If you have a storage tank/vessel and are removing liquid faster than it is inflowing, then the pressure above the liquid will decrease (the gas has to fill an increasing volume). Even large piping can have a problem withstanding partial/full vacuum. Note, we aren't necessary talking a full vacuum (0 psia), a few inches of water column vacuum can be enough to result in damage to storage tanks.
Vacuum breakers are installed to allow air or another gas to flow inside the tank/vessel to maintain sufficient pressure inside the tank/vessel to avoid damage.
Take a look at Protectoseal's or Shand and Jurs websites for more information.
Generically, a vacuum breaker is a valve-like device that will open an evacuated structure (vessel, casing, piping, ...) to a source of (relatively) higher pressure, usually (but not necessarily) atmospheric air.
A vacuum breaker might activated by a solenoid, or a spring, or manually.
The breaker might open upon a certain condition occuring, such as a low pressure limit, or an equipment shutdown, so as to prevent structural damage to due an abnormal differential pressure, or run-away rotation when equipment is unloaded, but still spinning (from its own momentum or residual stored energy). It might be a hand-cranked "vent" valve that allows an evacuated vessel to be opened without requiring excessive cracking forces.
What I know as a "vacuum breaker" is from steam turbine applications: Condensing exhaust casings operate well below atmospheric pressure - typically 1 or 2 psia (in other words 13 or 14 psi below atmospheric pressure). If the turbine is "tripped" (shut down), a solenoid vacuum breaker in the exhaust casing will open and allow air to "break the vacuum", halt the residual expansion of steam, and also slow the turbine rotation due to the increased aerodynamic braking. This is important in case the resistance of the driven equipment suddenly drops, as with a generator going off-line without warning. The intention is to prevent the turbine from spinning to a destructive over-speed.
LOL TD2K, thats first answer was very logical and was my first choice too!!!
They also serve to allow a break in flow in pipelines that are under pressure and then subjected to unwanted vacuum. Such occurances are typical in water systems where hydraulic gradients fall below the pipe line for various hydraulic reasons and the pipe becomes a vacuum, sucking in other pipe lines attached to it. The water company does not want the water back once its in your property. The vacuum breaker allows air to enter the system and stop chancs for water to flow backwards.
Vacuum breakers are also installed on the steam side of equipment like air intake coils, and heat exchangers. If they're not there condensate can't drain out when the temperature control valve closes, even if everything is sloped for gravity drain to an atmospheric receiver.
