In my coffee machine there is a drip valve at the point the water comes out of the boiler which consists of a mushroom shaped round topped thing pushed into the hole with a spring. When the pressure exceeds the force exerted by the spring, water comes out of the hole, of course. When the pump is off, the valve stops water dripping or running out of the boiler.
There is also an over-pressure valve after the pump, with a spring, that opens at (supposedly) 9 bar and diverts excess water back to the tank and hence keeps the pressure in the boiler at 9 bar, which is the right pressure to force through the coffee and make delicious espresso.
I have a pressure gauge that screws onto the bit where the coffee goes, and when you put that in place the pressure builds up to the point where the check valve opens and you can see the pressure it's generating. Now, here's the confusion:
If I remove the ball valve the gauge reports 8 bar (close enough). But if I put the valve in, it reports 5 bar. Now - the over-pressure valve must be opening at 8 bar, and we know that's enough to open the drip valve, how come the pressure in the gauge does not go up to 8 bar? The drip valve should open until the pressure on both sides is the same, no? What am I missing?
I'm a little confused from your explanation and don't know anything about coffee machines but are you seeing the pressure drop across the coffee @5bar whereas @8bar is through the PRV and to wherever it vents the water/steam.
Little confused as well, you talk about drip valve and prv then say you remove a ball valve?
My guess is the water flows through the coffee quicker than the pump can top it up, so the pressure drop though the coffee/portafilter is 5 bar.
Little confused as well, you talk about drip valve and prv then say you remove a ball valve?
I presume it's a version where the boiler is pressurized and a valve is then opened between it and the coffee. Rather than one where you press a button and the pump starts.
Sorry, ball valve should read drip valve.
The experiment is done when there's no coffee in it. Just the pump, the over-pressure valve, the drip valve and the pressure gauge which blocks off the water flow. When you start the pump, it pushes water through the boiler, through the OPV, then the regulated side of that pipe goes through the boiler (which is just a heater really, it's not set to boil) and into the bit where the coffee would be. But it cannot escape due to the gauge which blocks the place where the coffee would normally come out of, so the incoming water pressurises the air that was in there until 5 bar is reached, then it stops. The pump is still running but all the water is presumably going back through the OPV.
I'd say the pump was knackered but it can generate 8bar without the drip valve in place.
I'll draw a picture.
"the pressure builds up to the point where the check valve opens"
Which one are you calling the check valve, the pump valve which returns to the tank, or the drip valve? If the ball valve you refer to is in the line to the brew group, it won't reach the max pressure as it's flowing into the coffee, unless by attaching your pressure gauge you've blocked that line off.
Sorry but the description needs to be a lot clearer!
Edit: You beat me to it!
Think the confusion is over why someone is so keen to understanding the how the machine makes the hot mud drinkable (and my confusion is why anyone would drink it...but I'm happy not to know).
Are you sure the pressure gauge is accurate?
Does it really matter? If it is making the stuff as you enjoy it then all good...
Could the pressure be dropping as the system itself is long enough for the pressure to drop?
What's the problem you're trying to solve, ie, why do you need to understand this? Is it academic curiosity (which I understand) or because you think the machine isn't working correctly, and if the latter, why do you think that?
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Pressure drop through the anti-drip valve if it takes 3bar to open it then it will let 5 bar through then the back pressure on the ball from that adds up to the 8 bar.
Well, 8 bar is a good pressure for coffee extraction, and 5 bar is too low at least for what I want.
I'm wondering if the drip valve is actually reducing the pressure to 5 bar where the coffee would be. Because I recently had a problem with the drip valve and spring necessitating a change.
@tinas right.. the spring pressure adds to the water pressure on the other side.
A bit odd though. I recently changed the spring in the OPV to a lower rate beacause it was reading 11 bar with the gauge which is too high. But maybe the new spring I have bought is stronger than the old one and now I need to put the higher OPV spring back.
If it was flowing that would be logical, but as it's a closed system and the pump is still running, seems to me it should still build up to full pressure after the spring-loaded valve.
Are you saying that the 5bar downstream + the spring load are enough to shut off the flow, i.e. prevent the spring-loaded valve from opening? Then you need more pressure upstream to get more than 5bar on the coffee side, assuming there's also the same back-pressure from the coffee side when brewing.
Then you need more pressure upstream to get more than 5bar on the coffee side, assuming there’s also the same back-pressure from the coffee side when brewing.
Hmm yes, that is a good question - the back pressure from the coffee side won't be the same because when actually brewing rather than doing this test the water is now flowing through the coffee (hopefully). That said, it's not flowing very quickly.
