OPV & Expansion Valve

Need help with equipment usage or want to share your latest discovery?
User avatar
BaristaBoy E61

#1: Post by BaristaBoy E61 »

Is an OPV & an Expansion Valve on a BD machine the same thing?
"You didn't buy an Espresso Machine - You bought a Chemistry Set!"

User avatar
Team HB

#2: Post by Jake_G »

Not necessarily.

The expansion valve is generally connected directly to the brew boiler and set to roughly 12 bar. Its purpose is to relieve the pressure caused by thermal expansion when cold water heats up. Since there ought not to be any air in a brew boiler, this expansion will cause pressure to rise until the volume is increased. The option without an expansion valve is that the weakest link in the hydraulic circuit ruptures and it's game over in short order. The expansion valve simply weeps a little bit of water out and saves from expensive repairs.

The OPV, on the other hand, is generally connected to the pump, or at least close to it. It is set to whatever you want your peak brew pressure to be. It usually never sees the expansion pressure because it is good practice to install a check valve between the pump and the brew boiler. This check valve prevents the expansion pressure from trying to back up through the pump and into the reservoir on reservoir-fed machines. There are a few posts on HB of folks troubleshooting bubbles and hot water in the reservoir due to the failure of this check valve.


- Jake
LMWDP #704

Cerini Coffee & Gifts: official US importer for Olympia Express
Sponsored by Cerini Coffee & Gifts

#3: Post by JRising »

But but but, what about the safety on the boiler, doesn't it get to be an OPV too?

Any valve that opens to relieve pressure above a set point, the expansion valve, the boiler safety valve, the brew circuit's OPV, even the pump's bypass relief, is an over pressure valve, but not every OPV is an expansion valve, or a boiler safety valve or a pump bypass...

OPV is a type of valve opened by pressure versus a spring.
Expansion Valve is a purpose of a valve, specifically the valve that lets pressure out of a boiler while the element is on. In most domestic machines it's a simple OPV, in far more expensive machines it is often a solenoid valve because the machine's are able to monitor temperature and pressure at important points and operate valves accordingly.

User avatar
BaristaBoy E61 (original poster)

#4: Post by BaristaBoy E61 (original poster) »

Thank you Jake and John for responding as I'm trying to understand the theory of the OPV devices. Sometimes it seems that the OPV is functioning as an Over Pressure Valve set at around 11-Bar and at other times it 'seems' that the same device is functioning as a Vacuum Relief Valve expelling air from boilers when heating up. In the case of our machine the same OPV is used for both boilers.

Then there's the Steam Boiler Safety Valve that you mentioned John, that is only on the Steam Boiler. This I'm well aware of as we once had a condition of 'Thermal Runaway' of the Steam Boiler, after a complete descaling, triggered (the right word) an activation of the Steam Boiler Safety Valve that almost sounded as if someone had fired a gun in the kitchen.

My question at this point is how does 1-OPV provide dual functions for both boilers; is this function simultaneous or sequential or what?

"You didn't buy an Espresso Machine - You bought a Chemistry Set!"

Team HB

#5: Post by Jeff »

The boiler generally has compressible air or steam in it, so thermal expansion is not anywhere near as much of an issue as with the water-filled brew system. The steam boiler is protected by a "pop-off" valve, often around 2-3 bar, depending on the machine's intended operating pressure.

The vacuum relief valve on the steam boiler opens as it cools, to equalize pressure when your machine is off.


#6: Post by JRising »

BaristaBoy E61 wrote: My question as this point is how does 1-OPV provide dual functions for both boilers; is this function simultaneous or sequential or what?
Aha, you have a Duetto III, and you're seeing the location of that expansion valve on a pipe between the two boilers. Yes, very cool engineering of a machine that's going to have a very stable brew boiler, but it isn't the simplest of machines.

What you actually have there is a system where the steam boiler has a pipe running through it, it has its own heat exchanger heating the water in a pipe between the pump and the brew boiler. You have to remember that even though the outside of the pipe is in the steam boiler, it is a separate system hydraulically. Brew water isn't leaking out to steam boiler, it's just getting nice and warm so that the PID can have less "cold inrush" to deal with when keeping the brew boiler at temperature.

So, that expansion valve that you see on it is really only connected to and only for the brew circuit. The check valve after the pump seals one end of that circuit, the closed brew valve seals the other. Without an expansion valve something would rupture with all that heat being added both at the pipe through the steam boiler and the brew boiler's heating element.

The safety valve on the steam boiler hydraulically prevents the steam boiler from going over pressure as you know from the shotgun noise. The breather valve/anti-vacuum valve is there to let the air out (Remember a good steam pocket in a steam boiler is actually water vapour, not air), you want the steam coming off the boiler at it reaches 100C to displace most of the air and then shut the anti-vacuum valve. You want the valve to open back up as it cools back down so you don't end up with a partial vacuum in the boiler and potentially suck milk up a frothing wand or even collapse a large copper boiler.
★ Helpful

User avatar
BaristaBoy E61 (original poster)

#7: Post by BaristaBoy E61 (original poster) »

Thank you John for your concise explanation.

I always look forward to and enjoy all that you write and contribute to on this website. It's most appreciated.
"You didn't buy an Espresso Machine - You bought a Chemistry Set!"