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Vacuum Breaker Basics – An interview

Product Announcement from KADANT

Vacuum breakers provide a simple, dependable way to relieve unwanted vacuum which may develop in a closed vessel or pipeline. Erich Bernhardt, sales and applications engineer at Kadant Johnson, provides insight into the basics of vacuum breaker use.

How do vacuum breakers provide vacuum protection? 
The vacuum breaker acts like a door that opens to let air in when vacuum is present in the system. When the system is at pressure, the differential between the system and the outside atmosphere keeps the vacuum breaker’s door closed. When the system is in vacuum, the differential between the outside atmosphere and the system opens the vacuum breaker’s door.

How do I know I need a vacuum breaker?  
Generally speaking, any closed system that the volume contained within can be reduced in some manner will have a vacuum generated should have a vacuum breaker on it for protection, unless that vacuum is desirable.

How do you install a vacuum breaker?  
We recommend:

    • Putting it at the highest point in the system
    • Installing it so it does not impinge into the flow
    • Piping the inlet – the port that it pulls air into –  in a safe direction so that it doesn’t drip on anything important when it eventually wears out
    • Goose-necking the inlet so that dirt or debris can’t fall into it
    • Using a pig-tail below it to isolate it from heat if the system is really hot

How does position matter?  
The position on the system is important on liquid systems because it’s possible for the vacuum breaker to drip water as it is pulling in air. Therefore we generally recommend vacuum breakers to be installed at the highest point.  However, in a gas system or bi-phase system, they should be installed in a location that remains dry inside the system.

The orientation (i.e., position the vacuum breaker is pointed) affects the cracking pressure, the vacuum level the vacuum breaker starts to open.  For 99% of applications, pressure really doesn’t come into play because the system can tolerate a lot more vacuum than it takes to start opening the vacuum breaker.

What size vacuum breaker do I need? 
To size a vacuum breaker, you should know three things:

(1) the source of the vacuum
(2) the speed of formation or amount of vacuum – converted to standard cubic feet per minute ideally
(3) the system’s tolerance for vacuum, i.e., how much vacuum can you have before you get bad effects?

What if the largest size vacuum breaker is too small for my application? Use more than one? 
It is completely acceptable to use multiple vacuum breakers to break the vacuum. On rare occasions, the required air flow to break the vacuum may be so large that using multiple vacuum breakers is not an ideal option.  In those cases, a large check valve can be used in place of a vacuum breaker.

If a vacuum breaker is too small, then the valve will not pass the amount of air required to break the vacuum. If a vacuum breaker is too large, then what? 
Even if a vacuum breaker is too small, sometimes it can still work.  If a vacuum breaker is too small, the vacuum in the system will increase drawing more air. If the source of vacuum is short lived, like steam collapsing after a heat exchanger is turned off, and the equipment can tolerate lots of vacuum, the vacuum breaker will eventually catch up.  Therefore, you often see small vacuum breakers on heat exchangers.  The shell of the exchanger may have a very high tolerance for vacuum even if the drainage of the condensate requires positive pressure. The vacuum level spikes but then drops to allow drainage.

If a vacuum breaker is too large, we would not foresee any operational issues.