A while back, a friend of mine asked me what happens if there is a loss of steam pressure in an operating naval boiler. Short answer: The generating tubes melt. The boiler could blow up.
Long answer: Take a look at the schematic of a 1200psi naval plant. look at the boiler on the upper left corner. See the circle in the boiler labelled "SD" and one labelled "WD" and the two curved lines between them? Those are the steam drum and the water drum. (Click on it to enlarge it)
Naval boilers do not have circulation pumps. There is a steam drum at the top, the water level is kept at the half-full mark. The water drum is at the bottom, it's also called the mud drum. The two drums are connected by the steam generating tubes and the downcomers. The generating tubes are in the firebox, the downcomers run between the boiler and the air casing. (There are also screenwall tubes, which protect the sides and rear wall of the boiler casing. Those tubes also generate steam, but not a significant amount.)
If you've ever closely watched a pot of water boil on a gas burner, you'll see there is a point where bubbles are coming off the bottom of the pot where the flames are hitting the underside of the pot. The cooler water goes down to where the flames aren't hitting the other side to replace the steam bubbles coming up, you are watching natural circulation. That's how a naval boiler keeps water circulating; slightly cooler water goes down the downcomers to the mud drum and then it replenishes the water in the generating tubes that has turned to steam.
Look again at the pot of water. When the little bubbles are coming off the bottom, that's called "nucleate boiling".
So let's start with a cold boiler and light fires. As the water heats, you get nucleate boiling off the sides of the tubes. The little bubbles go up the center of the tubes and rise to the steam drum. This is "bubble flow", the tubes are about 5% quality (5% steam) . As the water has more heat applied, the bubbles become larger and now you have slug flow. More heat and the center of the generating tubes is just steam with a ring of water around the inner wall of the tubes, which keeps the tubes cooled. This is annular flow and it's about 30% quality; you still have nucleate boiling going on and water is being replenished by natural circulation. The steam coming out of the generating tubes into the steam drum is heavily saturated with moisture.
This is as far as you can fire a boiler at normal pressures of 1,200psi, you will reach the "endpoint of combustion", you can't get any more fuel through the burner nozzles.
Now, say you lower the boiler's operating pressure below the operating set point. The water boils at lower levels of heat, so you can get more steam generated in the tubes. Then you can get to spray flow, where you still have nucleate boiling but the tubes are only just wetted on the inside and the steam is about 60% quality.
Lower the pressure a little more and you get to DNB or "departure from nucleate boiling", where there is no longer any water on the inside of the tubes. The steam is 100% quality or even slightly superheated, no moisture is carried over to the steam drum ("endpoint of carryover"). This is, to quote Rod Machado, A Bad Thing, for the generating tubes are designed to be cooled by the water inside of them. The tubes will start to melt.
If you can push it even hotter, you will generate so much steam that the downcomers will not be able to supply water to the water drum. This the endpoint of circulation and at this point, the damn thing may very well blow up. This is like having the water level in an old steam locomotive or traction engine drop below the crown sheet over the firebox.
So, to prevent that from happening, if the main steam pressure drops below 1080psi, the fireroom crew will call out a low pressure casualty and they will wrap up the boiler.
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