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From the workbench 2
Vacuum breakers
What are they?

A vacuum breaker allows the boiler of a HX machine or the steam boiler on a Marzocco heat up without creating a
"false pressure".   If the boiler was sealed then it would not give a true reading to both the pressure gauge and the
pressure switch.  For example when the boiler is filling with water for the first time it allows air to escape.  Again for
example when the boiler is heating and the water and air expand it allows the "expansion".  

Only when the steam pressure is sufficient does the boiler seal by closing the vacuum breaker.
The entire breaker. The bottom of the breaker - this is inside the boiler. The breaker components. The o-ring is clearly visible on the stem.
Teflon cone. The cone is forced up and seal with a gasket in the top of the body. Complete valve.
All the components. Teflon seal which mates with the o-ring. Complete valve.
Side by side comparison

To illustrate the point about height differences here is a side by side shot.
Problems and overhauling


Vacuum breakers are one of the easiest components to diagnose and restore.  


Problem - Sounds like a snake is trapped inside the machine.

Cause - 3 causes. 1. the washer or gasket sealing the breaker to the boiler has deteriorated over time  - as we
would expect.  
2. the viton or silicone o-ring on the stem has deteriorated and cannot make a seal.  3. Limescale or
dirt has been deposited on the top body of the breaker preventing the o-ring from making a seal.

Remedy - A rebuild and new gaskets is necessary.  Switch off machine and bleed off steam by opening steam
valves.  When pressure is at zero remove the valve.  Take apart, clean, descale and replace o-ring and the gasket
in between breaker and boiler.


Problem - Gauge says 1.5bar but no steam when you open the valve - usually when you switch on the machine.

Cause - Sticky stem, i.e. the stem is not dropping when the pressure drops.  This is preventing the air escaping
when it is first heating and creating a "false pressure". Sticky stems are caused by dirt, limescale and a bad o-ring.

Remedy -  As above, clean, descale,  replace stem o-ring and replace the gasket in between breaker and boiler.


Remember that the steam pressure inside a boiler is highly dangerous.  Never touch the vacuum breaker
without first depressurising first.
 
O-rings revisited Feb 05

Early December 2004 I started a small experiment.  I replaced the o-rings on a number of units around HK and used
different types of o-rings to see how each one held-up.  

All the machines were on 24/7 since December and yesterday I went around an inspected them all.  The Viton
o-rings had hardened and were barely making a seal.  So they only lasted about 60 days before they almost failed.  
The silicone rubber ones were still soft and making excellent seals.   

The silicone o-rings cost me 6 times as much as the viton but I wouldn't recommend the viton o-rings to anyone.  
This website is created by Paul Pratt, Hong Kong 2004-5. If you would like to use any of the images or text I
am sure I will say yes, but please ask first!  

Email me here.


L to R. new, cone, old.
A better perspective.
Vacuum breaker in operating position.
Viton and Silicone o-rings
Red - silicone.  Black - Viton.