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"False" pressure in boiler while heating up
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Discussions > Espresso > Machines > "False" pressure...  
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billc
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Joined: 15 Jan 2009
Posts: 130
Location: Seattle, Washington
Expertise: Professional

Espresso: CC1 - GS - GS3 - GB5
Grinder: Baratza - Mazzer-Marzocco
Drip: My own Creation
Posted Tue Oct 30, 2012, 7:50am
Subject: Re: "False" pressure in boiler while heating up
 

False pressure is caused by air in the steam boiler.  When the boiler heats, the hot water heats the air causing it to expand.  If your steam boiler pressure/temperature is controlled by a pressure switch, then the expanding air will increase the pressure until the pressure switch turns it off.  As an aside..... is your steam boiler is controlled by temperature then the pressure will be too great causing a false "Over Pressure".......

Once the valve is opened the expanded air is released through the steam wand.  Once the air is gone it can be replaced by water vapor.  The heating element adds energy to the water until boiling.  As the water boils it transitions to water vapor.  The water will boil until the amount of water vapor is great enough to increase the pressure in the boiler.  The increased pressure also increases the boiling temp of water.  Additional energy in added until the pressure is great enough to cause the pressure switch to turn off the heating element.  At this point the water and water vapor are at equilibrium and no boiling happens.  

The Vacuum breaker is a device that ensures that at no time will a boiler have negative pressure (Vacuum - usually during the cooling process when the water vapor turns back into water).  In the case of espresso machines we use the vacuum breaker for not only the intended purpose but also for an air escape during the heat up process.  This allows the air to escape during the heat up process (air is lighter than water so it exits first).  When the boiling water has made enough water vapor to fill the boiler it begins to escape through the vacuum breaker.  As the pressure builds inside the boiler the velocity that the water vapor escapes increases.  Eventually the increased velocity causes the little piston in the vacuum breaker to close and the pressure in the boiler keeps it closed.  The water vapor that escapes through the vacuum breaker then cools and condenses back into water and either drips inside the machine or is directed to the drain tray.

WHEW!  The cause of the leaking you described sounds like a leaking fill valve........ the valve leaks until the boiler is full.  Line pressure is usually greater than the relief pressure of the "pressure relief" valve so water exits freely.


BillC
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grumpybarista
Senior Member
grumpybarista
Joined: 19 Feb 2007
Posts: 224
Location: Detroit
Expertise: I live coffee

Espresso: LP Pub1
Grinder: Mazzer Mini w/ doser, NS...
Vac Pot: Yama 5-cup
Drip: French Press, aeropress
Roaster: Behmor 1600
Posted Tue Oct 30, 2012, 8:07am
Subject: Re: "False" pressure in boiler while heating up
 

GVDub Said:

MIght be scale on the fill probe, or it might have a loose connection. I'd check the connection and pull the probe to check for scale accumulation. According to the hydraulic schematic I found there's a boiler drain. You can also drain from the hot water spigot.

Posted October 29, 2012 link

I pulled the probe this morning before work and it shows pretty clean. I wasn't anticipating any scale since I'd recently done a grouphead cleaning and didn't see much then either. I did a "dip" test and couldn't really tell if the water level was any higher than the end of the probe, so I'm not sure why the sight glass indicator appears to show a high level. The electrical connection seemed okay, even if the terminal cover is pretty brittle and not providing much in the way of insulation or protection anymore. Still, the contact seems pretty good.

Will keep digging. I'm thinking of pull the sight glass and vacuum breaker off to inspect those later tonight. It's gym/bar night so I may not get to it today...
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grumpybarista
Senior Member
grumpybarista
Joined: 19 Feb 2007
Posts: 224
Location: Detroit
Expertise: I live coffee

Espresso: LP Pub1
Grinder: Mazzer Mini w/ doser, NS...
Vac Pot: Yama 5-cup
Drip: French Press, aeropress
Roaster: Behmor 1600
Posted Tue Oct 30, 2012, 8:25am
Subject: Re: "False" pressure in boiler while heating up
 

billc Said:

False pressure is caused by air in the steam boiler.  When the boiler heats, the hot water heats the air causing it to expand.  If your steam boiler pressure/temperature is controlled by a pressure switch, then the expanding air will increase the pressure until the pressure switch turns it off.  As an aside..... is your steam boiler is controlled by temperature then the pressure will be too great causing a false "Over Pressure".......

