, soldered into place by a layer of the low-melting-point alloy. When overheated, the plug does not release any steam or water until the alloy melts sufficiently to release the centre plug. The plug now fails dramatically, opening its entire bore immediately. This full-bore jet is then more likely to be noticed.[8]
Un-noticed melted plugs
A drawback to the device was found on 7 March 1948, when the firebox crown sheet of Princess Alexandra, a Coronation Pacific of the London, Midland and Scottish Railway, failed while hauling a passenger train from Glasgow to London. Enquiries established that both water gauges were defective and on a journey earlier that day one or both of the fusible plugs had melted, but this had gone unnoticed by the engine crew because of the strong draught carrying the escaping steam away from them.[9]
Maintenance
Alloy composition
Investigation showed the importance of the alloy on plug ageing. Alloys were initially favoured as they offered lower
Plug ageing
In the 1920s investigations by the U.S. Bureau of Standards, in conjunction with the Steamboat Inspection Service, found that in use encrustation and oxidation above the fusible core can increase the melting point of the device and prevent it from working when needed: melting points in excess of 2000 °F (1100 °C) in used examples have been found.[10] Typical current practice in locomotives requires new plugs to be inspected after "15 to 30 working days (dependent upon water condition and use of locomotive) or at least once every six months," depending on the boiler operating pressure and temperature.[12]
Other applications
The principle of the fusible plug is also applied to the transport of
Fusible plugs are common in aircraft wheels, typically in larger or high-performance aircraft. The very large thermal loads imposed by abnormal landing and braking conditions (such as a high-speed rejected takeoff, where an aircraft heavy with fuel must brake hard from a very high speed to a stop in a relatively short distance) can cause the already high pressure in the tyres to rise to the point that the tyre might burst, so fusible plugs are used as a relief mechanism. The vented gas may be directed to cool the braking surfaces.[15]
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.[16]
Automobile air conditioning systems were commonly fitted with fusible plugs, operating at 100–110 °C, but from concerns about the environmental effects of any released refrigerant gas this function has been taken over by an electrical switch.[17]
A patented (Patent published 1867) type of
Fusible plugs enhance the safety of Liquid fluoride thorium Nuclear Reactors by preventing overheating of the reactor. In the event that a limit temperature is reached, a fusible plug placed at the bottom of the reactor melts, allowing the fluid reactor fuel to drain into underground storage tanks, preventing nuclear meltdown.[20]
See also
References
- ^ Staff (1957). "The Boiler: Boiler Mountings and Details". Handbook for railway steam locomotive enginemen. London: British Transport Commission. p. 53.
- ISBN 0-582-12793-9.
- ^ Payton, Philip (2004). Trevithick, Richard (1771–1833). Oxford Dictionary of National Biography. Oxford University Press.
- OCLC 561620.
- ISBN 1-4255-0590-2.
- .
...want of water contributes only [insofar] as the metal may be heated and weakened thereby; that in no case of water on a heated part of the boiler can steam be generated in quantity so suddenly as to explode the boiler...
- ^ Hewison (1983: 116–117)
- ^ "Improved fusible plug for steam boilers". Scientific American. New York: Munn and company: 158. 1 September 1866.
- ISBN 0-7153-8305-1.
- ^ .
- OCLC 3351379.
- ^ "The management of steam locomotive boilers" (PDF). Sudbury, Suffolk, UK: Health and Safety Executive. 2007. pp. 22, 33. Archived from the original (PDF) on 2012-10-22. Retrieved 2011-04-22.
- ^ "Pressure container with thermoplastic fusible plug". United States Patent 4690295. Free Patents Online. 1987. Retrieved 2008-04-07.
- ISBN 978-0-470-18098-3.
- ^ "Tactics and Techniques — Undercarriages" (PDF). The Firefighter Initial Structured Learning Programme. Darlington, England: International Fire Training Centre. January 2003. Archived from the original (PDF) on February 5, 2022. Retrieved 22 February 2012.
- ISBN 0-7506-2530-9.
- ISBN 0-7506-6955-1.
- ^ "Patent 72,176 Fireproof safe". Commissioner of Patents annual report. Washington, DC: United States Patent Office. 17 December 1867.
- ^ "Improvement in fire-proof safes".
- ^ Juhasz, Albert J.; Rarick, Richard A.; Rangarajan, Rajmohan (2009-08-01). "High Efficiency Nuclear Power Plants Using Liquid Fluoride Thorium Reactor Technology (from NASA Technical Reports Server)". Archived from the original (PDF) on 2022-01-13. Retrieved 2022-08-14.