A fusible plug is a threaded metal cylinder usually of bronze

Bronze

Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...

, brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 or gunmetal

Gunmetal

Gunmetal, also known as yellow brass in the United States, is a type of bronze – an alloy of copper, tin, and zinc. Originally used chiefly for making guns, gunmetal was eventually superseded by steel...

, with a tapered hole drilled completely through its length. This hole is sealed with a metal of low melting point that flows away if a pre-determined, high temperature is reached. The initial use of the fusible plug was as a safety precaution against low water levels in steam engine boilers

Boiler

A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...

, but later applications extended its use to other closed vessels, such as air conditioning

Air conditioning

An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...

 systems and tanks for transporting corrosive

Corrosive

A corrosive substance is one that will destroy or irreversibly damage another surface or substance with which it comes into contact. The main hazards to people include damage to the eyes, the skin, and the tissue under the skin; inhalation or ingestion of a corrosive substance can damage the...

 or liquefied petroleum gasses.

Purpose

A fusible plug operates as a safety valve when dangerous temperatures, rather than dangerous pressures, are reached in a closed vessel. In steam engine

Steam engine

A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...

s the fusible plug is screwed into the crown sheet (the top plate) of the firebox, typically extending about an inch (25mm) 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 low-melting-point 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, which could result in catastrophic failure of the boiler. The temperature of the flue gases in a steam engine firebox can reach 1000 °F (550 °C), at which temperature copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, from which historically most fireboxes were made, softens to a state which can no longer sustain the boiler pressure and a severe explosion will result if water is not put into the boiler quickly and the fire removed or extinguished. The hole through the plug is too small to have any great effect in reducing the steam pressure and the small amount of water, if any, that passes through it is not expected to have any great impact in quenching the fire.

History

The device was invented in 1803 by Richard Trevithick

Richard Trevithick

Richard Trevithick was a British inventor and mining engineer from Cornwall. His most significant success was the high pressure steam engine and he also built the first full-scale working railway steam locomotive...

, the proponent of high-pressure (as opposed to atmospheric

Watt steam engine

The Watt steam engine was the first type of steam engine to make use of steam at a pressure just above atmospheric to drive the piston helped by a partial vacuum...

) 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.

Experiments

Experiments conducted by the Franklin Institute

Benjamin Franklin Institute of Technology

The Benjamin Franklin Institute of Technology in Boston, Massachusetts is one of New England's oldest colleges of engineering and technologies. The college was established with funds bequethed in Benjamin Franklin's will.-History:...

, Boston, in the 1830s had initially cast doubt on the practice of adding water as soon as the escape of steam through the device was noted. A steam boiler was fitted with a small observation window of glass and heated beyond its normal operating temperature

Operating temperature

An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...

 with the water level below the top of the firebox. When water was added it was found that the pressure rose suddenly and the observation glass shattered. The report concluded that the high temperature of the metal had vaporised the added water too quickly and that an explosion was the inevitable result. It was not until 1852 that this assumption was challenged: Thomas Redmond, one of the Institute's own inspectors, specifically ruled out this theory in his investigation into the boiler explosion on the steam ship Redstone on the Ohio River

Ohio River

The Ohio River is the largest tributary, by volume, of the Mississippi River. At the confluence, the Ohio is even bigger than the Mississippi and, thus, is hydrologically the main stream of the whole river system, including the Allegheny River further upstream...

 on 3 April that year. A 1907 investigation in Wales came to a similar conclusion: a steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

 belonging to the Rhymney Railway

Rhymney Railway

The Rhymney Railway was virtually a single stretch of main line, some fifty miles in length, by which the Rhymney Valley was connected to the docks at Cardiff in the county of Glamorgan, South Wales.-History:...

 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, was weakened by the heat of the fire 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. From then on it was accepted that the correct action in the event of the operation of the fusible plug was to add water.

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 begins 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.

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

London, Midland and Scottish Railway

The London Midland and Scottish Railway was a British railway company. It was formed on 1 January 1923 under the Railways Act of 1921, which required the grouping of over 120 separate railway companies into just four...

, failed while hauling a passenger train from Glasgow to London. Enquiries established that both water gauges

Sight glass

A sight glass or water gauge is a transparent tube through which the operator of a tank or boiler can observe the level of liquid contained within.-Liquid in tanks:...

 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.

Alloy composition

Investigation showed the importance of the alloy on plug ageing. Alloys were initially favoured as they offered lower eutectic melting points than pure metals. It was found though that alloys aged poorly and could encourage the development of a matrix of oxides on the water surface of the plug, this matrix having a dangerously high melting point that made the plug inoperable. In 1888 the US Steamboat Inspection Service made a requirement that plugs were to be made of pure banca tin and replaced annually. This avoided lead and also zinc

Zinc

Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

 contamination. Zinc contamination was regarded as so serious a problem that the case of the plugs was also changed from brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 (a copper-zinc alloy) to a zinc-free copper-tin bronze

Bronze

Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...

, to avoid the risk of zinc migrating from the housing into the alloy plug.

Plug ageing

In the 1920s investigations by the U.S. Bureau of Standards

National Institute of Standards and Technology

The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

, in conjunction with the Steamboat Inspection Service, found that in use encrustation

Limescale

Limescale is the hard, off-white, chalky deposit found in kettles, hot-water boilers and the inside of inadequately maintained hot-water central heating systems...

 and oxidation

Corrosion

Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

 above the fusible core can increase 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. 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.

Other applications

The principle of the fusible plug is also applied to the transport of liquefied petroleum gases, where fusible plugs (or small, exposed patches of the containers' lining membrane) are designed to melt or become porous if too high a temperature is reached: a controlled release, at a typical temperature of 250 °F (120 °C), is preferable to an explosive release (a "BLEVE

BLEVE

A boiling liquid expanding vapor explosion occurs when a vessel containing a pressurized liquid above its boiling point ruptures.-Mechanism:...

") at a higher temperature. Corrosive gas containers, such as those used for liquid chlorine

Chlorine

Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

, are fitted with one or more fusible plugs with an operating temperature of about 158 to 165 °F (70–74 °C).

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.

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

Refrigerant

A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...

 this function has been taken over by an electrical switch.

A patented type of fireproof safe uses a fusible plug to douse its contents with water if the external temperature gets too high.