Boilers don't normally explode due to weak or fatigued spots in the sheet metal of the boiler. They can rupture and spew out steam and hot water if this happens (which can be dangerous if someone is in front of this) but an explosion is unlikely as this rupture will relieve the boiler of its steam pressure. Explosions happen when the steam pressure builds so quickly that the limits of the sheet metal walls (even if they are in mint condition) are exceeded. This happens if the engineer is not paying attention to what he is doing.
If the water level in the boiler is above the crown sheet then the firebox is completely submerged in water. Because all of the firebox sides are under water the temperature of this metal stays relatively cool (around the boiling point of water) compared to the temperature of the fire inside. This happens because the water absorbs the heat from the metal keeping it cool.
(You can see this phenomenon in action by placing a paper cup half full of water into the hot coals of a fire. The paper of the cup will burn off down to the water line but the paper below the water line will not burn. This is because the water keeps the temperature of the paper below the paper's flash point.)
If the engineer allows the water level to drop too low then the crown sheet becomes exposed to the steam in the boiler. Without the insulating effect of the water the temperature of the crown sheet begins to climb. If the engineer notices the water level and adds more water to the boiler at this point the new, cold water will come in contact with a very hot crown sheet. This water is vaporized very quickly and the steam pressure rises rapidly (too fast for the safety valve to handle).
There is one safety device built in to try to prevent this from happening. This device is a fusible plug. The plug, which is screwed into a hole in the crown sheet, has a core made of a material with a low melting point (e.g. lead). The plug will melt through as the temperature of the crown sheet gets too high and allow steam to vent into the firebox extinguishing the fire.
This plug is not fail-safe and must be checked regularly. The material in the plug can corrode or the outside of the firebox can rust and scale over the plug. It is easy to open the hand hole located near the plug and replace a defective plug or simply clean away any rust.
In 1988 the Tennessee Boiler Board decided that all boilers over thirty years of age were unsafe and could not be operated in the presence of a large group of people. If an antique boiler is to be operated in this situation it would need to be inspected and certified for operation. Kay-Gee 1875 and other engines that operate in Tennessee must be inspected each year in order to carry steam at shows like the Tennessee-Kentucky Threshing Show. And, their boilers must also undergo an ultrasound inspection every few years to check for metal fatigue. Once the boiler passes inspection a safety pressure valve is installed that limits the amount of pressure the boiler can carry.
