I was once on a problem steam job where the water seemed to vanish from the boiler as soon as the pressure hit its high limit and the burner shut off. There was an automatic water feeder on this job and it kicked in when the water plunged out of the gauge glass. It added some water to the boiler, of course, and that raised the water level. Then the burner restarted.
A few minutes later, the pressuretrol sensed the high limit again and shut off the burner. Once again, the water vanished from the gauge glass and the feeder added even more water.
Before long, the boiler was flooded. And once the water level got near the top of the boiler, the steam started sucking great gobs of water into the system piping. That's when the fun started. The water flew down the mains and hammered like mad. The boiler went off on low water again. The feeder fed again.
This job was becoming a nightmare for the installer. Before I had gotten there, he had piped a check valve into the wet return. He figured that the water was backing out of the boiler, but the check valve didn't make a bit of difference because the water wasn't backing out of the boiler. It was rocking between the boiler and the gauge glass. But he couldn't see that from the outside. He eventually removed the check valve and called me.
To understand what's happening here, you have to think your way inside the boiler. A modern cast-iron boiler looks like a loaf of bread. The sections butt up against each other like slices of bread in that loaf. Each section is actually a tiny boiler. The water is on the inside of the section and the fire is on the outside.
When you pull all the sections of a steam boiler together, you create a big "pot" that's separated into chambers. You're going to make steam in each of those cast-iron chambers (sections). The heat from the fire will move between the sections, passing BTUs through the cast-iron pins as it goes.
Okay, now try to "think" like steam. It's born as tiny bubbles down near the fire because that's the hottest part of the boiler. When water flashes into steam, it occupies about 1,700 times the amount of space it takes up as water, so as a steam bubble forms, it shoves the water surrounding it out of the way.
When the steam bubble gets large enough, it breaks away from the iron casting and floats upward toward the surface of the water. Once there, and assuming the water is clean, the bubble breaks and the steam heads out into the system piping.
Now there's a lot of hydronic violence going on inside those cast iron sections. The steam bubbles are knocking into each other and shoving the surrounding water all over the place. The amount of steam you can make depends on the firing rate of the burner. The harder you fire, the more steam you'll make. And the wider the boiler sections are, the more orderly the rise of steam up through the water will be. Can you visualize that? Lots of bubbles need lots of space. The more space there is, the less violent the action will be.
If you cut open an old boiler and look inside, you'll get an idea of what I mean. Notice how wide the sections are? Modern steam boilers are much narrower than the boilers of Yesteryear. When you hear the terms "high-water-content" and "low-water-content" boilers, this is what manufacturers are talking about. The older boilers held more water because their sections were wider. They gave the rising steam bubbles more room to maneuver. Those old boilers were cast to be steam boilers, not hot water boilers.
But imagine that same amount of steam trying to rise through a boiler section that's half as wide as its older counterpart. The steam has to move faster, and the movement is going to be more violent. And this is where problems can start because when the steam forms inside a too-narrow section, it can emulsify the water and make it lighter than it ought to be. Can you see it in your mind's eye? The water inside the boiler is literally filled with steam. If the boiler were made of glass, it would look as though an air compressor was shooting bubbles into the water.
Now here's the key to the vanishing water problem. The formation of the steam in a too-narrow section can make the water inside the boiler rise up. The water in the boiler gauge glass, however, has no steam rising up through it. This water is a bit heavier than the steam-laden water inside the boiler, so it drops a bit to compensate for the difference in weight. What you're seeing in the gauge glass is actually a false water line. It's not representative of what's actually happening inside the boiler.
Now imagine what happens when the steam pressure reaches the high-limit setting of the pressuretrol. Suddenly, the burner shuts off and removes the heat needed to produce steam. The steam bubbles in the boiler water immediately begin collapsing, just as they do in a pot that's boiling over when you remove it from the stove. As the bubbles collapse inside the boiler, the water level plummets. The water in the gauge glass, which is denser (heavier) than the water in the boiler falls into the boiler to compensate for the falling boiler waterline.
It's like a scale that's trying to find a balance point, but from the outside, all you're seeing is one half of that scale. You're seeing the water momentarily vanish into the boiler when the burner shuts off on high limit. And if there's a feeder, you hear it click on. Before long, the boiler is flooded.
Dirt or oil on the surface of the water can aggravate this, of course, which is why it's so important for installers to clean and skim steam boilers. If the steam can't break free of the surface because there's a layer of scum floating there, the water will contain even more steam. It will become lighter and fall even further when the burner shuts off. Modern steam boilers are really sensitive to dirt in or on the water. Suggest to your contractor customers that they quote the charge for a good system cleaning as an extra, and tell them to explain the importance of all of this to their customer. Most folks will go for it once they understand the potential for problems if they don't. And the contractors stands a better chance of getting the order (and staying out of trouble afterwards!).
When it comes to steam boilers, it's not so much about the amount of water that's in a boiler as it is about the space in which the steam has to form. It's about the internal width of the boiler sections. If you compare today's boilers to older boilers you'll see what I mean. The next time you're looking at boilers, use a ruler. Look down into the steam nozzle and measure the internal width of the section. To get the highest possible efficiency, boiler manufacturers have to make their steam boilers smaller nowadays, but it pays to go with the ones that have the wider sections.