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On a steam locomotive (or indeed, any kind of steam engine, such as a traction engine), the failure of the crown sheet (the metal sheet at the top of the firebox)

This generally happens due to a low water level. The heat of the fire is generally quite sufficient to melt the metal of the firebox; the boiler water cools the metal. If no water covers the crown sheet, the metal will heat up and become soft. Since the boiler is at quite a considerable pressure (modern steam locomotives have a boiler pressure somewhere between 180 and 350 pounds per square inch), the metal sheets will start to deform and buckle, and ultimately fail completely. The staybolts that brace the firebox against the boiler pressure help, but if the metal is too soft, they cannot prevent it.

If the crown sheet breaks and steam and superheated water get out, the result is as if a bomb went off in the firebox. The steam is under considerable pressure and is suddenly free to expand; the hot water is at a temperature considerably hotter than the boiling point of water at atmospheric pressure, and will flash instantly to steam. Even a small crown sheet failure will most likely cause fatal burns and scalds to the crew. A large one may blow the boiler clean off the locomotive in a full boiler explosion.

In an attempt to limit the damage, many locomotives are fitted with a fusible plug screwed into a hole in the crown sheet. This is made of a metal of a lower melting point than the crown sheet metal; the idea is that if the crown sheet is exposed, the fusible plug will melt through long before the crown sheet proper. This will allow hot water and steam to escape in a more controlled fashion. It's still very dangerous, and is likely to severely burn the crew, but that jet of water and steam will extinguish the fire and most likely prevent a full-on boiler explosion and large loss of life among bystanders and passengers.

A steam locomotive is an incredibly dangerous piece of equipment, and has to be cared for properly and with great respect for the energy stored inside it. Handled poorly, it's a bomb that can go off at any time.

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