Fusible plug

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.

In steam boilers the fusible plug is screwed into the crown sheet (the top plate) of the firebox, typically extending about 1 in (25 mm) 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.

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.

[3][4] 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.

[5] It was not until 1852 that this assumption was challenged: Thomas Redmond, one of the Institute's inspectors, specifically ruled out this theory in his investigation into the boiler explosion on the steamship Redstone on the Ohio River on 3 April that year.

[6] 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.

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.