Don Y wrote:
On 11/23/2015 9:28 PM, Don Y wrote:

http://www.sprinklerwarehouse.com/DV-050-C-p/dv-050-c.htm

(Look at rightmost photo)

In operation, water pressure pushes that slug outward to seal the
hole so water does not escape the pipe (save for a little "spit" out
as the slug moves to close the hole). Once water pressure is
removed AND FALLS BELOW a couple of psi, the spring can once again
overcome that pressure and push the slug away from the hole, thereby
creating an opening, again, through which air can enter or water
exit.

In theory, these devices don't do anything in normal operation
(of the solenoid valves). The "irrigation controller" turns on
each solenoid based on a set of criteria (previous/current/predicted
weather conditions, the water needs of the plants that each "zone"
services, etc.). *As* the solenoid comes on, water flows through
the pipe building pressure as it does so. This quickly ends up
causing the "spitter" to close so no water leaves the pipe via
this "exit".

When the irrigation controller decides to turn the solenoid valve
off, water pressure downstream of the solenoid SHOULD fall
(water keeps oozing out "wherever" so the pressure falls as it
does so).

When the pressure falls enough, the spitter opens and provides
an exit for any remaining water "standing" in the pipe.

The controller eventually turns off the MASTER solenoid that
gates water to the entire irrigation (and hose bibb) system.
At that point, it deliberately opens OTHER valves to provide
a means for pressure remaining in the lines to be released.

In the event that the hose bibb downstream from a solenoid valve
is CLOSED, when the solenoid initially is opened (to supply water
to that zone/circuit), there will be no place for the water to
go! As such, the portion of pipe AFTER the solenoid will be
pressurized to the supply pressure through the solenoid valve.
This pressure will force the slug to plug the hole -- and,
remain in that plugged state as long as pressure persists.

With an IDEAL solenoid valve, closing the valve will trap this
pressure on the downstream side of the solenoid valve. The
slug will never move because it's always being forced closed.
Water will "stand" in the pipe *after* the solenoid.

By deliberately turning off the master supply valve and opening the
other (irrigation) valves, the pressure on the upstream side of
this particular solenoid valve will fall. So, if the controller
opens this solenoid valve at that time, the "standing" water
will have a means of finding pressure relief back into the supply
line.

I don't think the electric valves I have will open unless the supply pressure is
higher than the pressure on the other side of the valve.


This should open the "slug" and accelerate the draining of that
"standing" portion of the line.

Or, the open faucet relieves the pressure so the valves opens. If the faucet is
closed, that may not happen, depending on the design of your control valves.


After a short while, the irrigation controller can let all of the
valves return to their closed states -- confident that any water
remaining in the system has fallen to the lowest point *in* that
system.

I.e., I use the solenoid valves for the "other" zones as vents.

Which may not work.