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Gaggia Espresso machines w/out OPV - how is pressure regulated?
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AndyPanda
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Posted Sat Jun 23, 2012, 6:48pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

That valve (on the steam valve) actually works the opposite way.  Low pressure in the boiler pushes the ball into the seat which would seal the valve and stop any flow through the valve EXCEPT they deliberately designed a small groove into the seat so that it would leak on purpose at low pressure but at higher pressure the rubber ball compresses into the groove and seals the leak.  There are two reasons for the valve (depending on who you ask):

Some call it a "Self Prime Valve" - that allows the pump to fill up an empty boiler without any need to open the steam valve first to vent off the pressure

Some call it an "Anti Siphon Valve" to prevent vaccum inside a hot boiler as it is cooling so it cannot suck milk up the steam wand and into the boiler.  Instead it will suck water from the water tank.

Some machines have it  (depending on the year - I've got same models from different years some with some without) If buying the replacement steam valve from Gaggia, you can choose either version - with or without that valve.   Most users plug up the thing so it cannot vent hot water back to the water tank.

It should have very little impact on brew pressure unless it was really malfunctioning and leaking a significant amount all the time.
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SJM
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Posted Sun Jun 24, 2012, 8:00am
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

Over at the Gaggia Group, we highly recommend plugging up that SPV as one of the first modifications on your so-equipped Gaggia  (Classics do not have SPVs).  

One reason is that it is so easy and inexpensive and gives the new-to-modifications person a chance to dig in inside the machine.  Another is that if/as/when it fails (which is most likely will do eventually),  it sucks hot water back into the reservoir and could cause premature pump failure (pump isn't rated for hot water)....

One way to diagnose SPV failure is to feel the reservoir water;  if it gets warm/hot,  your SPV valve has most likely failed.  Another clue is that the tube that takes the water from the reservoir to the boiler will get milky and discolored from the heat of the water travelling back down into the reservoir.
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Espressoliver
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Posted Sun Jun 24, 2012, 8:57pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

Huh. That's embarrassing. Thanks for the correction.  Seems like I correctly remembered Hooke's Law (F=kx), but barely thought about x. Based on a quick consult with Wikipedia, x is the distance you compress a spring from its "equilibrium" state--and equilibrium exists whenever you stand on a spring until it compresses no further.  The constancy of an ideal spring is this, I think: If you're handed a ten pound bag of groceries, it doesn't matter whether you're 100lbs or 150lbs, you'll compress the spring an extra x inches,  to a new equilibrium point,  and the additional force at that point compared to the prior one, will be kx=10lbs. It's the same distance x, for an ideal spring, because ten pounds always requires ten pounds of force resisting it to stop its fall, and because for any given spring, k is a constant.  That's the principle of a simple bathroom scale, I suppose.

The spring valve spring isn't at equilibrium under a load, but at some arbitrary compression, based on how far you thread that part in. Still, thread it in farther by a distance x, you get kx more force pressing the ball against the boiler. (Like you said)
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Espressoliver
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Posted Mon Jun 25, 2012, 7:27pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

SJM Said:

Over at the Gaggia Group, we highly recommend plugging up that SPV as one of the first modifications on your so-equipped Gaggia  (Classics do not have SPVs).  

... it sucks hot water back into the reservoir and could cause premature pump failure...

Posted June 24, 2012 link


But it's not suction, right? According to how we've talked about its operation and purpose, water is *pushed* down the tube by positive pressure in the boiler.
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Espressoliver
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Posted Mon Jun 25, 2012, 7:35pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

AndyPanda Said:

That valve (on the steam valve) actually works the opposite way.  Low pressure in the boiler pushes the ball into the seat which would seal the valve and stop any flow through the valve EXCEPT they deliberately designed a small groove into the seat so that it would leak on purpose at low pressure but at higher pressure the rubber ball compresses into the groove and seals the leak.  There are two reasons for the valve (depending on who you ask):

Some call it a "Self Prime Valve" - that allows the pump to fill up an empty boiler without any need to open the steam valve first to vent off the pressure

Some call it an "Anti Siphon Valve" to prevent vaccum inside a hot boiler as it is cooling so it cannot suck milk up the steam wand and into the boiler.  Instead it will suck water from the water tank.

