Have you ever gone to start up a steam boiler and found the sight glass overfilled and the boiler completely full of water, even though the water level was normal the night before when you shut it down? Where did all the water come from and how did it get inside the boiler if the feedwater pumps were never on? This scenario is the usually the result of not having a functioning vacuum breaker on the boiler.
How a Vacuum Gets Created
Thinking about a steam system and all the pipes that are connected to the boiler, it’s easy to forget what’s actually inside those pipes: steam or air (depending on whether or not the boiler is running). Dry clean steam is as invisible as dry clean air, and this is why most people forget about it in a steam system. With the boiler off, the pipes will have air and maybe some water in them at atmospheric pressure (which is just the normal air pressure that you live in every day).
Now turn on the boiler—the fire is lit, and the water starts to boil. The water goes through a phase change and becomes steam. The steam rises out of the boiler and rushes into the pipes, slowly heating the pipes and condensing back into water.
But wait! The pipes were full of air when the boiler was started, and that air doesn’t magically disappear when steam travels into the pipes. Even though you can’t see steam or air, they cannot coexist in the same space. Steam will have to push air out of the system though air vents, a feedwater system, or steam traps. There are many different types of steam systems, but they all have to have measures for air removal.
Now that the air is removed from the system, only steam and water remain. This will continue until the boiler is shut down, at which point the water stops boiling and all the steam in the system loses its heat and goes through a phase change back to water. The total volume of steam is actually about 1700 times more than water, and so when steam condenses back into water, a lot of empty space is left inside the pipes and boiler. If no matter can occupy that empty space, the system will be pulled into a vacuum.
What the Vacuum Breaker Does
When in a vacuum the system will want to pull in any matter that can fill the vacuum and return the pressure to equilibrium at atmospheric pressure. The boiler usually overfills with water when in a vacuum, causing issues on the next startup such as water hammer in the steam system and having to drain down the boiler to the normal operating water level. This is where a vacuum breaker starts to work. Its function is to prevent the formation of a vacuum by automatically opening and allowing air back into the system. You can think of a vacuum breaker as a sort of check valve; it allows air into a boiler without allowing steam to escape.
How to Test It
The vacuum breaker should be installed on the boiler anywhere above the water line in the steam space and checked for proper function whenever the boilers are shutdown. You should be able to hear air rushing back into the system though the valve and if you put your finger over the hole in the valve as the system is cooling, you should feel your finger being sucked to the opening. This is a quick test method to make sure the vacuum breaker is operating properly.