What is the purpose of a vacuum breaker valve - Page 2

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Endo

#11: Post by Endo »

jlunavtgrad wrote:So would a PIDed steam boilers be better served by an OPV or a VBV?
The Vivaldi steam boiler has both a vacuum breaker and a over-pressure relief valve.

The OPV is simply there to avoid an explosion just in case the thermastat on the steam bolier fails. (Just like your home water heater).
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jlunavtgrad

#12: Post by jlunavtgrad »

Interesting, I'll try to see if my Duetto II has both an OVP and a VBV when I get home. My assumption is that the functions of both valves could be combined on steam boilers that are PIDed. If the OVP was set to a little over the normal operating steam pressure (~1.5 bars), then it would prevent vapor lock from being a problem without the need for a VBV. This could reduce the complexity/maintenance/cost of the machine by one valve.

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cbrucecampbell

#13: Post by cbrucecampbell »

As a participant in one of those referenced threads I don't think they helped me all that much.

IMHO, the confusion comes from the implied connection between the dual function of the breaker valve.

It opens to:

1) bleed "false pressure" at startup
and
2) to equalize pressure between the boiler and the atmosphere as the machine cools down (to prevent milk getting sucked up the steam wand I guess).

The two functions are not related, i.e, you can open up a cool boiler to equalize pressure; the pressurestat will still trip too early on a valveless boiler the next time it heats up.

The "false pressure" is just the result of the initial heating of the air and water - pre boiling. Has nothing to do with the partial vacuum that may or may not get created on cool down.

And if you keep your steam wand purged the opening of the valve at cool down is useless.

At least that's as far as my brain can wrap around it...
Bruce Campbell

Endo

#14: Post by Endo »

cbrucecampbell wrote:2) to equalize pressure between the boiler and the atmosphere as the machine cools down (to prevent milk getting sucked up the steam wand I guess).
I don't see the need for item 2. Who keeps their steam wand in milk when it's cooling down? I don't see why we are afraid of having a vacuum once the machine is shut off. I assume the vacuum disappears anyway after a few hours since we wouldn't be getting "false pressure" if the whole system was perfectly sealed.

Maybe it has something to do with pressure cycling and boiler fatique life? But that seems like a stretch.

I may just replace my VBV with a bolt plug when it eventually fails and see what happens.
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another_jim
Team HB

#15: Post by another_jim »

There's a myth that vacuum breakers were used to prevent boilers from buckling or imploding as steam condensed. But if a boiler can stand a few bar outward, it can stand a bar going inward. The term actually comes from plumbing valves meant to prevent water flow from stopping due to a siphon effect (i.e. water won't flow if the space it clears is not refilled and becomes a vacuum).

This has absolutely nothing at all to do with the espresso machine application, where its function during cool down is irrelevant, and it's there to prevent false pressure during heat up. It just so happens to be the same type of valve, hence the confusing name.

Edit: For those confused, "false pressure" is air pressure rather than steam pressure. Air expands and hits a level that will fool the pstat before the water even reaches boiling. A temperature PID would not be fooled and would not need a vacuum breaker (although the initial steam output would be thin). I have no clue where the air comes from if there is no vacuum breaker and the system stays sealed; but if the pressure isn't from water, it has to be from air.
Jim Schulman

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cbrucecampbell

#16: Post by cbrucecampbell »

Exactly. We should stop calling it a "vacuum breaker" valve. Technically it may be that, but that name misleads people who are trying to understand its purpose in espresso machines...
Bruce Campbell

Endo

#17: Post by Endo »

another_jim wrote: I have no clue where the air comes from if there is no vacuum breaker and the system stays sealed; but if the pressure isn't from water, it has to be from air.
I never owned a machine without a VBV. How long after cooldown can you open the steam wand and it still sucks in air? I would imagine the steam boiler leaks air to some extent and vacuum would be lost after a short period of time.

(BTW, this slow, inward leakage of air is what must cause false pressure once reheated.)

So I assume that's the answer to my original quiz question: The Vivaldi will continue to heat until it reaches the cut-off temperature and the steam boiler pressure will be HIGHER without the VBV releasing the extra air that leaked in during the vacuum created during shut-down. So, it's basically there to eliminate the extra step of purging the false pressure (which in this case, unlike the p-stat machines, is actually higher than the running pressure and could be dangerous). Nice pufffff I assume.
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pdx

#18: Post by pdx »

Its not just about pressurestats & false pressure.

Synessos (with pid boilers) have them. From the Synesso repair manual:

"In the event that your steam tank runs out of all pressure the vacuum breaker opens to allow air into the tank. If it fails to open and milk steaming is in progress the vacuum created by running out of steam will allow milk to be drawn through the steam wand and into the steam tank. This requires a thorough cleaning process."
Ben King.

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kschendel

#19: Post by kschendel »

HB wrote:...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?
Actually, it's all about the steam; the water has very little to do with it.

Let's start with a cold boiler. It's not 100% full, so you have some amount of water, with the rest of the space being taken up with air. The air will contain a certain amount of water vapor, but not much.

Now, you turn the machine on, and the boiler heats up. As it heats, two things happen. 1) the air (actually air + a little water vapor) pressure increases, because it's a gas, and in a constant volume gas pressure is proportional to temperature. 2) the vapor pressure of the water increases, meaning basically that more of the water is trying to turn into gaseous water vapor.

The air part of the pressure rise inside the boiler is undesirable, because the boiler is regulated by pressure, not temperature. What's going to happen is that the boiler heater will turn off prematurely because it's seeing a pressure component from the air. (and, if you ran the boiler purely on temperature, you'd over-pressurize it initially, and then after the air is eliminated, the pressure drop would throw the temps off anyway.)

So, you dump the air; either through the vacuum breaker valve, or by cracking the steam valve for a bit. Now, the boiler has nothing in it except water, partly liquid and partly gas, and for a constant pressure (and known volume) it stays at a known temperature. Which is why you can control boiler temp with a pressurestat. (Going back to the point previously, the pressurestat is set to assume the boiler is nothing but water; air in the mix throws the setting off, low.) All nice and predictable, and it doesn't change as you use steam from the boiler. (well, it does, but the heater soon gets things back to equilibrium.)

Now, you turn off the machine, and the boiler cools. Lower temperature, lower pressure; and this time since all you have in the boiler is water, the final gas pressure is basically the cold water's vapor pressure, which is much lower than normal atmospheric. You could obviously build boilers (and steam valves) to stand up to vacuum as well as pressure, but it's harder and not really worth the effort. Instead, air is let back into the boiler, either through the VBV or who-knows, and we're back to the beginning.

(It's been a long time since physics for me too, but I think I have this at least mostly right!)

Endo

#20: Post by Endo »

I think we moved on from the basic functioning of a VBV and p-stat on post #2 and are now discussing VBV when a PID is used.

Still, thanks for the refresher.
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