Some machines have it  (depending on the year - I've got same models from different years some with some without) If buying the replacement steam valve from Gaggia, you can choose either version - with or without that valve.   Most users plug up the thing so it cannot vent hot water back to the water tank.

It should have very little impact on brew pressure unless it was really malfunctioning and leaking a significant amount all the time.

Posted June 23, 2012 link


Actually, how you describe the operation is exactly what I was speculating, not the opposite, although the point about the partially sealed state I had no idea about. Anyway, thanks for the explanation. What's your idea of a malfunction that would allow more than the normal rate of flow during brewing? All I can think of is if the ball was blown clear down the tube into the reservoir.
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AndyPanda
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Posted Mon Jun 25, 2012, 8:10pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

There "shouldn't be" any flow at all through that valve during brewing.  You should only see a few bubbles going towards the tank when you are topping up a boiler that isn't full and a few bubbles towards the tank when a cool boiler that is very full starts heating up.  THen when a hot boiler is cooling down, you should see cool water from the tank bubbling the opposite direction up the tube and into the boiler.  

I said "shouldn't be" meaning in an ideal world - in the real world we almost always see some bubbles headed to the reservoir whenever the boiler is heating up - so the heating cooling cycle usually has some bubbles headed towards the water tank at the top of the heat cycle.

As much as people talk about hot water in the reservoir ruining their pump - it would take a lot of hot water to heat up the reservoir to the point of damaging the pump --- but on the other hand the idea of any water from the boiler going back to the fresh water supply doesn't sound very good to me.  So I generally plug the thing up.

If the ball gets old and hardens up - it might stop sealing - the seal depends on the ball being soft enough to squeeze into the groove (under pressure) and seal it.  Or I suppose it could get crusted with scale.  I can't imagine any condition that could push the ball through the hole and to the tank - the ball is the size of a pea and the hole is the size of a toothpick - it just couldn't fit through there without a lot more pressure than the pump can generate.
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AndyPanda
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AndyPanda
Joined: 12 Jul 2010
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Posted Mon Jun 25, 2012, 8:16pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

Espressoliver Said:

But it's not suction, right? According to how we've talked about its operation and purpose, water is *pushed* down the tube by positive pressure in the boiler.

Posted June 25, 2012 link

When the water is moving towards the reservoir - it is pressure (not suction) from either the pump or the heat of the boiler expanding the contents of the boiler pushing the water and air bubbles past the ball (through the "leak" in the seal) and towards the reservoir.

When the water is moving from the reservoir towards the boiler - then it is suction - that is because the cooling boiler creates a suction and that does pull the ball away from the seal and sucks water from the reservoir.

On a sealed boiler lever machine (like the La Pavoni or Olympia Cremina) when the boiler cools it will suck the piston up and raise the lever... or you can open the steam valve and it will suck in air as the boiler cools.   This is what would be a disaster if the steam wand were sitting in a pitcher of milk (though you'd have to be pretty sloppy to leave it like that) - it could suck milk up the wand and into the boiler (but easily solved - just don't do that).
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Espressoliver
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Posted Tue Jun 26, 2012, 7:35pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

AndyPanda Said:

When the water is moving towards the reservoir - it is pressure (not suction) from either the pump or the heat of the boiler expanding the contents of the boiler pushing the water and air bubbles past the ball (through the "leak" in the seal) and towards the reservoir.

When the water is moving from the reservoir towards the boiler - then it is suction - that is because the cooling boiler creates a suction and that does pull the ball away from the seal and sucks water from the reservoir.

