Calcification and "auto-drain" valves

We have reasonably hard water -- municipal well sourced along with CAP.
I've plumbed the "outdoor water supplies" (i.e., irrigation and hose bibbs) to use water directly from the municipal supply (i.e., NOT "locally" treated) as I don't want to be paying to "treat" water that's going out a garden hose, etc.
[I don't think the plants would like the extra salt, regardless]
There are ~20 solenoid operated valves buried around the yard (little concrete, covered vaults that I built for them). Each operates an irrigation "zone" *or* one of the 4 hose bibbs scattered around the perimeter.
The irrigation *lines* are all below grade. So, essentially safe from freezes. But, the hose bibbs require the water supply (exiting the solenoid valve) to climb to a height that is convenient for a human user to access. So, that portion of the plumbing that is above grade is exposed to the elements more than the rest of the lines which remain below grade.
These risers are 3/4" copper run through the "voids" in 8" "half (cinder) blocks" which have then been filled with "Nasty Foam" (TmReg) to improve insulation.
So, you have a length of copper pipe that exits the solenoid valve, rises through the surface of the soil *into* this foam-filled void and terminates at a hose bibb -- the valve of which might be open or closed
[no idea as to which; the whole point of the solenoid is to allow me to put a garden hose on a bibb, set the (manual) valve to an appropriate flow rate, and direct the hose's output to a particular spot in the yard that needs "supplemental watering". Then, use the solenoid to turn that water supply on or off.]
Of course, I don't want to risk the water standing in that pipe freezing and rupturing the pipe (I can't rely on it destroying the valve, instead :< ).
To that end, I installed these auto-drain valves (colloquially known as "spitters") on the "load" side of the solenoid. They resemble a PCV valve in construction: an opening which can be plugged by a slug of metal inside the valve body. A low tension spring holds the plug AWAY from the opening so it is normally open. The slightest water pressure acts to push the slug (plug) against the hole thereby sealing it.
When pressure is removed, the spring's force eventually overcomes that of the fading water pressure and the slug exposes the opening. As this is located at the low point in the plumbing, the water "standing" above it *should* drain out through this opening.
[This is a sort of bastardization of their use]
Over time, I suspect these will fail due to concretions from the water supply. Anyone know for a fact if this is true? I've located them in a way that allows me to service them down the road. But, I'm looking for FEWER chores, not MORE! :<
Alternatively, I'm considering forcing all of the solenoids *open* (after turning off the main supply) and hoping water will "seek its level" throughout the system -- instead of dealing with each of these little "stand pipes" in isolation.
[Or, even adding an "extra" valve whose sole purpose is to be the "low point" in the system so there is a place BELOW GRADE for all water to drain]
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On 11/22/15 4:04 PM, Don Y wrote:

Is blowing the lines out with compressed air not feasible for some reason ?
(FWIW, I once had one of those "auto" valves stick open. What a mess !)
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On 11/22/2015 2:17 PM, Retired wrote:

How would I do that? This is intended for "unattended operation". E.g., today, irrigation zones 3, 7, 8 and 14 might be used -- individually or in some combination. Tomorrow, hose bibb #2 might see some use and the *solenoid* for bibb #4 might be engaged -- but, someone may have turned off the (manual) valve/bibb (because they decided this supplemental water was no longer needed yet hadn't got 'round to updating the "programmed watering schedule", yet).
So, if I force air through from the *supply*, there's no guarantee it will have a means of "exit" (assuming I deliberately open all desired solenoids).
And, I can't push air through from the "output" end (obvious reasons).

That's what worries me. These are hidden and below grade. I have made drainage provisions for a little "spitting" that is inevitable with each actuation of the device. But, if it failed open (even partially so), I'd never know it unless the valve vault overflowed with water (and, I happened to be outside to notice it!)
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