Yesterday evening we made some very satisfying adjustments on our friends three group Hydra espresso machine (made by Synesso). It helped much to be served a little buddy prior to engaging the work. The issue was a bleeding expansion valve (they all bleed, but this one was doing it too much). The expansion valve is somewhat a beast to open in place. it is perhaps the Largest brass valve in the entire machine, with the strongest spring. Each of the four tanks in a three group hydra have their own water inlet. On all inlets that lead to a grouphead there is a check valve followed by an expansion valve. Check valves only allow water movement in one direction. We want water going into the espresso machine, not back out of it (check valves are also found in all bicycle pumps and on all bicycle inner tube stems). Inside the checkvalve is either a ball or piston that seals up when pressure is applied in the direction the valve is supposed to check (this is also what keeps beer taps closed- pressure, these are check valves as well).
There are two kinds of tanks in an espresso machine- those completely full of water lead to the groupheads (where espresso is made), and those partially full of water (to create steam). Both of these systems need a way to safely release pressure and each has a valve that opens. These valves keep closed with a spring pressing a seal against an opening. A check valve operates in line with the flow of water, an expansion valve (or pressure release, or safety valve) is connected in line, but allows excess pressure to exit or escape the line. Air is a much better absorber of pressure than water. If we had a column of water and we tried to press that column from two ends and make it smaller we would have a very hard time, but with air there would be some give (why those fancy shoes use air instead of water as a cushion). Air has better cushioning effects versus water because it is a gas. Air and other gasses may pressurize to a much higher level because their molecules are loosely spaced and not as rigid. When one pressurizes a a super soaker it is air, not water that one is compacting. Think about pressurizing a solid on the other hand, like a rock (much harder to do). So the two different types of tanks in an espresso machine are very different. One is completely full of water, and the other half full of water and half full of gas (air, steam). The Expansion valve is named such because as cold water enters the hotter environment it expands, and there is not any air to cushion its expansion (it cannot move backwards past the check valve), so we release it from the system. There is a continual drip from this valve that is usually infrequent when not in use, and more frequent in use. The water is directed to a hidden drip pan that also collects water from the drip tray before sending it down a tube to a drain. Some people freak out when they look into their machine and see water dripping, this is a natural feeling to have, but everything is all right.
In our case the other night water was dripping very frequently, so we looked further. The water shut off, excess pressure released from the line, Expansion valve removed, we simply tightened the pressure of the spring and re installed. The sealing part is a disc of teflon held against an orifice (the exit hole to our closed system). Our goal is to have the teflon seat properly with pressure against the orifice. We only had to remove the valve completely because there were a few things in our way from getting a wrench in there to make adjustments. That this worked at all was a crapshoot, it often takes a little more to get a problem disc to seat well, and the brass could have pitted or fractured due to a casting or production issue (in this case we would install a new valve).
The beautiful part was that it was easy to access. In the end we did not open up the expansion valve, because it looked like we could just tighten it a little and have it seat. We were treated to a glass of Yamazaki 12 year after the work was done.