Posted June 25, 2012 link

You've lost me--so maybe you were describing it opposite to me after all. I have been picturing the ball as sealing when it moves toward the boiler, and as being pushed out of the way and unsealing whenever any pressure whatsoever exists in the boiler, such that some amount of water will flow to the reservoir. Are you saying it seals when it moves away from the boiler? Or are you saying it seals in both directions? If it's both directions, is the weak, low-pressure seal when the ball is pushed away from the reservoir? Thanks for your patience, if I haven't exhausted it yet. This would be much easier I suppose with a picture.
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AndyPanda
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AndyPanda
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Posted Tue Jun 26, 2012, 11:16pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

It seals when pressure in the boiler moves the ball away from the boiler.  The only reason there is any leakage to the water reservoir is that they deliberately designed a small leak into it so that it would not seal until the pressure built up a bit - the reason being that allows the pump to easily fill an empty boiler without opening the steam valve first (Self Prime).

If it weren't for that little deliberate leak (that leak is what makes it a Self Prime Valve) it would be a one way valve that only allowed water from the reservoir to enter the boiler when the boiler had a suction from cooling down and would not allow anything to move in the direction of the reservoir. (this would be the Anti Siphon Valve function).  

By putting the leak into it, they made it do both tasks ... Anti Siphon (allows a vacuum in the boiler to draw water from the reservoir so I cannot suck milk up the steam wand) and also be a Self Prime Valve allowing a little bit of pressure to bleed off making it easier for the pump to fill up an empty boiler - with the intent that once pressure started to build up, the rubber ball would deform and seal the leak completely.

Here is a picture of the nipple that goes from the steam valve back to the water tank - this is the sealing surface inside that the ball gets pressed up against by pressure inside the boiler.  You can see the little groove that is there to allow a small leak - but the rubber ball is soft enough to compress into that groove and seal it once the boiler is full.  It is only meant to bleed off all the air in the empty boiler - to make it easy for the pump to self prime.  That is the theory ... in the real world it leaks a few dribbles of hot water into the reservoir.

AndyPanda: SPV.jpg
(Click for larger image)
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Espressoliver
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Posted Wed Jun 27, 2012, 10:24am
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

AndyPanda Said:

It seals when pressure in the boiler moves the ball away from the boiler.  The only reason there is any leakage to the water reservoir is that they deliberately designed a small leak into it so that it would not seal until the pressure built up a bit - the reason being that allows the pump to easily fill an empty boiler without opening the steam valve first (Self Prime).

If it weren't for that little deliberate leak (that leak is what makes it a Self Prime Valve) it would be a one way valve that only allowed water from the reservoir to enter the boiler when the boiler had a suction from cooling down and would not allow anything to move in the direction of the reservoir. (this would be the Anti Siphon Valve function).  

By putting the leak into it, they made it do both tasks ... Anti Siphon (allows a vacuum in the boiler to draw water from the reservoir so I cannot suck milk up the steam wand) and also be a Self Prime Valve allowing a little bit of pressure to bleed off making it easier for the pump to fill up an empty boiler - with the intent that once pressure started to build up, the rubber ball would deform and seal the leak completely.

Here is a picture of the nipple that goes from the steam valve back to the water tank - this is the sealing surface inside that the ball gets pressed up against by pressure inside the boiler.  You can see the little groove that is there to allow a small leak - but the rubber ball is soft enough to compress into that groove and seal it once the boiler is full.  It is only meant to bleed off all the air in the empty boiler - to make it easy for the pump to self prime.  That is the theory ... in the real world it leaks a few dribbles of hot water into the reservoir.

Posted June 26, 2012 link

Ahh, finally, I think I've got it. Thanks a bunch! I've been confused about this thing for years and even argued with others about it online, and nobody clued me in to my false image of the thing. Now that I know what it does, it seems like sort of an elegant solution to the problems it's designed to solve.
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