What is the purpose of a vacuum breaker valve

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jlunavtgrad

#1: Post by jlunavtgrad »

I understand what a vacuum breaker valve is and how one works, but why is it needed? I've heard they prevent vapor lock, but I'm not sure I understand how a vacuum could prevent a steam boiler from heating up and operating normally. As the water gets to 250 degrees the steam will fill the vacuum right?

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another_jim
Team HB

#2: Post by another_jim »

The miracle of Google site search reveals these amazing facts:
here and here.

Whenever you post a question, you take some time from everyone who reads it, and a lot of time from everyone who answers. Moreover, you have to wait to get the answer. If you search, you stop wasting everyone elses time, and get instant gratification.
Jim Schulman

Endo

#3: Post by Endo »

Fair enough (all you smarty-pants types.........) :D

Here's something that might baffle a Google search but is still a question many owners of some of the new machines WITHOUT p-stats on the steam boiler (like the Alex Duetto II and Vivaldi) might be asking themselves.

Quiz: Do they also need a vacuum breaker? And if so, why?
"Disclaimer: All troll-like comments are my way of discussing"

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cannonfodder
Team HB

#4: Post by cannonfodder »

No machine needs a vacuum breaker, but they make life more convenient by eliminating the false pressure.
Dave Stephens

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networkcrasher

#5: Post by networkcrasher »

Endo wrote:Quiz: Do they also need a vacuum breaker? And if so, why?
Also, there's no reason to assume the method of temperature control of a boiler has an impact on how vacuum is released once the boiler cools down. AFAIK, the P-Stats are a closed system, not allowing any transfer of pressure to atmosphere, thus they wouldn't allow for vacuum relief upon cool down.

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HB
Admin

#6: Post by HB »

another_jim wrote:The miracle of Google site search reveals these amazing facts...
I had no trouble finding how they work, but not a good "physics level" explanation of why it's necessary.

True confession... My days of college physics are long behind me. However, I believe the vacuum forms because the water contracts as it cools. Of course some of the suspended water in the saturated steam returns to liquid, but not enough to offset the reduced volume of the cooled water, hence a vacuum.

At the moment, why this situation does not neatly reverse itself upon reheating eludes me. Anyone care to explain why it doesn't?
Dan Kehn

jlunavtgrad

#7: Post by jlunavtgrad »

Jim, thanks for the links. I tried searching but so many forums here have the words "vacuum" or "vacuum breaker valve." Next time I'll be more diligent.

Endo

#8: Post by Endo »

HB wrote:At the moment, why this situation does not neatly reverse itself upon reheating eludes me. Anyone care to explain why it doesn't?
I find it easier to explain this way:

What is the noise you hear before the vacuum breaker closes? Air escaping (of course). If this air wasn't to escape (either through the valve or by purging the wand), this extra volume of air would result in a higher boiler pressure than if the system was sealed, and therefore trip the p-stat earlier. This extra pressure created by this purged volume of air (pre-boiling air) is called "false pressure".
"Disclaimer: All troll-like comments are my way of discussing"

jlunavtgrad

#9: Post by jlunavtgrad »

I think endo raises a good question about PIDed boilers. Since the PID would not be affected by a false pressure reading it would heat the water up to 250 degrees and raise the pressure in the boiler beyond its 1.3 bar setting. It's been a while since I used PV=nRT so I'm not going to try to calculate the pressure it would get to. If it's too high, it could damage the machine or risk injuring an unsuspecting operator. But I guess SOP is to purge the water out to the drip tray.

So would a PIDed steam boilers be better served by an OPV or a VBV?

jlunavtgrad

#10: Post by jlunavtgrad »

HB wrote:At the moment, why this situation does not neatly reverse itself upon reheating eludes me. Anyone care to explain why it doesn't?
The only way I can explain it on a machine like the Maximatic is if the vacuum causes air to get sucked in over time.