Ted S posted:Sometimes the crown sheet would just collapse and water would douse the fire without a catastrophic explosion. Some locos had fusible plugs that would melt to achieve this goal. I think this is what happened on the Gettysburg RR in the 1990s, which led to increased oversight of steam locos by the FRA. Hopefully Rich Melvin or someone who has in-cab experience will explain better.
Ted, I'm sorry, but your explanation is not quite correct. If the Crown Sheet fails, there will definitely be an explosion. If a Fusible Plug fails, there might be an explosion. They are two different things.
Water under pressure does not boil at 212 degrees F. At 250 psi the boiling point of water is 401 degrees F. When a crown sheet fails, all that pressure is suddenly released. When the pressure is released, all that 401 degree water instantly turns to steam! That is what causes the violent explosion...the expansion of the water from a liquid to a gas.
A fusible plug is like a "soft plug" in an automobile engine block. If the crown sheet gets too hot, these plugs melt and theoretically allow water to spray into the firebox, thus putting out the fire. However, if there is no water above the crown to start with, there is no water to be sprayed into the fire! In this case, the steel in the crown sheet soon fails, with a resulting explosion.
Here's a video that will explain what a crown sheet is and where it's located in a steam locomotive.
This video, along with dozens of others, are available to all OGR Digital Subscribers in the OGR Video Digital Library.