Once the valve is opened the expanded air is released through the steam wand.  Once the air is gone it can be replaced by water vapor.  The heating element adds energy to the water until boiling.  As the water boils it transitions to water vapor.  The water will boil until the amount of water vapor is great enough to increase the pressure in the boiler.  The increased pressure also increases the boiling temp of water.  Additional energy in added until the pressure is great enough to cause the pressure switch to turn off the heating element.  At this point the water and water vapor are at equilibrium and no boiling happens.  

The Vacuum breaker is a device that ensures that at no time will a boiler have negative pressure (Vacuum - usually during the cooling process when the water vapor turns back into water).  In the case of espresso machines we use the vacuum breaker for not only the intended purpose but also for an air escape during the heat up process.  This allows the air to escape during the heat up process (air is lighter than water so it exits first).  When the boiling water has made enough water vapor to fill the boiler it begins to escape through the vacuum breaker.  As the pressure builds inside the boiler the velocity that the water vapor escapes increases.  Eventually the increased velocity causes the little piston in the vacuum breaker to close and the pressure in the boiler keeps it closed.  The water vapor that escapes through the vacuum breaker then cools and condenses back into water and either drips inside the machine or is directed to the drain tray.

WHEW!  The cause of the leaking you described sounds like a leaking fill valve........ the valve leaks until the boiler is full.  Line pressure is usually greater than the relief pressure of the "pressure relief" valve so water exits freely.

Posted October 30, 2012 link

Thanks for the breakdown, Bill! Based on this, I'm thinking the vacuum breaker might be faulty, AND I've got a separate fill valve that may also be faulty. As I said, I will keep plugging away, but probably after bar night :)
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grumpybarista
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grumpybarista
Joined: 19 Feb 2007
Posts: 224
Location: Detroit
Expertise: I live coffee

Espresso: LP Pub1
Grinder: Mazzer Mini w/ doser, NS...
Vac Pot: Yama 5-cup
Drip: French Press, aeropress
Roaster: Behmor 1600
Posted Thu Nov 1, 2012, 9:46pm
Subject: Re: "False" pressure in boiler while heating up
 

GVDub Said:

MIght be scale on the fill probe, or it might have a loose connection. I'd check the connection and pull the probe to check for scale accumulation. According to the hydraulic schematic I found there's a boiler drain. You can also drain from the hot water spigot.

Posted October 29, 2012 link

Okay, I finally found some time tonight to diagnose further. I pulled the vacuum breaker and found it to be stuck with some scale build up. Scrubbed it clean and made sure if actioned well. But I was also puzzled about the apparent high water level in the tank; the sight glass still showed an over-full tank. So I decided to drain the tank a bit but the water came out very slowly via the drain. I opened up the lower tube to the sight glass and removed it from the tank. Hardly any water. I thought perhaps instead of a full tank, it was an empty tank. But I noticed some scale buildup there too and as soon as I prodded it with a rod, boy did the water come gushing out and all over my floor! I quickly re-attached the tube connecting the tank to the lower sight glass fitting to stop the "bleeding". This, however, did serve to lower the level to about mid-glass. And to mop up the floor, which was also kind of overdue.

I made sure everything was connected and tight and I flipped the machine back on. After about 10 minutes I heard the vacuum breaker seal up, after which the pressure gauge began to rise. I let it pressurize to it's stopping point (around 1.4 bar) and tested the hot water tap and steam wand. Like a charm! No "false" pressure this time. The vacuum breaker did its job. I decided, while everything was still open, to lower the pressurestat to 1.2 bar. I also ran several bursts of hot water to see if and when the pump kicked in, and to how high it would return the level. So far it would go down about a quarter inch above the MIN line and refill to about halfway between MIN and MAX.

Next step, I guess, is to see if it will still creep up to beyond the max level. If so, then definitely I would suspect a leaky fill valve, as billc mentioned.

At a minimum, I have seen that the entire system will need a good descaling soon. From a few minor meddlings I hadn't noticed much scale before (cleaned out the entire grouphead about two months ago), but this time I saw quite a bit at at least one tank opening.