The reason I got the gauge out to check is that having replaced the spring I found the water flowed through the coffee I was actually making, when the grind and coffee were the same as they had been before the spring was replaced, which suggests pressure had somehow dropped.
Which spring have you replaced, the OPV one or the drip valve one?
I replaced the OPV one a while ago, to bring the brewing pressure at the coffee down to 9 bar which seemed to improve the coffee brewing a lot - it was about 12 or so which pushed the water through too fast.
Then, the other day I accidentally left it on with the steam setting selected for a whole morning. This kept everything really hot, and after that somehow the water would not flow through any more at all, even with no coffee. I am not sure why - perhaps the high temperature affected the rubber in the drip valve, no idea. I cannot explain that. So then I replaced the drip valve rubber mushroom bit and spring, and then somehow the water would barely flow through the coffee. So I got the gauge out and observed the different readings as above, and I was puzzled.
I think I will go down now and put the 12 bar spring back into the OPV and see how it goes. It's possible the old drip valve was weakened somehow or just a weaker part to begin with.
The site selling the OPV springs as mods, incidentally, is selling them for a more expensive machine which has a solenoid instead of the drop valve to shut off the flow.
If it was flowing that would be logical, but as it’s a closed system and the pump is still running, seems to me it should still build up to full pressure after the spring-loaded valve.
Are you saying that the 5bar downstream + the spring load are enough to shut off the flow, i.e. prevent the spring-loaded valve from opening? Then you need more pressure upstream to get more than 5bar on the coffee side, assuming there’s also the same back-pressure from the coffee side when brewing.
Nope, it's an issue you see in industrial applications too.
E.g. if that was the PRV across the shutdown valves on a pipeline and there's one every kilometer, and several km of pipe you end up having to add them together.
So if the design pressures of a 10km pipe is 200psi, you set the nearest valve at 100psi, then the next 10 valves at 10psi so that the last one opens at 200.
If you don't do that, and set them all to 200psi you get a range of outcomes from the valve flanges cracking to bits of plant launching themselves in search of extra terrestrial life.
Original spring back in, pressure at the gauge is back up to 12 bar. Too high.
Something wierd is going on with that anti-drip valve then.
My suspicion is that it's meant to drop the pressure by 3bar, but it's sometimes sticking open slightly which lets the pressure bleed through when you have the guage on it.
Under normal conditions when the water is allowed to flow it wouldn't matter so much.
With a closed system with no flow, the pressure builds up below the drip valve. If the spring pressure is equivalent to 3 bar, then when the pressure below it just over 5 bar, the valve will see just over 8 bar from below, so will close. It's an artificial situation because normally there's no pressure below the valve.
Can you cut a bit off the new spring to stiffen it a bit?
So if the design pressures of a 10km pipe is 200psi, you set the nearest valve at 100psi, then the next 10 valves at 10psi so that the last one opens at 200.
I assume that's related to momentum in the pipe. It's a big problem in long water mains, eg, from a reservoir to a city. If you close the valve at the city end suddenly, the pressure at that end is huge.
It’s an artificial situation because normally there’s no pressure below the valve.
Well yes, it is an artificial situation, but there is pressure below the valve normally. The packed in coffee is what restricts the water flow and this is what creates pressure below the valve. The pressure drop is across the coffee grounds. This is why it's such a faff, because you have to have the right amount, the correct grain size, and the right oil content in the coffee to get the optimum flow rate.
Something wierd is going on with that anti-drip valve then.
TINAS has the correct answer. I put the previous OPV spring in which is meant to be rated for 9 bar, and I tried again with the gauge but it read 5 bar again. But suddenly, there's a pop and a hiss and it jumped up to 8 bar. Checked everything, tried again and 8 bar again. So I made a coffee and realised I'd left the grinder on the coarser setting.
I think that something must have have been blocked around the drop valve housing where the water is meant to flow around the valve, I don't know. But I'm not taking it apart again to check 🙂
I assume that’s related to momentum in the pipe. It’s a big problem in long water mains, eg, from a reservoir to a city. If you close the valve at the city end suddenly, the pressure at that end is huge.
That's called surge and is a different issue to do with the momentum of the fluid in the line.
To solve that you usually specify slow closing valves which take a long time to close, giving the fluid in the pipe time to slow down gradually.
Sounds more like a flow restriction somewhere, if the flow is poor with no coffee. Could the long heating of the boiler have displaced a piece of scale which has partially blocked the outlet?
But is it making coffee how you like it? If so maybe leave it be, if not crack on with the tinkering but don't worry too much about the actual numbers just use them as a guide for more or less until it makes the coffee how you like it (assuming you don't overpressurise and make a bomb).
Pressure relief type valves don't always make sense. That's why process engineers spend their days puzzling over them.
