A fusible plug is a threaded cylinder of metal, usually bronze, brass or gunmetal, 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, but later applications extended its use to other closed vessels, such as air conditioning systems and tanks for transporting corrosive or liquefied petroleum gases.
Purpose
A fusible plug operates as a safety valve when dangerous temperatures, rather than dangerous pressures, are reached in a closed vessel. In steam boilers 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 it. 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, 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, 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.
Experiments
Experiments conducted by the Franklin Institute, 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 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 on 3 April that year. A 1907 investigation in Wales came to a similar conclusion: a steam locomotive belonging to the Rhymney Railway 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.