Thanks to all for the tips and the step-by-step help. I hope this exercise helps someone else in the future as well, and it has made me feel a little smarter about my machine.

Phil
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GVDub
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Joined: 25 Jan 2008
Posts: 849
Location: Los Angeles, CA
Expertise: I live coffee

Espresso: Londinium I, Arrarex...
Grinder: Gaggia MD85, Dienes Mokka,...
Drip: Behmor Brazen, Abid Clever
Roaster: Behmor 1600
Posted Fri Nov 2, 2012, 7:22am
Subject: Re: "False" pressure in boiler while heating up
 

Glad to hear that at least the false pressure issue is sorted. Been looking at a Pub that someone's got locally "partially restored" and at how much my wife will kill me if I pick it up.
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grumpybarista
Senior Member
grumpybarista
Joined: 19 Feb 2007
Posts: 224
Location: Detroit
Expertise: I live coffee

Espresso: LP Pub1
Grinder: Mazzer Mini w/ doser, NS...
Vac Pot: Yama 5-cup
Drip: French Press, aeropress
Roaster: Behmor 1600
Posted Fri Nov 2, 2012, 9:38am
Subject: Re: "False" pressure in boiler while heating up
 

Good luck with the Pub. I like mine -- it's my first HX -- but as I bought it used I've had to tinker with it here and there (not that I mind). The only downside is the asthetic compared to some other machines. But at $800 I thought it was a good deal and I haven't had any issues keeping up with demand when I invite friends over for group coffee socials. It'll run constantly for a couple hours during those events and has very quick recovery times. In fact, the only time it's ever lost its "oomph" was when I was trying to steam up about a half gallon of chocolate milk for my kids and their friends. But then again, a HALF GAL is quite a lot to steam up!
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GVDub
Senior Member


Joined: 25 Jan 2008
Posts: 849
Location: Los Angeles, CA
Expertise: I live coffee

Espresso: Londinium I, Arrarex...
Grinder: Gaggia MD85, Dienes Mokka,...
Drip: Behmor Brazen, Abid Clever
Roaster: Behmor 1600
Posted Fri Nov 2, 2012, 10:05am
Subject: Re: "False" pressure in boiler while heating up
 

The machine I'm looking at is a 2-group for such a stupid low price that I have to wonder just how "partial" the restoration is. Also have to wonder if it's possible to convert the 2-group to run on 110v (run only one heater? ) since I have to 220v circuit and the landlord is unlikely to install one.
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grumpybarista
Senior Member
grumpybarista
Joined: 19 Feb 2007
Posts: 224
Location: Detroit
Expertise: I live coffee

Espresso: LP Pub1
Grinder: Mazzer Mini w/ doser, NS...
Vac Pot: Yama 5-cup
Drip: French Press, aeropress
Roaster: Behmor 1600
Posted Fri Nov 2, 2012, 12:21pm
Subject: Re: "False" pressure in boiler while heating up
 

GVDub Said:

Also have to wonder if it's possible to convert the 2-group to run on 110v (run only one heater? ) since I have to 220v circuit and the landlord is unlikely to install one.

Posted November 2, 2012 link

I made up a quick solution for 220V on a 2-group Rancilio Epoca, by running off my dryer's outlet (btw, I guess I lied about the HX being my first: I had a small espresso bar for bit and used the 2-group for that; I meant that the Pavoni is my first one for my home :)  Anyway, I'm not sure if you can access the dryer (electric) outlet for that, or if you want to even try it. Mine was in the basement and I only set it up that way to keep it running in order to sell it.

Good luck!
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GVDub
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Joined: 25 Jan 2008
Posts: 849
Location: Los Angeles, CA
Expertise: I live coffee

Espresso: Londinium I, Arrarex...
Grinder: Gaggia MD85, Dienes Mokka,...
Drip: Behmor Brazen, Abid Clever
Roaster: Behmor 1600
Posted Fri Nov 2, 2012, 8:09pm
Subject: Re: "False" pressure in boiler while heating up
 

Unfortunately, in our apartment there is neither washer nor dryer and the stove is gas. I'm hoping that the next place we move into will have a 220 outlet I can use and something that will pass for shop space so I can adopt some poor old commercial machine and restore it.
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