This may be a stupid question, but I'm curious none-the-less.

I've read the repair manual, and I know that the expansion valve relieves pressure within the boiler at 12 bar (or should if set properly).

My question is this: If one is using a PID, where the temp is stable, and the heating elements only kick on briefly, the pressure should never reach that high, am I wrong for assuming this?

The background to this question is that, I've played around with my expansion valve a little, and don't see a noticeable change in pressure.

At Idle: the brew portion reads: 2bar
At Brew: 8.5bar
After Brew: Heating element on: 3bar

If it never reaches 12 bar, what use is the expansion valve? Am I missing something here or is the PID in a way creating a system that doesn't necessarily need an expansion valve unless there is a failure?

Thanks,
Justin

Assume for the moment that the total volume of 75 F water required to fill your brew system is 7.0 litres. That water weighs 15.4 lbs. Now take that same 15.4 lbs and raise it up to 225 F and it will do its best to occupy 7.3 litres. Yes, the boiler system itself increases in physical volumetric capacity from 75 to 225 but not nearly enough to cover the efforts the water is making in its expansion (the key word).

Hence, the need for an expansion valve to release some of that water to prevent component distortion/failure.

Note that I have left off/rounded off lots of digits because they are unnecessary to illustrate the expansion and we don't need a math lesson at this hour.

Eric,

Thanks for the illustration, I'm pretty sure I understand the reasoning behind it, but I also have yet to see my pressure in the tank raise above 9bar, which everything I've read, it "should" slowly rise to about 12bar, after pulling a shot, this being because cold water enters, heater kicks on, and temp starts to raise. I'm wondering does the fact that I run my boiler on a PID make a system that will never see 12bar, or is there a possibility there is a problem with my expansion valve, as to why i'm never reaching that 12bar, even with the valve completely close, or completely open...

Euology101 wrote:Eric, I'm wondering does the fact that I run my boiler on a PID make a system that will never see 12bar, or is there a possibility there is a problem with my expansion valve, as to why i'm never reaching that 12bar, even with the valve completely close, or completely open...

The PID control has nothing to do with it. Is your expansion valve adjustable? If it is adjust it so that when you heat up your machine from cold the expansion valve starts leaking water between 10.5-12 bar. Higher then the extraction pump pressure, lower then the opening pressure of the group solenoid. When properly adjusted the brew boiler pressure gauge will usually read in this range when the machine is idle although that can vary.

. . . should slowly rise to about 12bar, after pulling a shot, . . .

Probably not - the thermal expansion "experience" is at a very minimum during a typical shot event. In reality, instant step changes in thermodynamic properties do not occur but, for the moment, assuming they do, you're starting with 7000 ml of water @ 225 F, losing 75 ml of it, and adding 75 ml of water at 75 F. These numbers are in the ballpark - not necessarily 100% accurate. Assume an instant mix of those 75 ml @75F and the 6925 ml @ 225F results in a boiler temp of 223.3 if you had NO heater action during those 25 seconds.

Whether a machine is controlled via PID or pressurestat has no bearing on the thermal expansion event. The most pronounced thermal expansion takes place as the machine's brew boiler goes from a 75F state to a 225F state and this is when you should see the controlled leakage from the expansion valve.

As has often been stated, the brew circuit pressure gage reading is typically not relevant unless you are passing water through the grouphead for whatever reason. In this particular situation, it is relevant to the proper setting of the expansion valve . . . but you know that .

JohnB. wrote:The PID control has nothing to do with it. Is your expansion valve adjustable? If it is adjust it so that when you heat up your machine from cold the expansion valve starts leaking water between 10.5-12 bar. Higher then the extraction pump pressure, lower then the opening pressure of the group solenoid. When properly adjusted the brew boiler pressure gauge will usually read in this range when the machine is idle although that can vary.

Expansion valve is adjustable.

I never shut the machine off... so starting from cold is something I may have to try when I shut it down for a cleaning.

Your last sentence is what I find most people say, but this is not true of my machine, which is more or less, what concerns me or makes me think. When idle, my machine drops down to 3bar, on the pressure gauge. Although I haven't watched it from a cold start, I don't think I've ever seen my pressure gauge reach 10-12.

All this being said, what effect does any of this have on the system while brewing? Any? I'm getting 9bar when brewing, (gauge is reading this, but I can't quantify it as I don't have a PF with a pressure gauge at the moment) but what does not having the 10-12 at idle do the system, if anything?

I've also read that if the expansion valve is not properly adjusted, there is a high pitched "squeal" during brewing? My pump makes a high pitched noise, but to be honest I'm not sure if that normal or not, seeing that I've never had a machine with an exposed pump, one of which is currently just sitting on the floor, without an dampening, or anything.

Euology101 wrote: I've also read that if the expansion valve is not properly adjusted, there is a high pitched "squeal" during brewing? My pump makes a high pitched noise, but to be honest I'm not sure if that normal or not, seeing that I've never had a machine with an exposed pump, one of which is currently just sitting on the floor, without an dampening, or anything.

The rotary pump will have a pressure adjustment. As long as the output from the rotary pump never hits the threshold of the expansion valve, there won't be any effect of the expansion valve while pulling a shot.

If the expansion valve threshold is adjusted below the max pressure output from the pump, then either the pump is set too high or the expansion valve is set too low.

allon wrote:If the expansion valve threshold is adjusted below the max pressure output from the pump, then either the pump is set too high or the expansion valve is set too low.

Okay, so this is sort of where I'm going with this, unless I start the machine from cold, to see where the threshold of the expansion valve is, I'll never know if the expansion valve is set properly, and not in fact "limiting" the pump correct? because no matter how many shots I pull, how much cold water I pull into the system, I never see my gauge read above ~9bar. So I'm wondering if the expansion valve, is a faulty...

Anyone know a way to test this? or should I just turn the machine off over night and then watch it while it heats up?

Euology101 wrote:or should I just turn the machine off over night and then watch it while it heats up?

Bingo!! What could be easier?

If the expansion valve exhausts into the drip tray, you could run the pump against a blind basket a la backflush; if the expansion valve doesn't drip, then it isn't limiting the pump pressure.