The Power of Steam – Part 1, by A.Y.
…through the flues, and out the stack, heating water as they move. The water ABSOLUTELY MUST stay above the top of the firebox, which is called the crown sheet. If it does, everything goes well and the engine operates like it should. If the water drops below the crown sheet and exposes the top of it to steam, the extreme heat from the fire will begin to melt the steel top of the crown sheet. When the steel finally gives way, all that hot water in the boiler (far hotter than boiling temperature) will instantly turn into steam when depressurized and blow the boiler up into the air and into little pieces, taking the crew with it. This is called a boiler explosion, and the most important thing to take away from this is once again: Do NOT EVER let the water drop below the top of the crown sheet….