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About 3-way Solenoid Valve Operation

Postby MarcoChao on Wed Mar 02, 2011 8:35 am

Hello guys!

A couple of days ago, I did a little inspection of my beloved Linea to see if she needs a descale. I took the brewhead 3-way solenoid valve off, and surprisingly I found the spring around that solenoid nucleus is a lot softer than I would expect. I know the pressure from brewing boiler could get about 12 bars when water expands, so how could this spring hold that much pressure? It is kind of stupid to ask, but I just really don't understand it. I also noticed there is another harder spring that holds the viton seal of solenoid nucleus, but it seems it has nothing to do with the functioning of solenoid valve.

I am curious, so I just throw the question out for whoever cares, thanks!
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Postby HB on Wed Mar 02, 2011 9:14 am

The purpose of the spring is to return the solenoid's piston to the "rest" position, not resist water pressure. To put it another way, the 3-way valve is a cylinder with holes drilled in its side; energizing the coil moves the piston to one end, thereby blocking one pathway and unblocking another. When power is removed, the spring pushes the piston back to its rest position.
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Postby MarcoChao on Wed Mar 02, 2011 10:11 am

Thanks a lot for the reply!

So, did you mean there is no pressure at all against the piston when it is in the rest position?
I don't understand, because I suppose there is nothing blocks the water away from the piston. And if the water does contact with the piston, then the piston must experience the water pressure at some level as the expansion valve (or OPV) does. As you know, the spring in the expansion valve is very hard, one can hardly compress it. :?
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Postby kmills on Wed Mar 02, 2011 10:38 am

When the solenoid is active (during extraction), the coil pushes the piston to the closed position using electromagnetic force. This force takes the place of the stiff spring you are referring to and resists the extraction pressure. When de-energized, the magnetic force is removed and the light spring returns the piston to the open position and lets the water flow out. This is a "normally open" (NO) type solenoid. A boiler fill type solenoid will work in reverse, opening when energized (well you can use NO, but that would degrade its service life and the programming for the controller is different, so dont just swap them).
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Postby shadowfax on Wed Mar 02, 2011 12:17 pm

kmills wrote:(well you can use NO, but that would degrade its service life and the programming for the controller is different, so dont just swap them)

Not to mention what would happen with a NO solenoid valve on your autofill if you have a plumbed-in machine and you turn it off!
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Postby kmills on Wed Mar 02, 2011 12:20 pm

details details!
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Postby erics on Wed Mar 02, 2011 12:49 pm

I am curious, so I just throw the question out for whoever cares, thanks!

The pressure from the brew boiler (~12 bar - expansion issues) acts on the solenoid piston via a ~ 1.00 mm hole. The Linea's "hole" may/may not be 1.00 mm, but close enough :)

12 bar = ~174 pounds per square inch
1.00 mm diameter = ~ 0.00122 square inches
Force on piston = ~0.21 lbs = ~ 3.4 ounces

When the valve assembly is screwed in place, the spring gets compressed such that its opposing force is greater than 3.4 ounces.
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Postby jarviscochrane on Wed Mar 02, 2011 10:26 pm

If my exhaust pipe from the group drips during warm up instead of out the thermal expansion valve, could it be that the spring in the 3 way solenoid is not correctly returning the valve to the normally closed position?
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Postby decaf_Ed on Wed Mar 02, 2011 11:38 pm

Many of those valves have an elastomeric seal at both ends of the plunger. One possible cause for a drip is a worn seal at the bottom end, such as the lavender colored parts here:Image
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Postby shadowfax on Wed Mar 02, 2011 11:50 pm

I think the material in that picture is ruby, like the stuff they cut high-precision gicleurs out of. I'm curious how elastomeric it is, but certainly it's supposed to seal. Most of the cores on the 3-way valves I have seen (including my Elektra and GS3) are viton, which is definitely elastomeric.

Anyway, I'd add to the diagnosis of worn seals (probably much more likely with viton than ruby!) that another possibility is scale buildup on the seal. You can diagnose either pretty easy with a quick disassembly.
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