Re: End-of-tank-dump -- Vacuum after shut down

["S. Hieber" <shieber at yahoo_com>]

If you've had enough of this subject skip to the next
message.  But if you wonder what's going on - - - -

Kevin Buckley has a possibly mysterious situation when he
shuts off the the CO2 tank valve and solenoid on his
compressed CO2 injection system:

> . . .The setup is - CO2 tank, tank shutoff valve,
> regulator +
> gauges + needle
> valve (it's a combined assembly), solenoid, check
> valve#1, bubble-counter,
> check valve#2, reactor. . .
> . . .when the CO2 is shut off, the bubble counter
> water flow back up the
> "inlet" (which as stated above is below the water 
> level).  . .if I didn't have check valve#1, the water 
> would go
> back through the
> solenoid & into the regulator!
> 
> By the way - I added check valve #2 because without it
> the water being
> "sucked" back down towards the cylinder also sucks water
> back from the
> reactor into the bubble counter which eventually results
> in the bubble
> counter becoming full with water (the water enters
> through the "outlet" &,
> of course, when the CO2 flow reverses it can't get back
> out again).

The collapse of the tubing between the solenoid and the 1st
check valve is intriguing.  One supposition is that the
water in the CO2 tubing is absorbing the CO2 in the gas
line and thereby causing a vacuum to occur, collapsing the
tubing between the solenoid and the 1st check valve.  I
have difficulty with this notion (so what?) because I would
expect it to be more likely that the relative vacuum on the
supply side of the bubble counter is (or is part of) what
is causing the water to flow back in the first place.  But
I wouldn't expect the pressure in the tubing between the
solenoid and the 1st check valve to be a vacuum *or
substantially less than room air pressure* (sufficient to
collapse the tubing).

When the second check valve is open, the water pump
pressure is pushing against the CO2 system and the CO2
(low-side) pressure keeps the water from coming out the
reactor and into the CO2 system.  When  the CO2 tank is
closed off, or the solenoid is closed, that pressure should
decline since CO2 can still escape from the tubing after
the solenoid and into the reactor.  But only up to a point.
 If the CO2 system has no leaks, then the CO2 pressure
after the solenoid will decline to a point that the pump
pressure exceeds the pump will begin pushing water back
into the CO2 system  -- but only until a pressure balance
is acheived (and a positive pressure at that) or a check
valve blocks the flow.

All of this seems to happen with Kevin's system *except* 
there is this additional thing of the relative vacuum on
the tubing between the solenoid and the 1st check valve.  I
don't think water in the CO2 line is absorbing so much CO2
that it is causing a vacuum, the check valve is blocking
the water from that tubing.

If the solenoid valve is such that power is used to open
its valve, then the heat generated could be enough to
slightly warm the tubing attached to it and the gas that
just passed through it.  When the solendoid is shut down
(powered off) and its valve is closed and the check valve
is closed (due to pump pressure), then the tubing cools off
to room temp and that might cause a vacuum in the piece of
hose in contact with the solenoid (but only because it is
closed off on the other end by the check valve).  I'd put
money on this guess, so to speak, but I've lost bets before
;-)

Scott H.

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